Added an "argument prefix" generator to the C# context so typemaps can add prefixes before arguments are generated.

src/Generator/Generators/CSharp/CSharpMarshal.cs

@@ -1,6 +1,8 @@
-using System.Collections.Generic;
+using System;
+using System.Collections.Generic;
 using CppSharp.AST;
 using CppSharp.Types;
+using Type = CppSharp.AST.Type;
 
 namespace CppSharp.Generators.CSharp
 {
@@ -16,12 +18,14 @@ namespace CppSharp.Generators.CSharp
             : base(driver)
         {
             Kind = CSharpMarshalKind.Unknown;
+            ArgumentPrefix = new TextGenerator();
             Cleanup = new TextGenerator();
         }
 
         public CSharpMarshalKind Kind { get; set; }
         public QualifiedType FullType;
 
+        public TextGenerator ArgumentPrefix { get; private set; }
         public TextGenerator Cleanup { get; private set; }
     }
 

src/Generator/Generators/CSharp/CSharpTextTemplate.cs

@@ -1589,6 +1589,7 @@ namespace CppSharp.Generators.CSharp
             }
 
             var needsFixedThis = needsInstance && isValueType;
+            var @params = GenerateFunctionParamsMarshal(parameters, function);
 
             Class retClass = null;
             if (function.HasHiddenStructParameter)
@@ -1602,15 +1603,17 @@ namespace CppSharp.Generators.CSharp
                     retClass.OriginalName);
             }
 
-                retType.Type = new BuiltinType(PrimitiveType.Void);
-                needsReturn = false;
-            }
-
-            var @params = GenerateFunctionParamsMarshal(parameters, function);
+            var names = new List<string>();
+            foreach (var param in @params)
+            {
+                var name = string.Empty;
+                if (param.Context != null
+                    && !string.IsNullOrWhiteSpace(param.Context.ArgumentPrefix))
+                    name += param.Context.ArgumentPrefix;
 
-            var names = (from param in @params
-                         where !param.Param.Ignore
-                         select Helpers.SafeIdentifier(param.Name)).ToList();
+                name += Helpers.SafeIdentifier(param.Name);
+                names.Add(name);
+            }
 
             if (function.HasHiddenStructParameter)
             {