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OpCodes.Switch Field

Implements a jump table.

[Visual Basic]
Public Shared ReadOnly Switch As OpCode
[C#]
public static readonly OpCode Switch;
[C++]
public: static OpCode Switch;
[JScript]
public static var Switch : OpCode;

Remarks

The following table lists the instruction's hexadecimal and Microsoft Intermediate Language (MSIL) assembly format, along with a brief reference summary:

Format Assembly Format Description
45 < unsigned int32 > < int32 > ... < int32 > switch (N, t1, t2 ... tN) Jumps to one of N values.

The stack transitional behavior, in sequential order, is:

  1. A value is pushed onto the stack.
  2. The value is popped off the stack and execution is transferred to the instruction at the offset indexed by the value, where the value is less than N.

The switch instruction implements a jump table. The format of the instruction is an unsigned int32 representing the number of targets N, followed by N int32 values specifying jump targets. These targets are represented as offsets (positive or negative) from the beginning of the instruction following this switch instruction.

The switch instruction pops a value off the stack and compares it, as an unsigned integer, to N. If value is less than N, execution is transferred to the target indexed by value, where targets are numbered from 0 (for example, a value of 0 takes the first target, a value of 1 takes the second target, and so on). If the value is greater than or equal to N, execution continues at the next instruction (fall through).

If the target instruction has one or more prefix codes, control can only be transferred to the first of these prefixes.

Control transfers into and out of try, catch, filter, and finally blocks cannot be performed by this instruction. (Such transfers are severely restricted and must use the leave instruction instead).

The following Emit constructor overload can use the switch opcode. The Label[] argument is an array of Labels representing 32-bit offsets.

  • ILGenerator.Emit(OpCode, Label[])

Example

[Visual Basic, C#, C++] The following code sample illustrates the use of the Switch opcode to generate a jump table using an array of Label.

[Visual Basic] 

Imports System
Imports System.Threading
Imports System.Reflection
Imports System.Reflection.Emit

 _

Class DynamicJumpTableDemo
   
   Public Shared Function BuildMyType() As Type

      Dim myDomain As AppDomain = Thread.GetDomain()
      Dim myAsmName As New AssemblyName()
      myAsmName.Name = "MyDynamicAssembly"
      
      Dim myAsmBuilder As AssemblyBuilder = myDomain.DefineDynamicAssembly(myAsmName, _
                            AssemblyBuilderAccess.Run)
      Dim myModBuilder As ModuleBuilder = myAsmBuilder.DefineDynamicModule("MyJumpTableDemo")
      
      Dim myTypeBuilder As TypeBuilder = myModBuilder.DefineType("JumpTableDemo", _
                                 TypeAttributes.Public)
      Dim myMthdBuilder As MethodBuilder = myTypeBuilder.DefineMethod("SwitchMe", _
                        MethodAttributes.Public Or MethodAttributes.Static, _
                        GetType(String), New Type() {GetType(Integer)})
      
      Dim myIL As ILGenerator = myMthdBuilder.GetILGenerator()
      
      Dim defaultCase As Label = myIL.DefineLabel()
      Dim endOfMethod As Label = myIL.DefineLabel()
      
      ' We are initializing our jump table. Note that the labels
      ' will be placed later using the MarkLabel method. 

      Dim jumpTable() As Label = {myIL.DefineLabel(), _
                  myIL.DefineLabel(), _
                  myIL.DefineLabel(), _
                  myIL.DefineLabel(), _
                  myIL.DefineLabel()}
      
      ' arg0, the number we passed, is pushed onto the stack.
      ' In this case, due to the design of the code sample,
      ' the value pushed onto the stack happens to match the
      ' index of the label (in IL terms, the index of the offset
      ' in the jump table). If this is not the case, such as
      ' when switching based on non-integer values, rules for the correspondence
      ' between the possible case values and each index of the offsets
      ' must be established outside of the ILGenerator.Emit calls,
      ' much as a compiler would.

      myIL.Emit(OpCodes.Ldarg_0)
      myIL.Emit(OpCodes.Switch, jumpTable)
      
      ' Branch on default case
      myIL.Emit(OpCodes.Br_S, defaultCase)
      
      ' Case arg0 = 0
      myIL.MarkLabel(jumpTable(0))
      myIL.Emit(OpCodes.Ldstr, "are no bananas")
      myIL.Emit(OpCodes.Br_S, endOfMethod)
      
      ' Case arg0 = 1
      myIL.MarkLabel(jumpTable(1))
      myIL.Emit(OpCodes.Ldstr, "is one banana")
      myIL.Emit(OpCodes.Br_S, endOfMethod)
      
      ' Case arg0 = 2
      myIL.MarkLabel(jumpTable(2))
      myIL.Emit(OpCodes.Ldstr, "are two bananas")
      myIL.Emit(OpCodes.Br_S, endOfMethod)
      
      ' Case arg0 = 3
      myIL.MarkLabel(jumpTable(3))
      myIL.Emit(OpCodes.Ldstr, "are three bananas")
      myIL.Emit(OpCodes.Br_S, endOfMethod)
      
      ' Case arg0 = 4
      myIL.MarkLabel(jumpTable(4))
      myIL.Emit(OpCodes.Ldstr, "are four bananas")
      myIL.Emit(OpCodes.Br_S, endOfMethod)
      
      ' Default case
      myIL.MarkLabel(defaultCase)
      myIL.Emit(OpCodes.Ldstr, "are many bananas")
      
      myIL.MarkLabel(endOfMethod)
      myIL.Emit(OpCodes.Ret)
      
      Return myTypeBuilder.CreateType()

   End Function 'BuildMyType
    
   
   Public Shared Sub Main()

      Dim myType As Type = BuildMyType()
      
      Console.Write("Enter an integer between 0 and 5: ")
      Dim theValue As Integer = Convert.ToInt32(Console.ReadLine())
      
      Console.WriteLine("---")
      Dim myInstance As [Object] = Activator.CreateInstance(myType, New Object() {})
      Console.WriteLine("Yes, there {0} today!", myType.InvokeMember("SwitchMe", _
                         BindingFlags.InvokeMethod, Nothing, _
                             myInstance, New Object() {theValue}))

   End Sub 'Main

End Class 'DynamicJumpTableDemo


[C#] 

using System;
using System.Threading;
using System.Reflection;
using System.Reflection.Emit;

class DynamicJumpTableDemo

{

   public static Type BuildMyType()
   {
    AppDomain myDomain = Thread.GetDomain();
    AssemblyName myAsmName = new AssemblyName();
    myAsmName.Name = "MyDynamicAssembly";

    AssemblyBuilder myAsmBuilder = myDomain.DefineDynamicAssembly(
                        myAsmName,
                        AssemblyBuilderAccess.Run);
    ModuleBuilder myModBuilder = myAsmBuilder.DefineDynamicModule(
                        "MyJumpTableDemo");

    TypeBuilder myTypeBuilder = myModBuilder.DefineType("JumpTableDemo",
                            TypeAttributes.Public);
    MethodBuilder myMthdBuilder = myTypeBuilder.DefineMethod("SwitchMe", 
                             MethodAttributes.Public |
                             MethodAttributes.Static,
                                             typeof(string), 
                                             new Type[] {typeof(int)});

    ILGenerator myIL = myMthdBuilder.GetILGenerator();

    Label defaultCase = myIL.DefineLabel();    
    Label endOfMethod = myIL.DefineLabel();    

    // We are initializing our jump table. Note that the labels
    // will be placed later using the MarkLabel method. 

    Label[] jumpTable = new Label[] { myIL.DefineLabel(),
                      myIL.DefineLabel(),
                      myIL.DefineLabel(),
                      myIL.DefineLabel(),
                      myIL.DefineLabel() };

    // arg0, the number we passed, is pushed onto the stack.
    // In this case, due to the design of the code sample,
    // the value pushed onto the stack happens to match the
    // index of the label (in IL terms, the index of the offset
    // in the jump table). If this is not the case, such as
    // when switching based on non-integer values, rules for the correspondence
    // between the possible case values and each index of the offsets
    // must be established outside of the ILGenerator.Emit calls,
    // much as a compiler would.

    myIL.Emit(OpCodes.Ldarg_0);
    myIL.Emit(OpCodes.Switch, jumpTable);
    
    // Branch on default case
    myIL.Emit(OpCodes.Br_S, defaultCase);

    // Case arg0 = 0
    myIL.MarkLabel(jumpTable[0]); 
    myIL.Emit(OpCodes.Ldstr, "are no bananas");
    myIL.Emit(OpCodes.Br_S, endOfMethod);

    // Case arg0 = 1
    myIL.MarkLabel(jumpTable[1]); 
    myIL.Emit(OpCodes.Ldstr, "is one banana");
    myIL.Emit(OpCodes.Br_S, endOfMethod);

    // Case arg0 = 2
    myIL.MarkLabel(jumpTable[2]); 
    myIL.Emit(OpCodes.Ldstr, "are two bananas");
    myIL.Emit(OpCodes.Br_S, endOfMethod);

    // Case arg0 = 3
    myIL.MarkLabel(jumpTable[3]); 
    myIL.Emit(OpCodes.Ldstr, "are three bananas");
    myIL.Emit(OpCodes.Br_S, endOfMethod);

    // Case arg0 = 4
    myIL.MarkLabel(jumpTable[4]); 
    myIL.Emit(OpCodes.Ldstr, "are four bananas");
    myIL.Emit(OpCodes.Br_S, endOfMethod);

    // Default case
    myIL.MarkLabel(defaultCase);
    myIL.Emit(OpCodes.Ldstr, "are many bananas");

    myIL.MarkLabel(endOfMethod);
    myIL.Emit(OpCodes.Ret);
    
    return myTypeBuilder.CreateType();

   }

   public static void Main()
   {
    Type myType = BuildMyType();
    
    Console.Write("Enter an integer between 0 and 5: ");
    int theValue = Convert.ToInt32(Console.ReadLine());

    Console.WriteLine("---");
    Object myInstance = Activator.CreateInstance(myType, new object[0]);    
    Console.WriteLine("Yes, there {0} today!", myType.InvokeMember("SwitchMe",
                                 BindingFlags.InvokeMethod,
                                 null,
                                 myInstance,
                                 new object[] {theValue}));  
              
   }

}


[C++] 

#using <mscorlib.dll>

using namespace System;
using namespace System::Threading;
using namespace System::Reflection;
using namespace System::Reflection::Emit;

Type* BuildMyType() {
   AppDomain*  myDomain = Thread::GetDomain();
   AssemblyName* myAsmName = new AssemblyName();
   myAsmName->Name = S"MyDynamicAssembly";

   AssemblyBuilder*  myAsmBuilder = myDomain->DefineDynamicAssembly(myAsmName,
      AssemblyBuilderAccess::Run);
   ModuleBuilder*  myModBuilder = myAsmBuilder->DefineDynamicModule(S"MyJumpTableDemo");

   TypeBuilder*  myTypeBuilder = myModBuilder->DefineType(S"JumpTableDemo",
      TypeAttributes::Public);

   Type* temp0 [] = {__typeof(int)};
   MethodBuilder*  myMthdBuilder = myTypeBuilder->DefineMethod(
      S"SwitchMe",
      static_cast<MethodAttributes>(MethodAttributes::Public | MethodAttributes::Static),
      __typeof(String),
      temp0);

   ILGenerator*  myIL = myMthdBuilder->GetILGenerator();

   Label defaultCase = myIL->DefineLabel();
   Label endOfMethod = myIL->DefineLabel();

   // We are initializing our jump table. Note that the labels
   // will be placed later using the MarkLabel method.

   Label jumpTable [] = new Label[5];
   jumpTable[0] = myIL->DefineLabel();
   jumpTable[1] = myIL->DefineLabel();
   jumpTable[2] = myIL->DefineLabel();
   jumpTable[3] = myIL->DefineLabel();
   jumpTable[4] = myIL->DefineLabel();

   // arg0, the number we passed, is pushed onto the stack.
   // In this case, due to the design of the code sample,
   // the value pushed onto the stack happens to match the
   // index of the label (in IL terms, the index of the offset
   // in the jump table). If this is not the case, such as
   // when switching based on non-integer values, rules for the correspondence
   // between the possible case values and each index of the offsets
   // must be established outside of the ILGenerator::Emit calls,
   // much as a compiler would.

   myIL->Emit(OpCodes::Ldarg_0);
   myIL->Emit(OpCodes::Switch, jumpTable);

   // Branch on default case
   myIL->Emit(OpCodes::Br_S, defaultCase);

   // Case arg0 = 0
   myIL->MarkLabel(jumpTable[0]);
   myIL->Emit(OpCodes::Ldstr, S"are no bananas");
   myIL->Emit(OpCodes::Br_S, endOfMethod);

   // Case arg0 = 1
   myIL->MarkLabel(jumpTable[1]);
   myIL->Emit(OpCodes::Ldstr, S"is one banana");
   myIL->Emit(OpCodes::Br_S, endOfMethod);

   // Case arg0 = 2
   myIL->MarkLabel(jumpTable[2]);
   myIL->Emit(OpCodes::Ldstr, S"are two bananas");
   myIL->Emit(OpCodes::Br_S, endOfMethod);

   // Case arg0 = 3
   myIL->MarkLabel(jumpTable[3]);
   myIL->Emit(OpCodes::Ldstr, S"are three bananas");
   myIL->Emit(OpCodes::Br_S, endOfMethod);

   // Case arg0 = 4
   myIL->MarkLabel(jumpTable[4]);
   myIL->Emit(OpCodes::Ldstr, S"are four bananas");
   myIL->Emit(OpCodes::Br_S, endOfMethod);

   // Default case
   myIL->MarkLabel(defaultCase);
   myIL->Emit(OpCodes::Ldstr, S"are many bananas");

   myIL->MarkLabel(endOfMethod);
   myIL->Emit(OpCodes::Ret);

   return myTypeBuilder->CreateType();
}

int main() {
   Type*  myType = BuildMyType();

   Console::Write(S"Enter an integer between 0 and 5: ");
   int theValue = Convert::ToInt32(Console::ReadLine());

   Console::WriteLine(S"---");
   Object*  myInstance = Activator::CreateInstance(myType, new Object*[0]);

   Object* temp1 [] = {__box(theValue)};
   Console::WriteLine(S"Yes, there {0} today!", myType->InvokeMember(S"SwitchMe",
      BindingFlags::InvokeMethod,
      0,
      myInstance,
      temp1));
}

[JScript] No example is available for JScript. To view a Visual Basic, C#, or C++ example, click the Language Filter button Language Filter in the upper-left corner of the page.

Requirements

Platforms: Windows 98, Windows NT 4.0, Windows Millennium Edition, Windows 2000, Windows XP Home Edition, Windows XP Professional, Windows Server 2003 family

See Also

OpCodes Class | OpCodes Members | System.Reflection.Emit Namespace

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