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ILGenerator.Emit Method (OpCode)

Puts the specified instruction onto the stream of instructions.

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

public virtual void Emit (
	OpCode opcode
)
public void Emit (
	OpCode opcode
)
public function Emit (
	opcode : OpCode
)

Parameters

opcode

The Microsoft Intermediate Language (MSIL) instruction to be put onto the stream.

If the opcode requires an argument, the caller must ensure that the argument length matches the length of the declared parameter. Otherwise, results will be unpredictable. For example, if the Emit instruction requires a 2-byte operand and the caller supplies a 4-byte operand, the runtime will emit two additional bytes to the instruction stream. These extra bytes will be Nop instructions.

The instruction values are defined in OpCodes.

The code sample below demonstrates the use of Emit to generate MSIL output via an instance of ILGenerator.


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}));  
			  
   }

}


import System .* ;
import System.Threading .* ;
import System.Reflection .* ;
import System.Reflection.Emit .* ;

class DynamicJumpTableDemo
{
    public static Type BuildMyType()
    {
        AppDomain myDomain = System.Threading.Thread.GetDomain();
        AssemblyName myAsmName = new AssemblyName();
        myAsmName.set_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,
            String.class.ToType(),new Type[] { int.class.ToType() });
        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();
    } //BuildMyType    
   
   public static void main(String[] args)
   {
        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[]{(Int32)(theValue)}));
    } //main
} //DynamicJumpTableDemo

Windows 98, Windows 2000 SP4, Windows Millennium Edition, Windows Server 2003, Windows XP Media Center Edition, Windows XP Professional x64 Edition, Windows XP SP2, Windows XP Starter Edition

The .NET Framework does not support all versions of every platform. For a list of the supported versions, see System Requirements.

.NET Framework

Supported in: 2.0, 1.1, 1.0

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