How to fire an event at runtime

Question

I want to create an (emit) class that implements a runtime interface with C #.

I managed to emit methods and properties, but I could not issue an event.

Here is the code I want to emit:

Namespace: TestInterfaces, Class: TestIImplementMe.

public event EventHandler SimpleEvent; 

Here is the generated IL code:

 .event [mscorlib]System.EventHandler SimpleEvent { .addon instance void TestInterfaces.TestIImplementMe::add_SimpleEvent(class [mscorlib]System.EventHandler) .removeon instance void TestInterfaces.TestIImplementMe::remove_SimpleEvent(class [mscorlib]System.EventHandler) } .method public final hidebysig specialname newslot virtual instance void add_SimpleEvent ( class [mscorlib]System.EventHandler 'value' ) cil managed { // Method begins at RVA 0x330c // Code size 48 (0x30) .maxstack 3 .locals init ( [0] class [mscorlib]System.EventHandler, [1] class [mscorlib]System.EventHandler, [2] class [mscorlib]System.EventHandler, [3] bool ) IL_0000: ldarg.0 IL_0001: ldfld class [mscorlib]System.EventHandler TestInterfaces.TestIImplementMe::SimpleEvent IL_0006: stloc.0 // loop start (head: IL_0007) IL_0007: ldloc.0 IL_0008: stloc.1 IL_0009: ldloc.1 IL_000a: ldarg.1 IL_000b: call class [mscorlib]System.Delegate [mscorlib]System.Delegate::Combine(class [mscorlib]System.Delegate, class [mscorlib]System.Delegate) IL_0010: castclass [mscorlib]System.EventHandler IL_0015: stloc.2 IL_0016: ldarg.0 IL_0017: ldflda class [mscorlib]System.EventHandler TestInterfaces.TestIImplementMe::SimpleEvent IL_001c: ldloc.2 IL_001d: ldloc.1 IL_001e: call !!0 [mscorlib]System.Threading.Interlocked::CompareExchange<class [mscorlib]System.EventHandler>(!!0&, !!0, !!0) IL_0023: stloc.0 IL_0024: ldloc.0 IL_0025: ldloc.1 IL_0026: ceq IL_0028: ldc.i4.0 IL_0029: ceq IL_002b: stloc.3 IL_002c: ldloc.3 IL_002d: brtrue.s IL_0007 // end loop IL_002f: ret } // end of method TestIImplementMe::add_SimpleEvent .method public final hidebysig specialname newslot virtual instance void remove_SimpleEvent ( class [mscorlib]System.EventHandler 'value' ) cil managed { // Method begins at RVA 0x3348 // Code size 48 (0x30) .maxstack 3 .locals init ( [0] class [mscorlib]System.EventHandler, [1] class [mscorlib]System.EventHandler, [2] class [mscorlib]System.EventHandler, [3] bool ) IL_0000: ldarg.0 IL_0001: ldfld class [mscorlib]System.EventHandler TestInterfaces.TestIImplementMe::SimpleEvent IL_0006: stloc.0 // loop start (head: IL_0007) IL_0007: ldloc.0 IL_0008: stloc.1 IL_0009: ldloc.1 IL_000a: ldarg.1 IL_000b: call class [mscorlib]System.Delegate [mscorlib]System.Delegate::Remove(class [mscorlib]System.Delegate, class [mscorlib]System.Delegate) IL_0010: castclass [mscorlib]System.EventHandler IL_0015: stloc.2 IL_0016: ldarg.0 IL_0017: ldflda class [mscorlib]System.EventHandler TestInterfaces.TestIImplementMe::SimpleEvent IL_001c: ldloc.2 IL_001d: ldloc.1 IL_001e: call !!0 [mscorlib]System.Threading.Interlocked::CompareExchange<class [mscorlib]System.EventHandler>(!!0&, !!0, !!0) IL_0023: stloc.0 IL_0024: ldloc.0 IL_0025: ldloc.1 IL_0026: ceq IL_0028: ldc.i4.0 IL_0029: ceq IL_002b: stloc.3 IL_002c: ldloc.3 IL_002d: brtrue.s IL_0007 // end loop IL_002f: ret } // end of method TestIImplementMe::remove_SimpleEvent 

I managed to do everything except this line: IL_001e: call !! 0 [mscorlib] System.Threading.Interlocked :: CompareExchange (!! 0 &, !! 0, !! 0)

What's the point:!! 0, !! 0 &?

How do I call this method with IL emit?

I tried to call the method, but when I add the handler:

 obj.SimpleEvent += new EventHandler(obj_SimpleEvent); 

I get this exception:

 System.BadImageFormatException was unhandled Message=An attempt was made to load a program with an incorrect format. (Exception from HRESULT: 0x8007000B) Source=DataBuilderAssembly StackTrace: at NewTest.add_SimpleEvent(EventHandler value) at TestInterfaces.Program.Main(String[] args) in D:\Dev\Private\TestInterfaces\TestInterfaces\Program.cs:line 47 at System.AppDomain._nExecuteAssembly(RuntimeAssembly assembly, String[] args) at System.AppDomain.ExecuteAssembly(String assemblyFile, Evidence assemblySecurity, String[] args) at Microsoft.VisualStudio.HostingProcess.HostProc.RunUsersAssembly() at System.Threading.ThreadHelper.ThreadStart_Context(Object state) at System.Threading.ExecutionContext.Run(ExecutionContext executionContext, ContextCallback callback, Object state, Boolean ignoreSyncCtx) at System.Threading.ExecutionContext.Run(ExecutionContext executionContext, ContextCallback callback, Object state) at System.Threading.ThreadHelper.ThreadStart() 

Here is the code to emit an event adding method where

 methodName = "add_SimpleEvent"; actionHanler = typeof(Delegate).GetMethod("Combine", new Type[]{typeof(Delegate),typeof(Delegate)}); eventField = typeBuilder.DefineField(info.Name, info.EventHandlerType, FieldAttributes.Private); compareExchange = GetGenericMethod(typeof(Interlocked), "CompareExchange");// finds the correct method to call. // thanks to @marc, create the specific method with the correct type compareExchange = compareExchange.MakeGenericMethod(info.EventHandlerType); 

GenerateEventMethod

 private static MethodBuilder GenerateEventMethod(string methodName, MethodInfo actionHanler, TypeBuilder typeBuilder, EventInfo info, FieldBuilder eventField, MethodInfo compareExchange) { MethodBuilder method = typeBuilder.DefineMethod(methodName, InternalMethodsAttributes, null, new Type[] { info.EventHandlerType }); method.DefineParameter(0, ParameterAttributes.Retval, null); method.DefineParameter(1, ParameterAttributes.In, "value"); ILGenerator il = method.GetILGenerator(); il.DeclareLocal(info.EventHandlerType); il.DeclareLocal(info.EventHandlerType); il.DeclareLocal(info.EventHandlerType); il.DeclareLocal(typeof (bool)); Label loop = il.DefineLabel(); il.Emit(OpCodes.Ldarg_0); il.Emit(OpCodes.Ldfld, eventField); il.Emit(OpCodes.Stloc_0); il.MarkLabel(loop);// loop start (head: IL_0007) il.Emit(OpCodes.Ldloc_0); il.Emit(OpCodes.Stloc_1); il.Emit(OpCodes.Ldloc_1); il.Emit(OpCodes.Ldarg_1); il.Emit(OpCodes.Call, actionHanler); il.Emit(OpCodes.Castclass, info.EventHandlerType); il.Emit(OpCodes.Stloc_2); il.Emit(OpCodes.Ldarg_0); il.Emit(OpCodes.Ldflda, eventField); il.Emit(OpCodes.Ldloc_2); il.Emit(OpCodes.Ldloc_1); // How to do this? //IL_001e: call !!0 [mscorlib]System.Threading.Interlocked::CompareExchange<class [mscorlib]System.EventHandler`1<class [mscorlib]System.ConsoleCancelEventArgs>>(!!0&, !!0, !!0) //il.Emit(OpCodes.Call, !!0 compareExchange(!!0&,!!0, !!0)); il.Emit(OpCodes.Call, compareExchange); il.Emit(OpCodes.Stloc_0); il.Emit(OpCodes.Ldloc_0); il.Emit(OpCodes.Ldloc_1); il.Emit(OpCodes.Ceq); il.Emit(OpCodes.Ldc_I4_0); il.Emit(OpCodes.Ceq); il.Emit(OpCodes.Stloc_3); il.Emit(OpCodes.Ldloc_3); il.Emit(OpCodes.Brtrue_S, loop); // end loop il.Emit(OpCodes.Ret); return method; } 

GetGenericMethod (specifically for this problem):

 private static MethodInfo GetGenericMethod(Type type, string methodName) { var q = from m in type.GetMethods() where m.Name == methodName && m.IsGenericMethod select m; return q.FirstOrDefault(); } 

Thanks, Ophir

Answer 1

!!0 is T in CompareExchange<T> (i.e. !!0 is the first (zero) generic type of the type), and !!0& is ref T - but all of this simply indicates overloads. The important bit is ldflda (loads the ref-field), ldloc.2 and ldloc.1 in the lines before that and the fact that it returns T

Answer 2

I would suggest writing code in C #, compiling it, and then using one of your favorite .NET decompilers to check what the exact IL code is. After that, it's easy enough to create the same code dynamically.