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ILGenerator.BeginExceptionBlock Method

Begins an exception block for a non-filtered exception.

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

public virtual Label BeginExceptionBlock()

Return Value

Type: System.Reflection.Emit.Label
The label for the end of the block. This will leave you in the correct place to execute finally blocks or to finish the try.

Creating an exception block records some information, but does not actually emit any Microsoft intermediate language (MSIL) onto the stream.

The code sample below demonstrates the contextual usage of the BeginExceptionBlock method.

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

class ILThrowExceptionDemo 
   public static void Main()
      AppDomain current = AppDomain.CurrentDomain;
      AssemblyName myAsmName = new AssemblyName();
      myAsmName.Name = "AdderExceptionAsm";
      AssemblyBuilder myAsmBldr = current.DefineDynamicAssembly(myAsmName,

      ModuleBuilder myModBldr = myAsmBldr.DefineDynamicModule(myAsmName.Name, 
                         myAsmName.Name + ".dll");

      TypeBuilder myTypeBldr = myModBldr.DefineType("Adder");

      Type[] adderParams = new Type[] {typeof(int), typeof(int)};

      // This method will add two numbers which are 100 or less. If either of the 
      // passed integer vales are greater than 100, it will throw an exception.

      MethodBuilder adderBldr = myTypeBldr.DefineMethod("DoAdd",
                         MethodAttributes.Public |
      ILGenerator adderIL = adderBldr.GetILGenerator();

      // Types and methods used in the code to throw, catch, and 
      // display OverflowException. Note that if the catch block were 
      // for a more general type, such as Exception, we would need  
      // a MethodInfo for that type's ToString method. 
      Type overflow = typeof(OverflowException);
      ConstructorInfo exCtorInfo = overflow.GetConstructor(
                        new Type[]
      MethodInfo exToStrMI = overflow.GetMethod("ToString");
      MethodInfo writeLineMI = typeof(Console).GetMethod("WriteLine",
                        new Type[] 

      LocalBuilder tmp1 = adderIL.DeclareLocal(typeof(int));   
      LocalBuilder tmp2 = adderIL.DeclareLocal(overflow);

      // In order to successfully branch, we need to create labels 
      // representing the offset IL instruction block to branch to. 
      // These labels, when the MarkLabel(Label) method is invoked, 
      // will specify the IL instruction to branch to. 
      Label failed = adderIL.DefineLabel();
      Label endOfMthd = adderIL.DefineLabel();

      // Begin the try block.
      Label exBlock = adderIL.BeginExceptionBlock();

      // First, load argument 0 and the integer value of "100" onto the
      // stack. If arg0 > 100, branch to the label "failed", which is marked
      // as the address of the block that throws an exception. 
      adderIL.Emit(OpCodes.Ldc_I4_S, 100);
      adderIL.Emit(OpCodes.Bgt_S, failed); 

      // Now, check to see if argument 1 was greater than 100. If it was, 
      // branch to "failed." Otherwise, fall through and perform the addition,
      // branching unconditionally to the instruction at the label "endOfMthd".
      adderIL.Emit(OpCodes.Ldc_I4_S, 100);
      adderIL.Emit(OpCodes.Bgt_S, failed);

      // Store the result of the addition.
      adderIL.Emit(OpCodes.Stloc_S, tmp1);
      adderIL.Emit(OpCodes.Br_S, endOfMthd);

      // If one of the arguments was greater than 100, we need to throw an 
      // exception. We'll use "OverflowException" with a customized message.
      // First, we load our message onto the stack, and then create a new 
      // exception object using the constructor overload that accepts a 
      // string message. 
      adderIL.Emit(OpCodes.Ldstr, "Cannot accept values over 100 for add.");
      adderIL.Emit(OpCodes.Newobj, exCtorInfo);

      // We're going to need to refer to that exception object later, so let's 
      // store it in a temporary variable. Since the store function pops the 
      // the value/reference off the stack, and we'll need it to throw the 
      // exception, we will subsequently load it back onto the stack as well.

      adderIL.Emit(OpCodes.Stloc_S, tmp2);
      adderIL.Emit(OpCodes.Ldloc_S, tmp2);

      // Throw the exception now on the stack.


      // Start the catch block for OverflowException. 

      // When we enter the catch block, the thrown exception  
      // is on the stack. Store it, then load the format string 
      // for WriteLine.  
      adderIL.Emit(OpCodes.Stloc_S, tmp2);
      adderIL.Emit(OpCodes.Ldstr, "Caught {0}");

      // Push the thrown exception back on the stack, then  
      // call its ToString() method. Note that if this catch block 
      // were for a more general exception type, like Exception, 
      // it would be necessary to use the ToString for that type. 
      adderIL.Emit(OpCodes.Ldloc_S, tmp2);
      adderIL.EmitCall(OpCodes.Callvirt, exToStrMI, null);

      // The format string and the return value from ToString() are 
      // now on the stack. Call WriteLine(string, object). 
      adderIL.EmitCall(OpCodes.Call, writeLineMI, null);

      // Since our function has to return an integer value, we'll load -1 onto 
      // the stack to indicate an error, and store it in local variable tmp1. 
      adderIL.Emit(OpCodes.Stloc_S, tmp1);

      // End the exception handling block.


      // The end of the method. If no exception was thrown, the correct value 
      // will be saved in tmp1. If an exception was thrown, tmp1 will be equal 
      // to -1. Either way, we'll load the value of tmp1 onto the stack and return. 
      adderIL.Emit(OpCodes.Ldloc_S, tmp1);

      Type adderType = myTypeBldr.CreateType();

      object addIns = Activator.CreateInstance(adderType); 

      object[] addParams = new object[2];

      Console.Write("Enter an integer value: ");
      addParams[0] = (object)Convert.ToInt32(Console.ReadLine());

      Console.Write("Enter another integer value: ");
      addParams[1] = (object)Convert.ToInt32(Console.ReadLine());

      Console.WriteLine("If either integer was > 100, an exception will be thrown.");

      Console.WriteLine("{0} + {1} = {2}",
            addParams[0], addParams[1],

/* This code produces output similar to the following:

Enter an integer value: 24
Enter another integer value: 101
If either integer was > 100, an exception will be thrown.
Caught System.OverflowException: Arithmetic operation resulted in an overflow.
   at Adder.DoAdd(Int32 , Int32 )
 24 + 101 = -1

.NET Framework

Supported in: 4.6, 4.5, 4, 3.5, 3.0, 2.0, 1.1

.NET Framework Client Profile

Supported in: 4, 3.5 SP1

.NET for Windows Phone apps

Supported in: Windows Phone 8.1, Windows Phone Silverlight 8.1, Windows Phone Silverlight 8
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