Cómo: Definir un tipo genérico con Reflection Emit

Artículo
08/23/2008

Actualización: noviembre 2007

Este tema explica cómo crear un tipo genérico simple con dos parámetros de tipo, cómo aplicar restricciones de clases, restricciones de interfaz y restricciones especiales a los parámetros de tipo, y cómo crear miembros que utilicen los parámetros de tipo de la clase como tipos de parámetro y tipos devueltos.

Nota importante:
Un método no es genérico sólo porque pertenece a un tipo genérico y utiliza los parámetros de tipo de ese tipo genérico. Un método sólo es genérico si tiene su propia lista de parámetros de tipo. La mayoría de los métodos de tipos genéricos no son genéricos, como en este ejemplo. Para obtener un ejemplo de emisión de un método genérico, vea Cómo: Definir un método genérico con Reflection Emit.

Un método no es genérico sólo porque pertenece a un tipo genérico y utiliza los parámetros de tipo de ese tipo genérico. Un método sólo es genérico si tiene su propia lista de parámetros de tipo. La mayoría de los métodos de tipos genéricos no son genéricos, como en este ejemplo. Para obtener un ejemplo de emisión de un método genérico, vea Cómo: Definir un método genérico con Reflection Emit.

Para definir un tipo genérico

Defina un ensamblado dinámico denominado GenericEmitExample1. En este ejemplo, se ejecuta el ensamblado y se guarda en el disco, por lo que se especifica AssemblyBuilderAccess.RunAndSave.

Dim myDomain As AppDomain = AppDomain.CurrentDomain
Dim myAsmName As New AssemblyName("GenericEmitExample1")
Dim myAssembly As AssemblyBuilder = myDomain.DefineDynamicAssembly( _
    myAsmName, _
    AssemblyBuilderAccess.RunAndSave)

AppDomain myDomain = AppDomain.CurrentDomain;
AssemblyName myAsmName = new AssemblyName("GenericEmitExample1");
AssemblyBuilder myAssembly = 
    myDomain.DefineDynamicAssembly(myAsmName, 
        AssemblyBuilderAccess.RunAndSave);

AppDomain^ myDomain = AppDomain::CurrentDomain;
AssemblyName^ myAsmName = gcnew AssemblyName( L"GenericEmitExample1" );
AssemblyBuilder^ myAssembly = myDomain->DefineDynamicAssembly( 
    myAsmName, AssemblyBuilderAccess::RunAndSave );

Defina un módulo dinámico. Un ensamblado se compone de módulos ejecutables. Para un ensamblado de un único módulo, el nombre de módulo es el mismo que el del nombre de ensamblado y el nombre de archivo es el nombre del módulo más una extensión.

Dim myModule As ModuleBuilder = myAssembly.DefineDynamicModule( _
    myAsmName.Name, _
    myAsmName.Name & ".dll")

ModuleBuilder myModule = 
    myAssembly.DefineDynamicModule(myAsmName.Name, 
       myAsmName.Name + ".dll");

ModuleBuilder^ myModule = myAssembly->DefineDynamicModule( 
    myAsmName->Name, String::Concat( myAsmName->Name, L".dll" ) );

Defina una clase. En este ejemplo, la clase se denomina Sample.

Dim myType As TypeBuilder = myModule.DefineType( _
    "Sample", _
    TypeAttributes.Public)

TypeBuilder myType = 
    myModule.DefineType("Sample", TypeAttributes.Public);

TypeBuilder^ myType = myModule->DefineType( L"Sample", 
    TypeAttributes::Public );

Defina los parámetros de tipo genérico de Sample pasando una matriz de cadenas que contienen los nombres de los parámetros al método TypeBuilder.DefineGenericParameters. De esta forma la clase se vuelve de tipo genérico. El valor devuelto es una matriz de objetos GenericTypeParameterBuilder que representan los parámetros de tipo, que se pueden utilizar en el código emitido.

En el código siguiente, Sample se vuelve de tipo genérico con parámetros de tipo TFirst y TSecond. Para facilitar la lectura del código, cada constructor GenericTypeParameterBuilder se coloca en una variable con el mismo nombre que el del parámetro de tipo.

Dim typeParamNames() As String = {"TFirst", "TSecond"}
Dim typeParams() As GenericTypeParameterBuilder = _
    myType.DefineGenericParameters(typeParamNames)

Dim TFirst As GenericTypeParameterBuilder = typeParams(0)
Dim TSecond As GenericTypeParameterBuilder = typeParams(1)

string[] typeParamNames = {"TFirst", "TSecond"};
GenericTypeParameterBuilder[] typeParams = 
    myType.DefineGenericParameters(typeParamNames);

GenericTypeParameterBuilder TFirst = typeParams[0];
GenericTypeParameterBuilder TSecond = typeParams[1];

array<String^>^typeParamNames = {L"TFirst",L"TSecond"};
array<GenericTypeParameterBuilder^>^typeParams = 
    myType->DefineGenericParameters( typeParamNames );

GenericTypeParameterBuilder^ TFirst = typeParams[0];
GenericTypeParameterBuilder^ TSecond = typeParams[1];

Agregue restricciones especiales a los parámetros de tipo. En este ejemplo, el parámetro de tipo TFirst está restringido a tipos que tienen constructores sin parámetros y a tipos de referencia.

TFirst.SetGenericParameterAttributes( _
    GenericParameterAttributes.DefaultConstructorConstraint _
    Or GenericParameterAttributes.ReferenceTypeConstraint)

TFirst.SetGenericParameterAttributes(
    GenericParameterAttributes.DefaultConstructorConstraint |
    GenericParameterAttributes.ReferenceTypeConstraint);

TFirst->SetGenericParameterAttributes( 
    GenericParameterAttributes::DefaultConstructorConstraint | 
    GenericParameterAttributes::ReferenceTypeConstraint 
);

También tiene la opción de agregar restricciones de clase y de interfaz a los parámetros de tipo. En este ejemplo, el parámetro de tipo TFirst está restringido a los tipos derivados de la clase base representada por el objeto Type contenido en la variable baseType, y que implementan las interfaces cuyos tipos están contenidos en las variables interfaceA e interfaceB. Vea el ejemplo de código para la declaración y la asignación de estas variables.
```
TSecond.SetBaseTypeConstraint(baseType)
Dim interfaceTypes() As Type = {interfaceA, interfaceB}
TSecond.SetInterfaceConstraints(interfaceTypes)
```
```
TSecond.SetBaseTypeConstraint(baseType);
Type[] interfaceTypes = {interfaceA, interfaceB};
TSecond.SetInterfaceConstraints(interfaceTypes);
```
```
array<Type^>^interfaceTypes = { interfaceA, interfaceB };
TSecond->SetInterfaceConstraints( interfaceTypes );
TSecond->SetBaseTypeConstraint( baseType );
```

Defina un campo. En este ejemplo, el tipo del campo viene especificado por el parámetro de tipo TFirst. GenericTypeParameterBuilder deriva de Type, por lo que puede usar parámetros de tipo genéricos siempre que se pueden utilizar tipos.

Dim exField As FieldBuilder = _
    myType.DefineField("ExampleField", TFirst, _
        FieldAttributes.Private)

FieldBuilder exField = 
    myType.DefineField("ExampleField", TFirst, 
        FieldAttributes.Private);

FieldBuilder^ exField = 
    myType->DefineField("ExampleField", TFirst, 
        FieldAttributes::Private);

Defina un método que utilice los parámetros de tipo del tipo genérico. Observe que tales métodos no son genéricos a menos que tengan sus propias listas de parámetros de tipo. El código siguiente define un método static (Shared en Visual Basic) que toma una matriz de TFirst y devuelve una List<TFirst> (List(Of TFirst) en Visual Basic) que contiene todos los elementos de la matriz. Para definir este método, es necesario crear el tipo List<TFirst> llamando a MakeGenericType en la definición de tipo genérico, List<T>. (Se omite T cuando se utiliza el operador typeof (GetType en Visual Basic) para obtener la definición de tipo genérico.) El tipo de parámetro se crea utilizando el método MakeArrayType.

Dim listOf As Type = GetType(List(Of ))
Dim listOfTFirst As Type = listOf.MakeGenericType(TFirst)
Dim mParamTypes() As Type = { TFirst.MakeArrayType() }

Dim exMethod As MethodBuilder = _
    myType.DefineMethod("ExampleMethod", _
        MethodAttributes.Public Or MethodAttributes.Static, _
        listOfTFirst, _
        mParamTypes)

Type listOf = typeof(List<>);
Type listOfTFirst = listOf.MakeGenericType(TFirst);
Type[] mParamTypes = {TFirst.MakeArrayType()};

MethodBuilder exMethod = 
    myType.DefineMethod("ExampleMethod", 
        MethodAttributes.Public | MethodAttributes.Static, 
        listOfTFirst, 
        mParamTypes);

Type^ listOf = List::typeid;
Type^ listOfTFirst = listOf->MakeGenericType(TFirst);
array<Type^>^ mParamTypes = { TFirst->MakeArrayType() };

MethodBuilder^ exMethod = 
    myType->DefineMethod("ExampleMethod", 
        MethodAttributes::Public | MethodAttributes::Static, 
        listOfTFirst, 
        mParamTypes);

Emita el cuerpo del método. El cuerpo del método consta de tres códigos de operación que cargan la matriz de entrada en la pila, llaman al constructor List<TFirst> que toma IEnumerable<TFirst> (que realiza el trabajo de poner en la lista los elementos de entrada), y vuelven (dejando en la pila el nuevo objeto List<T>). La parte difícil de emitir este código es la de obtener el constructor.

El método GetConstructor no se admite en un GenericTypeParameterBuilder, por lo que no es posible de obtener directamente el constructor de List<TFirst>. En primer lugar, es necesario obtener el constructor de la definición de tipo genérico List<T> y luego llamar a un método que lo convierta en el correspondiente constructor de List<TFirst>.

El constructor utilizado para este código toma una IEnumerable<T>. Observe, sin embargo, que ésta no es la definición de tipo genérico de la interfaz genérica IEnumerable<T>, sino que el parámetro de tipo T de List<T> debe sustituirse para el parámetro de tipo T de IEnumerable<T>. (Esto puede parecer confuso sólo porque ambos tipos tienen parámetros de tipo denominados T. Por eso en este ejemplo de código se utilizan los nombres TFirst y TSecond.) Para obtener el tipo del argumento del constructor, comience con la definición de tipo genérico IEnumerable<T> y llame a MakeGenericType con el primer parámetro de tipo genérico de List<T>. La lista de argumentos de constructores se debe pasar como una matriz, con un único argumento en este caso.

Nota:

La definición de tipo genérico se expresa como IEnumerable<> cuando se utiliza el operador typeof de C# o IEnumerable(Of ) cuando se usa el operador GetType de Visual Basic.

Ahora es posible obtener el constructor de List<T> llamando a GetConstructor en la definición de tipo genérico. Para convertir este constructor en el constructor correspondiente de List<TFirst>, pase List<TFirst> y el constructor desde List<T> al método TypeBuilder.GetConstructor(Type, ConstructorInfo) estático.
```
Dim ilgen As ILGenerator = exMethod.GetILGenerator()

Dim ienumOf As Type = GetType(IEnumerable(Of ))
Dim listOfTParams() As Type = listOf.GetGenericArguments()
Dim TfromListOf As Type = listOfTParams(0)
Dim ienumOfT As Type = ienumOf.MakeGenericType(TfromListOf)
Dim ctorArgs() As Type = { ienumOfT }

Dim ctorPrep As ConstructorInfo = _
    listOf.GetConstructor(ctorArgs)
Dim ctor As ConstructorInfo = _
    TypeBuilder.GetConstructor(listOfTFirst, ctorPrep)

ilgen.Emit(OpCodes.Ldarg_0)
ilgen.Emit(OpCodes.Newobj, ctor)
ilgen.Emit(OpCodes.Ret)
```
```
ILGenerator ilgen = exMethod.GetILGenerator();

Type ienumOf = typeof(IEnumerable<>);
Type TfromListOf = listOf.GetGenericArguments()[0];
Type ienumOfT = ienumOf.MakeGenericType(TfromListOf);
Type[] ctorArgs = {ienumOfT};

ConstructorInfo ctorPrep = listOf.GetConstructor(ctorArgs);
ConstructorInfo ctor = 
    TypeBuilder.GetConstructor(listOfTFirst, ctorPrep);

ilgen.Emit(OpCodes.Ldarg_0);
ilgen.Emit(OpCodes.Newobj, ctor);
ilgen.Emit(OpCodes.Ret);
```
```
ILGenerator^ ilgen = exMethod->GetILGenerator();

Type^ ienumOf = IEnumerable::typeid;
Type^ TfromListOf = listOf->GetGenericArguments()[0];
Type^ ienumOfT = ienumOf->MakeGenericType(TfromListOf);
array<Type^>^ ctorArgs = {ienumOfT};

ConstructorInfo^ ctorPrep = listOf->GetConstructor(ctorArgs);
ConstructorInfo^ ctor = 
    TypeBuilder::GetConstructor(listOfTFirst, ctorPrep);

ilgen->Emit(OpCodes::Ldarg_0);
ilgen->Emit(OpCodes::Newobj, ctor);
ilgen->Emit(OpCodes::Ret);
```

Nota:
La definición de tipo genérico se expresa como IEnumerable<> cuando se utiliza el operador typeof de C# o IEnumerable(Of ) cuando se usa el operador GetType de Visual Basic.

Cree el tipo y guarde el archivo.

Dim finished As Type = myType.CreateType()
myAssembly.Save(myAsmName.Name & ".dll")

Type finished = myType.CreateType();
myAssembly.Save(myAsmName.Name+".dll");

Type^ finished = myType->CreateType();
myAssembly->Save( String::Concat( myAsmName->Name, L".dll" ) );

Invoque el método. ExampleMethod no es genérico, pero el tipo al que pertenece sí es genérico; por tanto, para obtener una MethodInfo que se pueda invocar es necesario crear un tipo construido a partir de la definición de tipo de Sample. El tipo construido utiliza la clase Example, que satisface las restricciones de TFirst porque es un tipo de referencia y tiene un constructor predeterminado sin parámetros, y la clase ExampleDerived que satisface las restricciones de TSecond. (El código para ExampleDerived se puede encontrar en la sección de código de ejemplo.) Estos dos tipos se pasan a MakeGenericType para crear el tipo construido. MethodInfo se obtiene entonces utilizando el método GetMethod.
```
Dim typeArgs() As Type = _
    { GetType(Example), GetType(ExampleDerived) }
Dim constructed As Type = finished.MakeGenericType(typeArgs)
Dim mi As MethodInfo = constructed.GetMethod("ExampleMethod")
```
```
Type[] typeArgs = {typeof(Example), typeof(ExampleDerived)};
Type constructed = finished.MakeGenericType(typeArgs);
MethodInfo mi = constructed.GetMethod("ExampleMethod");
```
```
array<Type^>^ typeArgs = 
    { Example::typeid, ExampleDerived::typeid };
Type^ constructed = finished->MakeGenericType(typeArgs);
MethodInfo^ mi = constructed->GetMethod("ExampleMethod");
```

El código siguiente crea una matriz de objetos Example, coloca esa matriz en una matriz del tipo Object que representa los argumentos del método que se va a invocar y los pasa al método Invoke(Object, array<Object[]). El primer argumento del método Invoke es una referencia nula porque el método es static.

Dim input() As Example = { New Example(), New Example() }
Dim arguments() As Object = { input }

Dim listX As List(Of Example) = mi.Invoke(Nothing, arguments)

Console.WriteLine(vbLf & _
    "There are {0} elements in the List(Of Example).", _
    listX.Count _ 
)

Example[] input = {new Example(), new Example()};
object[] arguments = {input};

List<Example> listX = 
    (List<Example>) mi.Invoke(null, arguments);

Console.WriteLine(
    "\nThere are {0} elements in the List<Example>.", 
    listX.Count);

array<Example^>^ input = { gcnew Example(), gcnew Example() };
array<Object^>^ arguments = { input };

List<Example^>^ listX = 
    (List<Example^>^) mi->Invoke(nullptr, arguments);

Console::WriteLine(
    "\nThere are {0} elements in the List<Example>.", 
    listX->Count);

Ejemplo

El ejemplo de código siguiente define una clase denominada Sample, junto con una clase base y dos interfaces. El programa define dos parámetros de tipo genérico para Sample, convirtiéndolo en un tipo genérico. Los parámetros de tipo son lo único que hace que un tipo sea genérico. El programa lo indica mostrando un mensaje de prueba antes y después de la definición de los parámetros de tipo.

El parámetro de tipo TSecond se utiliza para explicar las restricciones de clase e interfaz, usando la clase y las interfaces base y, por último, el parámetro de tipo TFirst se emplea para ilustrar las restricciones especiales.

El ejemplo de código define un campo y un método que utiliza los parámetros de tipo de la clase para el tipo de campo y para el parámetro y el tipo de valor devuelto del método.

Después de haberse creado la clase Sample, se invoca el método.

El programa incluye un método que muestra información sobre un tipo genérico y un método que muestra las restricciones especiales en un parámetro de tipo. Estos métodos se utilizan para mostrar información sobre la clase Sample terminada.

El programa guarda en disco el módulo terminado como GenericEmitExample1.dll para que pueda abrirlo con el Desensamblador de MSIL (Ildasm.exe) y examinar el código MSIL de la clase Sample.

Imports System
Imports System.Reflection
Imports System.Reflection.Emit
Imports System.Collections.Generic

' Define a trivial base class and two trivial interfaces 
' to use when demonstrating constraints.
'
Public Class ExampleBase
End Class

Public Interface IExampleA
End Interface

Public Interface IExampleB
End Interface

' Define a trivial type that can substitute for type parameter 
' TSecond.
'
Public Class ExampleDerived
    Inherits ExampleBase
    Implements IExampleA, IExampleB
End Class

Public Class Example
    Public Shared Sub Main()
        ' Define a dynamic assembly to contain the sample type. The
        ' assembly will not be run, but only saved to disk, so
        ' AssemblyBuilderAccess.Save is specified.
        '
        Dim myDomain As AppDomain = AppDomain.CurrentDomain
        Dim myAsmName As New AssemblyName("GenericEmitExample1")
        Dim myAssembly As AssemblyBuilder = myDomain.DefineDynamicAssembly( _
            myAsmName, _
            AssemblyBuilderAccess.RunAndSave)

        ' An assembly is made up of executable modules. For a single-
        ' module assembly, the module name and file name are the same 
        ' as the assembly name. 
        '
        Dim myModule As ModuleBuilder = myAssembly.DefineDynamicModule( _
            myAsmName.Name, _
            myAsmName.Name & ".dll")

        ' Get type objects for the base class trivial interfaces to
        ' be used as constraints.
        '
        Dim baseType As Type = GetType(ExampleBase)
        Dim interfaceA As Type = GetType(IExampleA)
        Dim interfaceB As Type = GetType(IExampleB)

        ' Define the sample type.
        '
        Dim myType As TypeBuilder = myModule.DefineType( _
            "Sample", _
            TypeAttributes.Public)

        Console.WriteLine("Type 'Sample' is generic: {0}", _
            myType.IsGenericType)

        ' Define type parameters for the type. Until you do this, 
        ' the type is not generic, as the preceding and following 
        ' WriteLine statements show. The type parameter names are
        ' specified as an array of strings. To make the code
        ' easier to read, each GenericTypeParameterBuilder is placed
        ' in a variable with the same name as the type parameter.
        ' 
        Dim typeParamNames() As String = {"TFirst", "TSecond"}
        Dim typeParams() As GenericTypeParameterBuilder = _
            myType.DefineGenericParameters(typeParamNames)

        Dim TFirst As GenericTypeParameterBuilder = typeParams(0)
        Dim TSecond As GenericTypeParameterBuilder = typeParams(1)

        Console.WriteLine("Type 'Sample' is generic: {0}", _
            myType.IsGenericType)

        ' Apply constraints to the type parameters.
        '
        ' A type that is substituted for the first parameter, TFirst,
        ' must be a reference type and must have a parameterless
        ' constructor.
        TFirst.SetGenericParameterAttributes( _
            GenericParameterAttributes.DefaultConstructorConstraint _
            Or GenericParameterAttributes.ReferenceTypeConstraint)

        ' A type that is substituted for the second type
        ' parameter must implement IExampleA and IExampleB, and
        ' inherit from the trivial test class ExampleBase. The
        ' interface constraints are specified as an array 
        ' containing the interface types.
        TSecond.SetBaseTypeConstraint(baseType)
        Dim interfaceTypes() As Type = {interfaceA, interfaceB}
        TSecond.SetInterfaceConstraints(interfaceTypes)

        ' The following code adds a private field named ExampleField,
        ' of type TFirst.
        Dim exField As FieldBuilder = _
            myType.DefineField("ExampleField", TFirst, _
                FieldAttributes.Private)

        ' Define a Shared method that takes an array of TFirst and 
        ' returns a List(Of TFirst) containing all the elements of 
        ' the array. To define this method it is necessary to create
        ' the type List(Of TFirst) by calling MakeGenericType on the
        ' generic type definition, List(Of T). (The T is omitted with
        ' the GetType operator when you get the generic type 
        ' definition.) The parameter type is created by using the
        ' MakeArrayType method. 
        '
        Dim listOf As Type = GetType(List(Of ))
        Dim listOfTFirst As Type = listOf.MakeGenericType(TFirst)
        Dim mParamTypes() As Type = { TFirst.MakeArrayType() }

        Dim exMethod As MethodBuilder = _
            myType.DefineMethod("ExampleMethod", _
                MethodAttributes.Public Or MethodAttributes.Static, _
                listOfTFirst, _
                mParamTypes)

        ' Emit the method body. 
        ' The method body consists of just three opcodes, to load 
        ' the input array onto the execution stack, to call the 
        ' List(Of TFirst) constructor that takes IEnumerable(Of TFirst),
        ' which does all the work of putting the input elements into
        ' the list, and to return, leaving the list on the stack. The
        ' hard work is getting the constructor.
        ' 
        ' The GetConstructor method is not supported on a 
        ' GenericTypeParameterBuilder, so it is not possible to get 
        ' the constructor of List(Of TFirst) directly. There are two
        ' steps, first getting the constructor of List(Of T) and then
        ' calling a method that converts it to the corresponding 
        ' constructor of List(Of TFirst).
        '
        ' The constructor needed here is the one that takes an
        ' IEnumerable(Of T). Note, however, that this is not the 
        ' generic type definition of IEnumerable(Of T); instead, the
        ' T from List(Of T) must be substituted for the T of 
        ' IEnumerable(Of T). (This seems confusing only because both
        ' types have type parameters named T. That is why this example
        ' uses the somewhat silly names TFirst and TSecond.) To get
        ' the type of the constructor argument, take the generic
        ' type definition IEnumerable(Of T) (expressed as 
        ' IEnumerable(Of ) when you use the GetType operator) and 
        ' call MakeGenericType with the first generic type parameter
        ' of List(Of T). The constructor argument list must be passed
        ' as an array, with just one argument in this case.
        ' 
        ' Now it is possible to get the constructor of List(Of T),
        ' using GetConstructor on the generic type definition. To get
        ' the constructor of List(Of TFirst), pass List(Of TFirst) and
        ' the constructor from List(Of T) to the static
        ' TypeBuilder.GetConstructor method.
        '
        Dim ilgen As ILGenerator = exMethod.GetILGenerator()

        Dim ienumOf As Type = GetType(IEnumerable(Of ))
        Dim listOfTParams() As Type = listOf.GetGenericArguments()
        Dim TfromListOf As Type = listOfTParams(0)
        Dim ienumOfT As Type = ienumOf.MakeGenericType(TfromListOf)
        Dim ctorArgs() As Type = { ienumOfT }

        Dim ctorPrep As ConstructorInfo = _
            listOf.GetConstructor(ctorArgs)
        Dim ctor As ConstructorInfo = _
            TypeBuilder.GetConstructor(listOfTFirst, ctorPrep)

        ilgen.Emit(OpCodes.Ldarg_0)
        ilgen.Emit(OpCodes.Newobj, ctor)
        ilgen.Emit(OpCodes.Ret)

        ' Create the type and save the assembly. 
        Dim finished As Type = myType.CreateType()
        myAssembly.Save(myAsmName.Name & ".dll")

        ' Invoke the method.
        ' ExampleMethod is not generic, but the type it belongs to is
        ' generic, so in order to get a MethodInfo that can be invoked
        ' it is necessary to create a constructed type. The Example 
        ' class satisfies the constraints on TFirst, because it is a 
        ' reference type and has a default constructor. In order to
        ' have a class that satisfies the constraints on TSecond, 
        ' this code example defines the ExampleDerived type. These
        ' two types are passed to MakeGenericMethod to create the
        ' constructed type.
        '
        Dim typeArgs() As Type = _
            { GetType(Example), GetType(ExampleDerived) }
        Dim constructed As Type = finished.MakeGenericType(typeArgs)
        Dim mi As MethodInfo = constructed.GetMethod("ExampleMethod")

        ' Create an array of Example objects, as input to the generic
        ' method. This array must be passed as the only element of an 
        ' array of arguments. The first argument of Invoke is 
        ' Nothing, because ExampleMethod is Shared. Display the count
        ' on the resulting List(Of Example).
        ' 
        Dim input() As Example = { New Example(), New Example() }
        Dim arguments() As Object = { input }

        Dim listX As List(Of Example) = mi.Invoke(Nothing, arguments)

        Console.WriteLine(vbLf & _
            "There are {0} elements in the List(Of Example).", _
            listX.Count _ 
        )

        DisplayGenericParameters(finished)
    End Sub

    Private Shared Sub DisplayGenericParameters(ByVal t As Type)

        If Not t.IsGenericType Then
            Console.WriteLine("Type '{0}' is not generic.")
            Return
        End If
        If Not t.IsGenericTypeDefinition Then _
            t = t.GetGenericTypeDefinition()

        Dim typeParameters() As Type = t.GetGenericArguments()
        Console.WriteLine(vbCrLf & _
            "Listing {0} type parameters for type '{1}'.", _
            typeParameters.Length, t)

        For Each tParam As Type In typeParameters

            Console.WriteLine(vbCrLf & "Type parameter {0}:", _
                tParam.ToString())

            For Each c As Type In tParam.GetGenericParameterConstraints()
                If c.IsInterface Then
                    Console.WriteLine("    Interface constraint: {0}", c)
                Else
                    Console.WriteLine("    Base type constraint: {0}", c)
                End If
            Next 

            ListConstraintAttributes(tParam)
        Next tParam
    End Sub

    ' List the constraint flags. The GenericParameterAttributes
    ' enumeration contains two sets of attributes, variance and
    ' constraints. For this example, only constraints are used.
    '
    Private Shared Sub ListConstraintAttributes(ByVal t As Type)

        ' Mask off the constraint flags. 
        Dim constraints As GenericParameterAttributes = _
            t.GenericParameterAttributes And _
            GenericParameterAttributes.SpecialConstraintMask

        If (constraints And GenericParameterAttributes.ReferenceTypeConstraint) _
                <> GenericParameterAttributes.None Then _
            Console.WriteLine("    ReferenceTypeConstraint")

        If (constraints And GenericParameterAttributes.NotNullableValueTypeConstraint) _
                <> GenericParameterAttributes.None Then _
            Console.WriteLine("    NotNullableValueTypeConstraint")

        If (constraints And GenericParameterAttributes.DefaultConstructorConstraint) _
                <> GenericParameterAttributes.None Then _
            Console.WriteLine("    DefaultConstructorConstraint")

    End Sub 

End Class

' This code example produces the following output:
'
'Type 'Sample' is generic: False
'Type 'Sample' is generic: True
'
'There are 2 elements in the List(Of Example).
'
'Listing 2 type parameters for type 'Sample[TFirst,TSecond]'.
'
'Type parameter TFirst:
'    ReferenceTypeConstraint
'    DefaultConstructorConstraint
'
'Type parameter TSecond:
'    Interface constraint: IExampleA
'    Interface constraint: IExampleB
'    Base type constraint: ExampleBase

using System;
using System.Reflection;
using System.Reflection.Emit;
using System.Collections.Generic;

// Define a trivial base class and two trivial interfaces 
// to use when demonstrating constraints.
//
public class ExampleBase {}

public interface IExampleA {}

public interface IExampleB {}

// Define a trivial type that can substitute for type parameter 
// TSecond.
//
public class ExampleDerived : ExampleBase, IExampleA, IExampleB {}


public class Example
{
    public static void Main()
    {
        // Define a dynamic assembly to contain the sample type. The
        // assembly will not be run, but only saved to disk, so
        // AssemblyBuilderAccess.Save is specified.
        //
        AppDomain myDomain = AppDomain.CurrentDomain;
        AssemblyName myAsmName = new AssemblyName("GenericEmitExample1");
        AssemblyBuilder myAssembly = 
            myDomain.DefineDynamicAssembly(myAsmName, 
                AssemblyBuilderAccess.RunAndSave);

        // An assembly is made up of executable modules. For a single-
        // module assembly, the module name and file name are the same 
        // as the assembly name. 
        //
        ModuleBuilder myModule = 
            myAssembly.DefineDynamicModule(myAsmName.Name, 
               myAsmName.Name + ".dll");

        // Get type objects for the base class trivial interfaces to
        // be used as constraints.
        //
        Type baseType = typeof(ExampleBase);
        Type interfaceA = typeof(IExampleA);
        Type interfaceB = typeof(IExampleB);

        // Define the sample type.
        //
        TypeBuilder myType = 
            myModule.DefineType("Sample", TypeAttributes.Public);

        Console.WriteLine("Type 'Sample' is generic: {0}", 
            myType.IsGenericType);

        // Define type parameters for the type. Until you do this, 
        // the type is not generic, as the preceding and following 
        // WriteLine statements show. The type parameter names are
        // specified as an array of strings. To make the code
        // easier to read, each GenericTypeParameterBuilder is placed
        // in a variable with the same name as the type parameter.
        // 
        string[] typeParamNames = {"TFirst", "TSecond"};
        GenericTypeParameterBuilder[] typeParams = 
            myType.DefineGenericParameters(typeParamNames);

        GenericTypeParameterBuilder TFirst = typeParams[0];
        GenericTypeParameterBuilder TSecond = typeParams[1];

        Console.WriteLine("Type 'Sample' is generic: {0}", 
            myType.IsGenericType);

        // Apply constraints to the type parameters.
        //
        // A type that is substituted for the first parameter, TFirst,
        // must be a reference type and must have a parameterless
        // constructor.
        TFirst.SetGenericParameterAttributes(
            GenericParameterAttributes.DefaultConstructorConstraint |
            GenericParameterAttributes.ReferenceTypeConstraint);

        // A type that is substituted for the second type
        // parameter must implement IExampleA and IExampleB, and
        // inherit from the trivial test class ExampleBase. The
        // interface constraints are specified as an array 
        // containing the interface types.
        TSecond.SetBaseTypeConstraint(baseType);
        Type[] interfaceTypes = {interfaceA, interfaceB};
        TSecond.SetInterfaceConstraints(interfaceTypes);

        // The following code adds a private field named ExampleField,
        // of type TFirst.
        FieldBuilder exField = 
            myType.DefineField("ExampleField", TFirst, 
                FieldAttributes.Private);

        // Define a static method that takes an array of TFirst and 
        // returns a List<TFirst> containing all the elements of 
        // the array. To define this method it is necessary to create
        // the type List<TFirst> by calling MakeGenericType on the
        // generic type definition, List<T>. (The T is omitted with
        // the typeof operator when you get the generic type 
        // definition.) The parameter type is created by using the
        // MakeArrayType method. 
        //
        Type listOf = typeof(List<>);
        Type listOfTFirst = listOf.MakeGenericType(TFirst);
        Type[] mParamTypes = {TFirst.MakeArrayType()};

        MethodBuilder exMethod = 
            myType.DefineMethod("ExampleMethod", 
                MethodAttributes.Public | MethodAttributes.Static, 
                listOfTFirst, 
                mParamTypes);

        // Emit the method body. 
        // The method body consists of just three opcodes, to load 
        // the input array onto the execution stack, to call the 
        // List<TFirst> constructor that takes IEnumerable<TFirst>,
        // which does all the work of putting the input elements into
        // the list, and to return, leaving the list on the stack. The
        // hard work is getting the constructor.
        // 
        // The GetConstructor method is not supported on a 
        // GenericTypeParameterBuilder, so it is not possible to get 
        // the constructor of List<TFirst> directly. There are two
        // steps, first getting the constructor of List<T> and then
        // calling a method that converts it to the corresponding 
        // constructor of List<TFirst>.
        //
        // The constructor needed here is the one that takes an
        // IEnumerable<T>. Note, however, that this is not the 
        // generic type definition of IEnumerable<T>; instead, the
        // T from List<T> must be substituted for the T of 
        // IEnumerable<T>. (This seems confusing only because both
        // types have type parameters named T. That is why this example
        // uses the somewhat silly names TFirst and TSecond.) To get
        // the type of the constructor argument, take the generic
        // type definition IEnumerable<T> (expressed as 
        // IEnumerable<> when you use the typeof operator) and 
        // call MakeGenericType with the first generic type parameter
        // of List<T>. The constructor argument list must be passed
        // as an array, with just one argument in this case.
        // 
        // Now it is possible to get the constructor of List<T>,
        // using GetConstructor on the generic type definition. To get
        // the constructor of List<TFirst>, pass List<TFirst> and
        // the constructor from List<T> to the static
        // TypeBuilder.GetConstructor method.
        //
        ILGenerator ilgen = exMethod.GetILGenerator();

        Type ienumOf = typeof(IEnumerable<>);
        Type TfromListOf = listOf.GetGenericArguments()[0];
        Type ienumOfT = ienumOf.MakeGenericType(TfromListOf);
        Type[] ctorArgs = {ienumOfT};

        ConstructorInfo ctorPrep = listOf.GetConstructor(ctorArgs);
        ConstructorInfo ctor = 
            TypeBuilder.GetConstructor(listOfTFirst, ctorPrep);

        ilgen.Emit(OpCodes.Ldarg_0);
        ilgen.Emit(OpCodes.Newobj, ctor);
        ilgen.Emit(OpCodes.Ret);

        // Create the type and save the assembly. 
        Type finished = myType.CreateType();
        myAssembly.Save(myAsmName.Name+".dll");

        // Invoke the method.
        // ExampleMethod is not generic, but the type it belongs to is
        // generic, so in order to get a MethodInfo that can be invoked
        // it is necessary to create a constructed type. The Example 
        // class satisfies the constraints on TFirst, because it is a 
        // reference type and has a default constructor. In order to
        // have a class that satisfies the constraints on TSecond, 
        // this code example defines the ExampleDerived type. These
        // two types are passed to MakeGenericMethod to create the
        // constructed type.
        //
        Type[] typeArgs = {typeof(Example), typeof(ExampleDerived)};
        Type constructed = finished.MakeGenericType(typeArgs);
        MethodInfo mi = constructed.GetMethod("ExampleMethod");

        // Create an array of Example objects, as input to the generic
        // method. This array must be passed as the only element of an 
        // array of arguments. The first argument of Invoke is 
        // null, because ExampleMethod is static. Display the count
        // on the resulting List<Example>.
        // 
        Example[] input = {new Example(), new Example()};
        object[] arguments = {input};

        List<Example> listX = 
            (List<Example>) mi.Invoke(null, arguments);

        Console.WriteLine(
            "\nThere are {0} elements in the List<Example>.", 
            listX.Count);

        DisplayGenericParameters(finished);
    }

    private static void DisplayGenericParameters(Type t)
    {
        if (!t.IsGenericType)
        {
            Console.WriteLine("Type '{0}' is not generic.");
            return;
        }
        if (!t.IsGenericTypeDefinition) 
        {
            t = t.GetGenericTypeDefinition();
        }

        Type[] typeParameters = t.GetGenericArguments();
        Console.WriteLine("\nListing {0} type parameters for type '{1}'.",
            typeParameters.Length, t);

        foreach( Type tParam in typeParameters )
        {
            Console.WriteLine("\r\nType parameter {0}:", tParam.ToString());

            foreach( Type c in tParam.GetGenericParameterConstraints() )
            {
                if (c.IsInterface)
                {
                    Console.WriteLine("    Interface constraint: {0}", c);
                }
                else
                {
                    Console.WriteLine("    Base type constraint: {0}", c);
                }
            }

            ListConstraintAttributes(tParam);
        }
    }

    // List the constraint flags. The GenericParameterAttributes
    // enumeration contains two sets of attributes, variance and
    // constraints. For this example, only constraints are used.
    //
    private static void ListConstraintAttributes(Type t)
    {
        // Mask off the constraint flags. 
        GenericParameterAttributes constraints = 
            t.GenericParameterAttributes & GenericParameterAttributes.SpecialConstraintMask;

        if ((constraints & GenericParameterAttributes.ReferenceTypeConstraint)
            != GenericParameterAttributes.None) 
        {
            Console.WriteLine("    ReferenceTypeConstraint");
        }

        if ((constraints & GenericParameterAttributes.NotNullableValueTypeConstraint)
            != GenericParameterAttributes.None) 
        {
            Console.WriteLine("    NotNullableValueTypeConstraint");
        }

        if ((constraints & GenericParameterAttributes.DefaultConstructorConstraint)
            !=GenericParameterAttributes.None) 
        {
            Console.WriteLine("    DefaultConstructorConstraint");
        }
    }
}

/* This code example produces the following output:

Type 'Sample' is generic: False
Type 'Sample' is generic: True

There are 2 elements in the List<Example>.

Listing 2 type parameters for type 'Sample[TFirst,TSecond]'.

Type parameter TFirst:
    ReferenceTypeConstraint
    DefaultConstructorConstraint

Type parameter TSecond:
    Interface constraint: IExampleA
    Interface constraint: IExampleB
    Base type constraint: ExampleBase
 */

using namespace System;
using namespace System::Reflection;
using namespace System::Reflection::Emit;
using namespace System::Collections::Generic;

// Dummy class to satisfy TFirst constraints.
//
public ref class Example {};

// Define a trivial base class and two trivial interfaces 
// to use when demonstrating constraints.
//
public ref class ExampleBase {};
public interface class IExampleA {};
public interface class IExampleB {};

// Define a trivial type that can substitute for type parameter 
// TSecond.
//
public ref class ExampleDerived : ExampleBase, IExampleA, IExampleB {};

// List the constraint flags. The GenericParameterAttributes
// enumeration contains two sets of attributes, variance and
// constraints. For this example, only constraints are used.
//
static void ListConstraintAttributes( Type^ t )
{
   // Mask off the constraint flags. 
   GenericParameterAttributes constraints = 
       t->GenericParameterAttributes & 
       GenericParameterAttributes::SpecialConstraintMask;

   if ((constraints & GenericParameterAttributes::ReferenceTypeConstraint)
           != GenericParameterAttributes::None)
       Console::WriteLine( L"    ReferenceTypeConstraint");

   if ((constraints & GenericParameterAttributes::NotNullableValueTypeConstraint)
           != GenericParameterAttributes::None)
       Console::WriteLine( L"    NotNullableValueTypeConstraint");

   if ((constraints & GenericParameterAttributes::DefaultConstructorConstraint)
           != GenericParameterAttributes::None)
       Console::WriteLine( L"    DefaultConstructorConstraint");
}

static void DisplayGenericParameters( Type^ t )
{
   if (!t->IsGenericType)
   {
       Console::WriteLine( L"Type '{0}' is not generic." );
       return;
   }
   if (!t->IsGenericTypeDefinition)
       t = t->GetGenericTypeDefinition();

   array<Type^>^ typeParameters = t->GetGenericArguments();
   Console::WriteLine( L"\r\nListing {0} type parameters for type '{1}'.", 
       typeParameters->Length, t );

   for each ( Type^ tParam in typeParameters )
   {
       Console::WriteLine( L"\r\nType parameter {0}:", 
           tParam->ToString() );

       for each (Type^ c in tParam->GetGenericParameterConstraints())
       {
           if (c->IsInterface)
               Console::WriteLine( L"    Interface constraint: {0}", c);
           else
               Console::WriteLine( L"    Base type constraint: {0}", c);
       }
       ListConstraintAttributes(tParam);
   }
}

void main()
{
   // Define a dynamic assembly to contain the sample type. The
   // assembly will be run and also saved to disk, so
   // AssemblyBuilderAccess.RunAndSave is specified.
   //
   AppDomain^ myDomain = AppDomain::CurrentDomain;
   AssemblyName^ myAsmName = gcnew AssemblyName( L"GenericEmitExample1" );
   AssemblyBuilder^ myAssembly = myDomain->DefineDynamicAssembly( 
       myAsmName, AssemblyBuilderAccess::RunAndSave );

   // An assembly is made up of executable modules. For a single-
   // module assembly, the module name and file name are the same 
   // as the assembly name. 
   //
   ModuleBuilder^ myModule = myAssembly->DefineDynamicModule( 
       myAsmName->Name, String::Concat( myAsmName->Name, L".dll" ) );

   // Get type objects for the base class trivial interfaces to
   // be used as constraints.
   //
   Type^ baseType = ExampleBase::typeid; 
   Type^ interfaceA = IExampleA::typeid; 
   Type^ interfaceB = IExampleB::typeid;

   // Define the sample type.
   //
   TypeBuilder^ myType = myModule->DefineType( L"Sample", 
       TypeAttributes::Public );

   Console::WriteLine( L"Type 'Sample' is generic: {0}", 
       myType->IsGenericType );

   // Define type parameters for the type. Until you do this, 
   // the type is not generic, as the preceding and following 
   // WriteLine statements show. The type parameter names are
   // specified as an array of strings. To make the code
   // easier to read, each GenericTypeParameterBuilder is placed
   // in a variable with the same name as the type parameter.
   // 
   array<String^>^typeParamNames = {L"TFirst",L"TSecond"};
   array<GenericTypeParameterBuilder^>^typeParams = 
       myType->DefineGenericParameters( typeParamNames );

   GenericTypeParameterBuilder^ TFirst = typeParams[0];
   GenericTypeParameterBuilder^ TSecond = typeParams[1];

   Console::WriteLine( L"Type 'Sample' is generic: {0}", 
       myType->IsGenericType );

   // Apply constraints to the type parameters.
   //
   // A type that is substituted for the first parameter, TFirst,
   // must be a reference type and must have a parameterless
   // constructor.
   TFirst->SetGenericParameterAttributes( 
       GenericParameterAttributes::DefaultConstructorConstraint | 
       GenericParameterAttributes::ReferenceTypeConstraint 
   );

   // A type that is substituted for the second type
   // parameter must implement IExampleA and IExampleB, and
   // inherit from the trivial test class ExampleBase. The
   // interface constraints are specified as an array
   // containing the interface types. 
   array<Type^>^interfaceTypes = { interfaceA, interfaceB };
   TSecond->SetInterfaceConstraints( interfaceTypes );
   TSecond->SetBaseTypeConstraint( baseType );

   // The following code adds a private field named ExampleField,
   // of type TFirst.
   FieldBuilder^ exField = 
       myType->DefineField("ExampleField", TFirst, 
           FieldAttributes::Private);

   // Define a static method that takes an array of TFirst and 
   // returns a List<TFirst> containing all the elements of 
   // the array. To define this method it is necessary to create
   // the type List<TFirst> by calling MakeGenericType on the
   // generic type definition, generic<T> List. 
   // The parameter type is created by using the
   // MakeArrayType method. 
   //
   Type^ listOf = List::typeid;
   Type^ listOfTFirst = listOf->MakeGenericType(TFirst);
   array<Type^>^ mParamTypes = { TFirst->MakeArrayType() };

   MethodBuilder^ exMethod = 
       myType->DefineMethod("ExampleMethod", 
           MethodAttributes::Public | MethodAttributes::Static, 
           listOfTFirst, 
           mParamTypes);

   // Emit the method body. 
   // The method body consists of just three opcodes, to load 
   // the input array onto the execution stack, to call the 
   // List<TFirst> constructor that takes IEnumerable<TFirst>,
   // which does all the work of putting the input elements into
   // the list, and to return, leaving the list on the stack. The
   // hard work is getting the constructor.
   // 
   // The GetConstructor method is not supported on a 
   // GenericTypeParameterBuilder, so it is not possible to get 
   // the constructor of List<TFirst> directly. There are two
   // steps, first getting the constructor of generic<T> List and then
   // calling a method that converts it to the corresponding 
   // constructor of List<TFirst>.
   //
   // The constructor needed here is the one that takes an
   // IEnumerable<T>. Note, however, that this is not the 
   // generic type definition of generic<T> IEnumerable; instead, the
   // T from generic<T> List must be substituted for the T of 
   // generic<T> IEnumerable. (This seems confusing only because both
   // types have type parameters named T. That is why this example
   // uses the somewhat silly names TFirst and TSecond.) To get
   // the type of the constructor argument, take the generic
   // type definition generic<T> IEnumerable and 
   // call MakeGenericType with the first generic type parameter
   // of generic<T> List. The constructor argument list must be passed
   // as an array, with just one argument in this case.
   // 
   // Now it is possible to get the constructor of generic<T> List,
   // using GetConstructor on the generic type definition. To get
   // the constructor of List<TFirst>, pass List<TFirst> and
   // the constructor from generic<T> List to the static
   // TypeBuilder.GetConstructor method.
   //
   ILGenerator^ ilgen = exMethod->GetILGenerator();

   Type^ ienumOf = IEnumerable::typeid;
   Type^ TfromListOf = listOf->GetGenericArguments()[0];
   Type^ ienumOfT = ienumOf->MakeGenericType(TfromListOf);
   array<Type^>^ ctorArgs = {ienumOfT};

   ConstructorInfo^ ctorPrep = listOf->GetConstructor(ctorArgs);
   ConstructorInfo^ ctor = 
       TypeBuilder::GetConstructor(listOfTFirst, ctorPrep);

   ilgen->Emit(OpCodes::Ldarg_0);
   ilgen->Emit(OpCodes::Newobj, ctor);
   ilgen->Emit(OpCodes::Ret);

   // Create the type and save the assembly. 
   Type^ finished = myType->CreateType();
   myAssembly->Save( String::Concat( myAsmName->Name, L".dll" ) );

   // Invoke the method.
   // ExampleMethod is not generic, but the type it belongs to is
   // generic, so in order to get a MethodInfo that can be invoked
   // it is necessary to create a constructed type. The Example 
   // class satisfies the constraints on TFirst, because it is a 
   // reference type and has a default constructor. In order to
   // have a class that satisfies the constraints on TSecond, 
   // this code example defines the ExampleDerived type. These
   // two types are passed to MakeGenericMethod to create the
   // constructed type.
   //
   array<Type^>^ typeArgs = 
       { Example::typeid, ExampleDerived::typeid };
   Type^ constructed = finished->MakeGenericType(typeArgs);
   MethodInfo^ mi = constructed->GetMethod("ExampleMethod");

   // Create an array of Example objects, as input to the generic
   // method. This array must be passed as the only element of an 
   // array of arguments. The first argument of Invoke is 
   // null, because ExampleMethod is static. Display the count
   // on the resulting List<Example>.
   // 
   array<Example^>^ input = { gcnew Example(), gcnew Example() };
   array<Object^>^ arguments = { input };

   List<Example^>^ listX = 
       (List<Example^>^) mi->Invoke(nullptr, arguments);

   Console::WriteLine(
       "\nThere are {0} elements in the List<Example>.", 
       listX->Count);

   DisplayGenericParameters(finished);
}

/* This code example produces the following output:

Type 'Sample' is generic: False
Type 'Sample' is generic: True

There are 2 elements in the List<Example>.

Listing 2 type parameters for type 'Sample[TFirst,TSecond]'.

Type parameter TFirst:
    ReferenceTypeConstraint
    DefaultConstructorConstraint

Type parameter TSecond:
    Interface constraint: IExampleA
    Interface constraint: IExampleB
    Base type constraint: ExampleBase
 */

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Vea también

Cómo: Definir un tipo genérico con Reflection Emit

Para definir un tipo genérico

Ejemplo

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Conceptos

Referencia

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