ILGenerator.Emit Method (OpCode, array<Label[])

Parameters

• opcode
  Type: System.Reflection.Emit.OpCode
  The MSIL instruction to be emitted onto the stream.

• labels
  Type: array<System.Reflection.Emit.Label[]
  The array of label objects that forms the jump table.

Imports System.Reflection
Imports System.Reflection.Emit

Class Example

   Public Shared Sub Demo(ByVal outputBlock As System.Windows.Controls.TextBlock)

      Dim myDomain As AppDomain = AppDomain.CurrentDomain
      Dim myAsmName As New AssemblyName("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()

      ' Initialize the 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 method argument, 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)

      Dim myType As Type = myTypeBuilder.CreateType()

      Dim testValues() As Integer = { 3, 500, 0 }
      Dim obj As Object = Activator.CreateInstance(myType)
      For Each testValue As Integer In testValues

         Dim result As Object = _
            myType.InvokeMember("SwitchMe", _
                                BindingFlags.InvokeMethod, _
                                Type.DefaultBinder, _
                                obj, _
                                New Object() { testValue })

         outputBlock.Text &= String.Format("Yes, there {0} today!", result) & vbLf
      Next

   End Sub 
End Class 

' This example produces the following output:
'
'Yes, there are three bananas today!
'Yes, there are many bananas today!
'Yes, there are no bananas today!

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

class Example
{
   public static void Demo(System.Windows.Controls.TextBlock outputBlock)
   {
      AppDomain myDomain = AppDomain.CurrentDomain;
      AssemblyName myAsmName = new AssemblyName("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();

      // Initialize the jump table. Note that the labels
      // will be placed later using the MarkLabel method. 
      Label[] jumpTable = {myIL.DefineLabel(), 
                           myIL.DefineLabel(), 
                           myIL.DefineLabel(), 
                           myIL.DefineLabel(), 
                           myIL.DefineLabel()};

      // arg0, the method argument, 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);

      Type myType = myTypeBuilder.CreateType();

      int[] testValues = {3, 500, 0};
      object obj = Activator.CreateInstance(myType);
      foreach( int testValue in testValues )
      {
         object result = 
            myType.InvokeMember("SwitchMe", 
                                BindingFlags.InvokeMethod, 
                                Type.DefaultBinder, 
                                obj, 
                                new object[] { testValue });

         outputBlock.Text += String.Format("Yes, there {0} today!\n", result);
      }
   }
}

/* This example produces the following output:

Yes, there are three bananas today!
Yes, there are many bananas today!
Yes, there are no bananas today!
 */