Metodo BeginCatchBlock

Metodo ILGenerator.BeginCatchBlock

Inizia un blocco catch.

Spazio dei nomi: System.Reflection.Emit
Assembly: mscorlib (in mscorlib.dll)

public virtual void BeginCatchBlock (
	Type exceptionType
)
public void BeginCatchBlock (
	Type exceptionType
)
public function BeginCatchBlock (
	exceptionType : Type
)

Parametri

exceptionType

Oggetto Type che rappresenta l'eccezione.

Tipo di eccezioneCondizione

ArgumentException

Il blocco catch si trova all'interno di un'eccezione filtrata.

ArgumentNullException

exceptionType è riferimento null (Nothing in Visual Basic) e il blocco di filtro delle eccezioni non ha restituito un valore che indica che i blocchi devono essere eseguiti fino all'individuazione del blocco catch.

NotSupportedException

Il flusso MSIL (Microsoft Intermediate Language) generato non si trova attualmente in un blocco di eccezioni.

Crea un'istruzione di diramazione alla fine del blocco di eccezioni corrente.

NotaNota

Se il blocco di eccezioni dei filtri restituisce constant exception_execute_handler (vedere la documentazione relativa a Common Language Infrastructure Instruction Set), l'argomento di BeginCatchBlock non verrà verificato.

Nell'esempio di codice riportato di seguito viene illustrato l'utilizzo contestuale del metodo BeginCatchBlock.


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

class ILThrowExceptionDemo {

   public static Type BuildAdderType() {

  	AppDomain myDomain = Thread.GetDomain();
  	AssemblyName myAsmName = new AssemblyName();
  	myAsmName.Name = "AdderExceptionAsm";
  	AssemblyBuilder myAsmBldr = myDomain.DefineDynamicAssembly(myAsmName,
					       	     AssemblyBuilderAccess.Run);

  	ModuleBuilder myModBldr = myAsmBldr.DefineDynamicModule("AdderExceptionMod");

  	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 |
						   	MethodAttributes.Static,
						   	typeof(int),
						   	adderParams);
	ILGenerator adderIL = adderBldr.GetILGenerator();

	// 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.

	ConstructorInfo exCtorInfo = typeof(OverflowException).GetConstructor(
								new Type[]
								{typeof(string)});
	MethodInfo exToStrMI = typeof(OverflowException).GetMethod("ToString");
	MethodInfo writeLineMI = typeof(Console).GetMethod("WriteLine",
								new Type[] 
								{typeof(string),
								 typeof(object)});
								   								
	LocalBuilder tmp1 = adderIL.DeclareLocal(typeof(int));	
	LocalBuilder tmp2 = adderIL.DeclareLocal(typeof(OverflowException));

	Label failed = adderIL.DefineLabel();
	Label endOfMthd = adderIL.DefineLabel();

	// 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.

	Label exBlock = adderIL.BeginExceptionBlock();
	adderIL.Emit(OpCodes.Ldarg_0);
	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.Ldarg_1);
	adderIL.Emit(OpCodes.Ldc_I4_S, 100);
	adderIL.Emit(OpCodes.Bgt_S, failed);

	adderIL.Emit(OpCodes.Ldarg_0);
	adderIL.Emit(OpCodes.Ldarg_1);
	adderIL.Emit(OpCodes.Add_Ovf_Un);
	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.MarkLabel(failed);
	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 currently atop the stack.

	adderIL.ThrowException(typeof(OverflowException));
		
	// Start the catch block.

	adderIL.BeginCatchBlock(typeof(OverflowException));

	// First, we'll load a "wrapper" string, and then perform a 
	// late-bound call to the ToString() method of OverflowException,
	// passing it the exception object we stored in local variable tmp2.

	adderIL.Emit(OpCodes.Ldstr, "{0}");
	adderIL.Emit(OpCodes.Ldloc_S, tmp2);
	adderIL.EmitCall(OpCodes.Callvirt, exToStrMI, null);

	// Now, we should have the "wrapper" string atop the stack,
	// along with the string result of the ToString() call. All
	// conditions are met to 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.Ldc_I4_M1);
	adderIL.Emit(OpCodes.Stloc_S, tmp1);

	// End the exception handling block.

	adderIL.EndExceptionBlock();	

	// 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.MarkLabel(endOfMthd);
	adderIL.Emit(OpCodes.Ldloc_S, tmp1);
	adderIL.Emit(OpCodes.Ret);
	
	return myTypeBldr.CreateType();

   }

   public static void Main() {

	Type adderType = BuildAdderType();

	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("---");

	Console.WriteLine("{0} + {1} = {2}",
				addParams[0], addParams[1],
				adderType.InvokeMember("DoAdd",
					BindingFlags.InvokeMethod,
					null,
					addIns,
					addParams)); 
            
   }

}


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

.NET Framework non supporta tutte le versioni di ciascuna piattaforma. Per un elenco delle versioni supportate, vedere Requisiti di sistema.

.NET Framework

Supportato in: 2.0 1.1 1.0

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