ILGenerator.Emit Method (OpCode, Label)

 

Puts the specified instruction onto the Microsoft intermediate language (MSIL) stream and leaves space to include a label when fixes are done.

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

public virtual void Emit(
	OpCode opcode,
	Label label
)

Parameters

opcode
Type: System.Reflection.Emit.OpCode

The MSIL instruction to be emitted onto the stream.

label
Type: System.Reflection.Emit.Label

The label to which to branch from this location.

The instruction values are defined in the OpCodes enumeration.

Labels are created using DefineLabel, and their location within the stream is fixed by using MarkLabel. If a single-byte instruction is used, the label can represent a jump of at most 127 bytes along the stream. opcode must represent a branch instruction. Because branches are relative instructions, label will be replaced with the correct offset to branch during the fixup process.

The code sample below illustrates the creation of a dynamic method with a jump table. The jump table is built using an array of Label.


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}));  

   }

}

.NET Framework
Available since 1.1
Portable Class Library
Supported in: portable .NET platforms
Silverlight
Available since 2.0
Windows Phone Silverlight
Available since 7.1
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