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OpCodes.Switch Field

Implements a jump table.

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

public static readonly OpCode Switch

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

  • ILGenerator.Emit(OpCode, Label[])

The following code sample illustrates the use of the Switch opcode to generate a jump table 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

Supported in: 4, 3.5, 3.0, 2.0, 1.1, 1.0

.NET Framework Client Profile

Supported in: 4, 3.5 SP1

Windows 7, Windows Vista SP1 or later, Windows XP SP3, Windows XP SP2 x64 Edition, Windows Server 2008 (Server Core not supported), Windows Server 2008 R2 (Server Core supported with SP1 or later), Windows Server 2003 SP2

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

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