Type::Missing Field

 

Represents a missing value in the Type information. This field is read-only.

Namespace:   System
Assembly:  mscorlib (in mscorlib.dll)

public:
static initonly Object^ Missing

Field Value

Type: System::Object^

Use the Missing field for invocation through reflection to obtain the default value of a parameter. If the Missing field is passed in for a parameter value and there is no default value for that parameter, an ArgumentException is thrown.

The following code example shows the use of the Missing field to invoke a method with its default arguments.

#using <System.dll>

using namespace System;
using namespace System::Reflection;
using namespace System::CodeDom::Compiler;

ref class Example
{
public:
    static void Main()
    {
        // VB source for example. Not all versions of CS and CPP compilers
        // support optional arguments.
        String^ codeLines =
            "Imports System\n\n" +
            "Public Class OptionalArg\n" +
            "  Public Sub MyMethod(ByVal a As Integer, _\n" +
            "    Optional ByVal b As Double = 1.2, _\n" +
            "    Optional ByVal c As Integer = 1)\n\n" +
            "    Console.WriteLine(\"a = \" & a & \" b = \" & b & \" c = \" & c)\n" +
            "  End Sub\n" +
            "End Class\n";

        // Generate a OptionalArg instance from the source above.
        Object^ o = GenerateObjectFromSource("OptionalArg", codeLines, "VisualBasic");
        Type^ t;

        t = o->GetType();
        BindingFlags bf = BindingFlags::Public | BindingFlags::Instance |
            BindingFlags::InvokeMethod | BindingFlags::OptionalParamBinding;

        t->InvokeMember("MyMethod", bf, nullptr, o, gcnew array<Object^> {10, 55.3, 12});
        t->InvokeMember("MyMethod", bf, nullptr, o, gcnew array<Object^> {10, 1.3, Type::Missing});
        t->InvokeMember("MyMethod", bf, nullptr, o, gcnew array<Object^> {10, Type::Missing, Type::Missing});
    }

private:
    static Object^ GenerateObjectFromSource(String^ objectName,
        String^ sourceLines, String^ providerName)
    {
        Object^ genObject = nullptr;
        CodeDomProvider^ codeProvider = CodeDomProvider::CreateProvider(providerName);
        CompilerParameters^ cp = gcnew CompilerParameters();

        cp->GenerateExecutable = false;
        cp->GenerateInMemory = true;

        CompilerResults^ results =
            codeProvider->CompileAssemblyFromSource(cp, sourceLines);
        if (results->Errors->Count == 0)
        {
            genObject = results->CompiledAssembly->CreateInstance(objectName);
        }

        return genObject;
    }
};

int main()
{
    Example::Main();
}

This code produces the following output:

a = 10 b = 55.3 c = 12

a = 10 b = 1.3 c = 1

a = 10 b = 1.2 c = 1

Universal Windows Platform
Available since 8
.NET Framework
Available since 1.1
Portable Class Library
Supported in: portable .NET platforms
Silverlight
Available since 2.0
Windows Phone Silverlight
Available since 7.0
Windows Phone
Available since 8.1
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