MethodBuilder myMthdBuilder = myTypeBuilder.DefineMethod("MyMethod", 
                     MethodAttributes.Public,
                     returnType, mthdParamTypes);
                               
   // We will assume that an external unmanaged type "LegacyNumber" has been loaded, and
   // that it has a method "ToString" which returns a string.

   MethodInfo unmanagedMthdMI = Type.GetType("LegacyNumber").GetMethod("ToString");
   ILGenerator myMthdIL = myMthdBuilder.GetILGenerator();

   // Code to emit various IL opcodes here ...

   // Load a reference to the specific object instance onto the stack.

   myMthdIL.Emit(OpCodes.Ldc_I4, addrOfLegacyNumberObject);
   myMthdIL.Emit(OpCodes.Ldobj, Type.GetType("LegacyNumber"));

   // Make the call to the unmanaged type method, telling it that the method is
   // the member of a specific instance, to expect a string 
   // as a return value, and that there are no explicit parameters.
   myMthdIL.EmitCalli(OpCodes.Calli, 
              System.Runtime.InteropServices.CallingConvention.ThisCall,
                  typeof(string),
              new Type[] {});

   // More IL code emission here ...