CppSharp/Mono.VisualC.Interop/ABI/CppAbi.cs

//
// Mono.VisualC.Interop.ABI.CppAbi.cs: Represents an abstract C++ ABI
//
// Author:
//   Alexander Corrado (alexander.corrado@gmail.com)
//
// Copyright (C) 2010 Alexander Corrado
//

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

using System.Collections.Generic;
using System.Runtime.InteropServices;

using Mono.VisualC.Interop.Util;

namespace Mono.VisualC.Interop.ABI {

        //FIXME: Exception handling, operator overloading etc.
        //FIXME: Allow interface to override default calling convention
        public abstract partial class CppAbi {

                protected ModuleBuilder impl_module;
                protected TypeBuilder impl_type;

                protected Type interface_type, layout_type, wrapper_type;
                protected string library, class_name;

                protected FieldBuilder typeinfo_field;
                protected ILGenerator ctor_il;

		protected MemberFilter vtable_override_filter = VTable.BindToSignatureAndAttribute;

		// Cache some frequently used methodinfos:
		private static readonly MethodInfo typeinfo_nativesize  = typeof (CppTypeInfo).GetProperty ("NativeSize").GetGetMethod ();
		private static readonly MethodInfo typeinfo_vtable      = typeof (CppTypeInfo).GetProperty ("VTable").GetGetMethod ();
		private static readonly MethodInfo typeinfo_adjvcall    = typeof (CppTypeInfo).GetMethod ("GetAdjustedVirtualCall");
		private static readonly MethodInfo vtable_initinstance  = typeof (VTable).GetMethod ("InitInstance");
		private static readonly MethodInfo vtable_resetinstance = typeof (VTable).GetMethod ("ResetInstance");

                // These methods might be more commonly overridden for a given C++ ABI:

                public virtual MethodType GetMethodType (MethodInfo imethod)
                {
                        if (imethod.IsDefined (typeof (ConstructorAttribute), false))
                                return MethodType.NativeCtor;
                        else if (imethod.Name.Equals ("Alloc"))
                                return MethodType.ManagedAlloc;
                        else if (imethod.IsDefined (typeof (DestructorAttribute), false))
                                return MethodType.NativeDtor;

                        return MethodType.Native;
                }

		// The members below must be implemented for a given C++ ABI:

		public abstract string GetMangledMethodName (MethodInfo methodInfo);
		public abstract CallingConvention? GetCallingConvention (MethodInfo methodInfo);

		// The ImplementClass overrides are the main entry point to the Abi API:

		private struct EmptyNativeLayout { }
		public Iface ImplementClass<Iface> (Type wrapperType, string lib, string className)
		{
			return this.ImplementClass<Iface,EmptyNativeLayout> (wrapperType, lib, className);
		}

                public virtual Iface ImplementClass<Iface, NLayout> (Type wrapperType, string lib, string className)
                        where NLayout : struct
                      //where Iface : ICppClassInstantiatable or ICppClassOverridable
                {
                        this.impl_module = CppLibrary.interopModule;
                        this.library = lib;
                        this.class_name = className;
                        this.interface_type = typeof (Iface);
                        this.layout_type = typeof (NLayout);
                        this.wrapper_type = wrapperType;

                        DefineImplType ();

                        var properties = GetProperties ();
                        var methods = GetMethods ();
 			CppTypeInfo typeInfo = MakeTypeInfo (methods.Where (m => IsVirtual (m)));


			// Implement all methods
			int vtableIndex = 0;
                        foreach (var method in methods)
                                DefineMethod (method, typeInfo, ref vtableIndex);

                        // Implement all properties
                        foreach (var property in properties)
                                DefineProperty (property);

                        ctor_il.Emit (OpCodes.Ret);


                        return (Iface)Activator.CreateInstance (impl_type.CreateType (), typeInfo);
                }

		protected virtual CppTypeInfo MakeTypeInfo (IEnumerable<MethodInfo> virtualMethods)
		{
			return new CppTypeInfo (this, virtualMethods, layout_type);
		}

		protected virtual IEnumerable<PropertyInfo> GetProperties ()
		{
			return ( // get all properties defined on the interface
			        from property in interface_type.GetProperties ()
			        select property
			       ).Union ( // ... as well as those defined on inherited interfaces
			        from iface in interface_type.GetInterfaces ()
			        from property in iface.GetProperties ()
			        select property
			       );
		}

		protected virtual IEnumerable<MethodInfo> GetMethods ()
		{
			// get all methods defined on inherited interfaces first
			var methods = (
				       from iface in interface_type.GetInterfaces ()
			               from method in iface.GetMethods ()
			               where !method.IsSpecialName
				       select method
			              ).Concat (
			               from method in interface_type.GetMethods ()
			               where !method.IsSpecialName
			               orderby method.MetadataToken
			               select method
			              );

			return methods;
		}

                protected virtual void DefineImplType ()
                {
                        string implTypeName = interface_type.Name + "_" + layout_type.Name + "_" + this.GetType ().Name + "_Impl";
                        impl_type = impl_module.DefineType (implTypeName, TypeAttributes.Class | TypeAttributes.Sealed);
                        impl_type.AddInterfaceImplementation (interface_type);

                        //vtable_field = impl_type.DefineField ("_vtable", typeof (VTable), FieldAttributes.InitOnly | FieldAttributes.Private);
                        //native_size_field = impl_type.DefineField ("_nativeSize", typeof (int), FieldAttributes.InitOnly | FieldAttributes.Private);
			typeinfo_field = impl_type.DefineField ("_typeInfo", typeof (CppTypeInfo), FieldAttributes.InitOnly | FieldAttributes.Private);

                        ConstructorBuilder ctor = impl_type.DefineConstructor (MethodAttributes.Public, CallingConventions.Standard,
                                                                              new Type[] { typeof (CppTypeInfo) });

                        ctor_il = ctor.GetILGenerator ();

			// this._typeInfo = (CppTypeInfo passed to constructor)
                        ctor_il.Emit (OpCodes.Ldarg_0);
                        ctor_il.Emit (OpCodes.Ldarg_1);
                        ctor_il.Emit (OpCodes.Stfld, typeinfo_field);

			/*
                         // this._vtable = (VTable passed to constructor)
                        ctor_il.Emit (OpCodes.Ldarg_0);
                        ctor_il.Emit (OpCodes.Ldarg_1);
                        ctor_il.Emit (OpCodes.Stfld, vtable_field);
                        // this._nativeSize = (native size passed to constructor)
                        ctor_il.Emit (OpCodes.Ldarg_0);
                        ctor_il.Emit (OpCodes.Ldarg_2);
                        ctor_il.Emit (OpCodes.Stfld, native_size_field);
                        */
                }

                protected virtual MethodBuilder DefineMethod (MethodInfo interfaceMethod, CppTypeInfo typeInfo, ref int vtableIndex)
                {
                        // 0. Introspect method
                        MethodType methodType = GetMethodType (interfaceMethod);
                        Type [] parameterTypes = ReflectionHelper.GetMethodParameterTypes (interfaceMethod);

                        // 1. Generate managed trampoline to call native method
                        MethodBuilder trampoline = GetMethodBuilder (interfaceMethod);

                        ILGenerator il = trampoline.GetILGenerator ();

                        if (methodType == MethodType.NoOp) {
                                // return NULL if method is supposed to return a value
                                // FIXME: this will make value types explode?
                                if (!interfaceMethod.ReturnType.Equals (typeof (void)))
                                        il.Emit (OpCodes.Ldnull);
                                il.Emit (OpCodes.Ret);
                                return trampoline;
                        } else if (methodType == MethodType.ManagedAlloc) {
                                EmitManagedAlloc (il, interfaceMethod);
                                il.Emit (OpCodes.Ret);
                                return trampoline;
                        }

                        bool isStatic = IsStatic (interfaceMethod);
                        LocalBuilder cppInstancePtr = null;
                        LocalBuilder nativePtr = null;

                        // If we're an instance method, load up the "this" pointer
                        if (!isStatic)
                        {
                                if (parameterTypes.Length < 1)
                                        throw new ArgumentException ("First argument to non-static C++ method must be IntPtr or CppInstancePtr.");

                                // 2. Load the native C++ instance pointer
                                EmitLoadInstancePtr (il, parameterTypes [0], out cppInstancePtr, out nativePtr);

                                // 3. Make sure our native pointer is a valid reference. If not, throw ObjectDisposedException
                                EmitCheckDisposed (il, nativePtr, methodType);
                        }

                        MethodInfo nativeMethod;

                        if (IsVirtual (interfaceMethod) && methodType != MethodType.NativeDtor)
                                nativeMethod = EmitPrepareVirtualCall (il, typeInfo, nativePtr, vtableIndex++);
			else
                                nativeMethod = GetPInvokeForMethod (interfaceMethod);

                        switch (methodType) {
                        case MethodType.NativeCtor:
                                EmitConstruct (il, nativeMethod, parameterTypes, nativePtr);
                                break;

                        case MethodType.NativeDtor:
                                EmitDestruct (il, nativeMethod, parameterTypes, cppInstancePtr, nativePtr);
                                break;

                        default:
                                EmitCallNative (il, nativeMethod, isStatic, parameterTypes, nativePtr);
                                break;

                        }

                        il.Emit (OpCodes.Ret);
                        return trampoline;
                }

                protected virtual PropertyBuilder DefineProperty (PropertyInfo property)
                {
                        if (property.CanWrite)
                                throw new InvalidProgramException ("Properties in C++ interface must be read-only.");

                        MethodInfo imethod = property.GetGetMethod ();
                        string methodName = imethod.Name;
                        string propName = property.Name;
                        Type retType = imethod.ReturnType;
                        FieldBuilder fieldData;

                        // C++ interface properties are either to return the CppTypeInfo or to access C++ fields
                        if (retType.IsGenericType && retType.GetGenericTypeDefinition ().Equals (typeof (CppField<>))) {

				// define a new field for the property
                               // fieldData = impl_type.DefineField ("__" + propName + "_Data", retType, FieldAttributes.InitOnly | FieldAttributes.Private);

                                // init our field data with a new instance of CppField
                                // first, get field offset
                                //ctor_il.Emit (OpCodes.Ldarg_0);

                                /* TODO: Code prolly should not emit hardcoded offsets n such, in case we end up saving these assemblies in the future.
                                *  Something more like this perhaps? (need to figure out how to get field offset padding into this)
                                *       ctorIL.Emit(OpCodes.Ldtoken, nativeLayout);
                                *       ctorIL.Emit(OpCodes.Call, typeof(Type).GetMethod("GetTypeFromHandle"));
                                *       ctorIL.Emit(OpCodes.Ldstr, propName);
                                *       ctorIL.Emit(OpCodes.Call, typeof(Marshal).GetMethod("OffsetOf"));
                                */
				/*
                                int fieldOffset = ((int)Marshal.OffsetOf (layout_type, propName)) + FieldOffsetPadding;
                                ctor_il.Emit (OpCodes.Ldc_I4, fieldOffset);
                                ctor_il.Emit (OpCodes.Newobj, retType.GetConstructor (new Type[] { typeof(int) }));

                                ctor_il.Emit (OpCodes.Stfld, fieldData);
                                */
				throw new NotImplementedException ("CppFields need to be reimplemented to use CppTypeInfo.");
                        } else if (retType.Equals (typeof (CppTypeInfo)))
				fieldData = typeinfo_field;
                          else
                                throw new InvalidProgramException ("Properties in C++ interface can only be of type CppField.");

                        PropertyBuilder fieldProp = impl_type.DefineProperty (propName, PropertyAttributes.None, retType, Type.EmptyTypes);

                        MethodAttributes methodAttr = MethodAttributes.Public | MethodAttributes.Virtual | MethodAttributes.SpecialName | MethodAttributes.HideBySig;
                        MethodBuilder fieldGetter = impl_type.DefineMethod (methodName, methodAttr, retType, Type.EmptyTypes);
                        ILGenerator il = fieldGetter.GetILGenerator ();

                        il.Emit (OpCodes.Ldarg_0);
                        il.Emit (OpCodes.Ldfld, fieldData);
                        il.Emit (OpCodes.Ret);

                        fieldProp.SetGetMethod (fieldGetter);

                        return fieldProp;
                }

                /**
                 * Implements the managed trampoline that will be invoked from the vtable by native C++ code when overriding
                 *  the specified C++ virtual method with the specified managed one.
                 */
                internal virtual Delegate GetManagedOverrideTrampoline (CppTypeInfo typeInfo, int vtableIndex)
                {
                        if (wrapper_type == null)
                                return null;

			MethodInfo interfaceMethod = typeInfo.VirtualMethods [vtableIndex];
                        MethodInfo targetMethod = FindManagedOverrideTarget (interfaceMethod);
                        if (targetMethod == null)
                                return null;

                        Type[] parameterTypes = GetParameterTypesForPInvoke (interfaceMethod).ToArray ();

                        // TODO: According to http://msdn.microsoft.com/en-us/library/w16z8yc4.aspx
                        // The dynamic method created with this constructor has access to public and internal members of all the types contained in module m.
                        // This does not appear to hold true, so we also disable JIT visibility checks.
                        DynamicMethod trampolineIn = new DynamicMethod (wrapper_type.Name + "_" + interfaceMethod.Name + "_FromNative", interfaceMethod.ReturnType,
                                                                        parameterTypes, typeof (CppInstancePtr).Module, true);

                        ReflectionHelper.ApplyMethodParameterAttributes (interfaceMethod, trampolineIn, true);
                        ILGenerator il = trampolineIn.GetILGenerator ();

                        // for static methods:
                        OpCode callInstruction = OpCodes.Call;
                        int argLoadStart = 1;

                        // for instance methods, we need an instance to call them on!
                        if (!targetMethod.IsStatic) {
                                callInstruction = OpCodes.Callvirt;
                                //argLoadStart = 1;

				il.Emit (OpCodes.Ldarg_0);
				il.Emit (OpCodes.Ldc_I4, typeInfo.NativeSize);

                                MethodInfo getManagedObj = typeof (CppInstancePtr).GetMethod ("GetManaged", BindingFlags.Static | BindingFlags.NonPublic).MakeGenericMethod (wrapper_type);
                                il.Emit (OpCodes.Call, getManagedObj);
                        }

                        for (int i = argLoadStart; i < parameterTypes.Length; i++) {
                                il.Emit (OpCodes.Ldarg, i);
                        }
                        il.Emit (OpCodes.Tailcall);
                        il.Emit (callInstruction, targetMethod);
                        il.Emit (OpCodes.Ret);

                        return trampolineIn.CreateDelegate (typeInfo.VTableDelegateTypes [vtableIndex]);
                }

                protected virtual MethodInfo FindManagedOverrideTarget (MethodInfo interfaceMethod)
                {
			// FIXME: Does/should this look in superclasses?
                        MemberInfo [] possibleMembers = wrapper_type.FindMembers (MemberTypes.Method, BindingFlags.Public | BindingFlags.NonPublic |
                                                                                BindingFlags.Instance | BindingFlags.Static, vtable_override_filter, interfaceMethod);

                        if (possibleMembers.Length > 1)
                                throw new InvalidProgramException ("More than one possible override found when binding virtual method: " + interfaceMethod.Name);
                        else if (possibleMembers.Length == 0)
                                return null;

                        return (MethodInfo)possibleMembers [0];
                }

                /**
                 * Defines a new MethodBuilder with the same signature as the passed MethodInfo
                 */
                protected virtual MethodBuilder GetMethodBuilder (MethodInfo interfaceMethod)
                {

                        Type [] parameterTypes = ReflectionHelper.GetMethodParameterTypes (interfaceMethod);
                        MethodBuilder methodBuilder = impl_type.DefineMethod (interfaceMethod.Name, MethodAttributes.Public | MethodAttributes.Virtual,
                                                                             interfaceMethod.ReturnType, parameterTypes);
                        ReflectionHelper.ApplyMethodParameterAttributes (interfaceMethod, methodBuilder, false);
                        return methodBuilder;
                }

                /**
                 * Defines a new MethodBuilder that calls the specified C++ (non-virtual) method using its mangled name
                 */
                protected virtual MethodBuilder GetPInvokeForMethod (MethodInfo signature)
                {
                        string entryPoint = GetMangledMethodName (signature);
                        if (entryPoint == null)
                                throw new NotSupportedException ("Could not mangle method name.");

                        Type [] parameterTypes = GetParameterTypesForPInvoke (signature).ToArray ();

                        MethodBuilder builder = impl_type.DefinePInvokeMethod ("__$" + signature.Name + "_Impl", library, entryPoint,
                                                                              MethodAttributes.Private | MethodAttributes.Static | MethodAttributes.PinvokeImpl,
                                                                              CallingConventions.Standard, signature.ReturnType, parameterTypes,
                                                                              GetCallingConvention (signature).Value, CharSet.Ansi);
                        builder.SetImplementationFlags (builder.GetMethodImplementationFlags () | MethodImplAttributes.PreserveSig);
                        ReflectionHelper.ApplyMethodParameterAttributes (signature, builder, true);
                        return builder;
                }

		/**
		 * Emits the IL to load the correct delegate instance and/or retrieve the MethodInfo from the VTable
		 * for a C++ virtual call.
		 */
		protected virtual MethodInfo EmitPrepareVirtualCall (ILGenerator il, CppTypeInfo typeInfo, LocalBuilder native, int vtableIndex)
		{
			Type vtableDelegateType = typeInfo.VTableDelegateTypes [vtableIndex];
			MethodInfo getDelegate  = typeinfo_adjvcall.MakeGenericMethod (vtableDelegateType);

			// this._typeInfo.GetAdjustedVirtualCall<T> (native, vtableIndex);
			il.Emit (OpCodes.Ldarg_0);
			il.Emit (OpCodes.Ldfld, typeinfo_field);
			il.Emit (OpCodes.Ldloc_S, native);
			il.Emit (OpCodes.Ldc_I4, vtableIndex);
			il.Emit (OpCodes.Callvirt, getDelegate);

			return ReflectionHelper.GetMethodInfoForDelegate (vtableDelegateType);
		}

                /**
                 *  Emits IL to allocate the memory for a new instance of the C++ class.
                 *  To complete method, emit OpCodes.Ret.
                 */
                protected virtual void EmitManagedAlloc (ILGenerator il, MethodInfo interfaceMethod)
                {
                        // this._typeInfo.NativeSize
			il.Emit (OpCodes.Ldarg_0);
			il.Emit (OpCodes.Ldfld, typeinfo_field);
			il.Emit (OpCodes.Callvirt, typeinfo_nativesize);

                        if (wrapper_type != null) {
                                // load managed wrapper
                                il.Emit (OpCodes.Ldarg_1);
                                il.Emit (OpCodes.Newobj, typeof (CppInstancePtr).GetConstructor (BindingFlags.Instance | BindingFlags.NonPublic, null,
                                                                                                new Type[] { typeof (int), typeof (object) }, null));
                        } else
                                il.Emit (OpCodes.Newobj, typeof (CppInstancePtr).GetConstructor (BindingFlags.Instance | BindingFlags.NonPublic, null,
                                                                                                new Type[] { typeof (int) }, null));
                }

                protected virtual void EmitConstruct (ILGenerator il, MethodInfo nativeMethod, Type [] parameterTypes,
                                                      LocalBuilder nativePtr)
                {
                        EmitCallNative (il, nativeMethod, false, parameterTypes, nativePtr);
                        EmitInitVTable (il, nativePtr);
                }

                protected virtual void EmitDestruct (ILGenerator il, MethodInfo nativeMethod, Type [] parameterTypes,
                                                     LocalBuilder cppInstancePtr, LocalBuilder nativePtr)
                {
                        // bail if we weren't alloc'd by managed code
                        Label bail = il.DefineLabel ();

                        il.Emit (OpCodes.Ldloca_S, cppInstancePtr);
                        il.Emit (OpCodes.Brfalse_S, bail); // <- FIXME? (would this ever branch?)
                        il.Emit (OpCodes.Ldloca_S, cppInstancePtr);
                        il.Emit (OpCodes.Call, typeof (CppInstancePtr).GetProperty ("IsManagedAlloc").GetGetMethod ());
                        il.Emit (OpCodes.Brfalse_S, bail);

                        EmitResetVTable (il, nativePtr);
                        EmitCallNative (il, nativeMethod, false, parameterTypes, nativePtr);

                        il.MarkLabel (bail);
                }

                /**
                 * Emits IL to call the native method. nativeMethod should be either a method obtained by
                 * GetPInvokeForMethod or the MethodInfo of a vtable method.
                 * To complete method, emit OpCodes.Ret.
                 */
                protected virtual void EmitCallNative (ILGenerator il, MethodInfo nativeMethod, bool isStatic, Type [] parameterTypes,
                                                       LocalBuilder nativePtr)
                {
                        int argLoadStart = 1; // For static methods, just strip off arg0 (.net this pointer)
                        if (!isStatic)
                        {
                                argLoadStart = 2; // For instance methods, strip off CppInstancePtr and pass the corresponding IntPtr
                                il.Emit (OpCodes.Ldloc_S, nativePtr);
                        }
                        for (int i = argLoadStart; i <= parameterTypes.Length; i++) {
				il.Emit (OpCodes.Ldarg, i);
				EmitSpecialParameterMarshal (il, parameterTypes [i - 1]);
			}

                        il.Emit (OpCodes.Call, nativeMethod);
                }

		// Note: when this is modified, usually GetParameterTypesForPInvoke types should be updated too
		protected virtual void EmitSpecialParameterMarshal (ILGenerator il, Type parameterType)
		{
			// auto marshal bool to C++ bool type (0 = false , 1 = true )
			if (parameterType.Equals (typeof (bool))) {
				Label isTrue = il.DefineLabel ();
				Label done = il.DefineLabel ();
				il.Emit (OpCodes.Brtrue_S, isTrue);
				il.Emit (OpCodes.Ldc_I4_0);
				il.Emit (OpCodes.Br_S, done);
				il.MarkLabel (isTrue);
				il.Emit (OpCodes.Ldc_I4_1);
				il.MarkLabel (done);
				//il.Emit (OpCodes.Conv_I1);
			}
			// auto marshal ICppObject
		}

		public virtual IEnumerable<Type> GetParameterTypesForPInvoke (MethodInfo method)
		{
			var originalTypes = ReflectionHelper.GetMethodParameterTypes (method);

			var pinvokeTypes = originalTypes.Transform (
			        For.AnyInputIn (typeof (bool)).Emit (typeof (byte)),

				// CppInstancePtr implements ICppObject
				For.InputsWhere ((Type t) => typeof (ICppObject).IsAssignableFrom (t)).Emit (typeof (IntPtr)),

			        For.UnmatchedInput<Type> ().Emit (t => t)
			);
			return pinvokeTypes;
		}

		/**
		 * Emits IL to load the VTable object onto the stack.
		 */
		protected virtual void EmitLoadVTable (ILGenerator il)
		{
			// this._typeInfo.VTable
			il.Emit (OpCodes.Ldarg_0);
                        il.Emit (OpCodes.Ldfld, typeinfo_field);
			il.Emit (OpCodes.Callvirt, typeinfo_vtable);
		}

                /**
                 * Emits IL to set the vtable pointer of the instance (if class has a vtable).
                 * This should usually happen in the managed wrapper of the C++ instance constructor.
                 */
                protected virtual void EmitInitVTable (ILGenerator il, LocalBuilder nativePtr)
                {
                        // this._typeInfo.VTable.InitInstance (nativePtr);
			EmitLoadVTable (il);
                        il.Emit (OpCodes.Ldloc_S, nativePtr);
                        EmitCallVTableMethod (il, vtable_initinstance, 2, false);
                }

                protected virtual void EmitResetVTable (ILGenerator il, LocalBuilder nativePtr)
                {
                        // this._typeInfo.VTable.ResetInstance (nativePtr);
                        EmitLoadVTable (il);
                        il.Emit (OpCodes.Ldloc_S, nativePtr);
                        EmitCallVTableMethod (il, vtable_resetinstance, 2, false);
                }

                /**
                 * A utility function to emit the IL for a vtable-dependant operation.
                 * In other words, classes with no virtual methods will not have vtables,
                 * so this method emits code to check for that and either throw an exception
                 * or do nothing if no vtable exists. To use, push the arguments to the method you
                 * want to call and pass the stackHeight for the call. If no vtable exists, this method
                 * will emit code to pop the arguments off the stack.
                 */
		protected virtual void EmitCallVTableMethod (ILGenerator il, MethodInfo method, int stackHeight,
		                                             bool throwOnNoVTable)
                {
                        // prepare a jump; do not call vtable method if no vtable
                        Label noVirt = il.DefineLabel ();
                        Label dontPushOrThrow = il.DefineLabel ();

                        EmitLoadVTable (il);
                        il.Emit (OpCodes.Brfalse_S, noVirt); // if (vtableInfo == null) goto noVirt

                        il.Emit (OpCodes.Callvirt, method); // call method
                        il.Emit (OpCodes.Br_S, dontPushOrThrow); // goto dontPushOrThrow

                        il.MarkLabel (noVirt);
                        // noVirt:
                        // since there is no vtable, we did not make the method call.
                        // pop arguments
                        for (int i = 0; i < stackHeight; i++)
                                il.Emit (OpCodes.Pop);

                        // if the method was supposed to return a value, we must
                        // still push something onto the stack
                        // FIXME: This is a kludge. What about value types?
                        if (!method.ReturnType.Equals (typeof (void)))
                                il.Emit (OpCodes.Ldnull);

                        if (throwOnNoVTable) {
                                il.Emit (OpCodes.Ldstr, "Native class has no VTable.");
                                il.Emit (OpCodes.Newobj, typeof (InvalidOperationException).GetConstructor(new Type[] {typeof (string)}));
                                il.Emit (OpCodes.Throw);
                        }

                        il.MarkLabel (dontPushOrThrow);
                }

                protected virtual void EmitLoadInstancePtr (ILGenerator il, Type firstParamType, out LocalBuilder cppip,
                                                            out LocalBuilder native)
                {
                        cppip = null;
                        native = null;

                        il.Emit (OpCodes.Ldarg_1);
                        if (firstParamType.Equals (typeof (CppInstancePtr))) {
                                cppip = il.DeclareLocal (typeof (CppInstancePtr));
                                native = il.DeclareLocal (typeof (IntPtr));
                                il.Emit (OpCodes.Stloc_S, cppip);
                                il.Emit (OpCodes.Ldloca_S, cppip);
                                il.Emit (OpCodes.Call, typeof (CppInstancePtr).GetProperty ("Native").GetGetMethod ());
                                il.Emit (OpCodes.Stloc_S, native);
                        } else if (firstParamType.Equals (typeof (IntPtr))) {
                                native = il.DeclareLocal (typeof (IntPtr));
                                il.Emit (OpCodes.Stloc_S, native);
                        } else if (firstParamType.IsByRef) {
                                native = il.DeclareLocal (firstParamType);
                                il.Emit (OpCodes.Stloc_S, native);
                        } else
                                throw new ArgumentException ("First argument to non-static C++ method must be byref, IntPtr or CppInstancePtr.");
                }

                protected virtual void EmitCheckManagedAlloc (ILGenerator il, LocalBuilder cppip)
                {
                        // make sure we were allocated by managed code
                        // if not, return
                        Label managedAlloc = il.DefineLabel ();
                        il.Emit (OpCodes.Ldloca_S, cppip);
                        il.Emit (OpCodes.Call, typeof (CppInstancePtr).GetProperty ("IsManagedAlloc").GetGetMethod ());
                        il.Emit (OpCodes.Brtrue_S, managedAlloc);
                        il.Emit (OpCodes.Ret);
                        il.MarkLabel (managedAlloc);
                }

                /**
                 * throw ObjectDisposedException if we have a null pointer for native
                 * however, allow destructor to be called even if we're disposed (just return immediately)
                 */
                protected virtual void EmitCheckDisposed (ILGenerator il, LocalBuilder native, MethodType methodType)
                {
                        Label validRef = il.DefineLabel ();
                        il.Emit (OpCodes.Ldloc_S, native);
                        il.Emit (OpCodes.Brtrue_S, validRef);
                        if (methodType == MethodType.NativeDtor) {
                                il.Emit (OpCodes.Ret);
                                il.MarkLabel (validRef);
                        } else {
                                il.Emit (OpCodes.Ldstr, String.Empty);
                                il.Emit (OpCodes.Newobj, typeof (ObjectDisposedException).GetConstructor (new Type[] { typeof(string) }));
                                il.Emit (OpCodes.Throw);
                                il.MarkLabel (validRef);
                        }
                }



        }

}