call by value: handle cctors with default parameters

Author: Fabio Anderegg

src/Generator/Generators/CSharp/CSharpMarshal.cs

@@ -777,10 +777,18 @@ private void MarshalRefClass(Class @class)
                                 Context.Parameter.Name}"", ""Cannot be null because it is passed by value."");");
 
                         var nativeClass = typePrinter.PrintNative(@class);
+
                         var cctorName = CSharpSources.GetFunctionNativeIdentifier(Context.Context, cctor);
+
+                        var defaultValue = "";
+                        var TypePrinter = new CSharpTypePrinter(Context.Context);
+                        var ExpressionPrinter = new CSharpExpressionPrinter(TypePrinter);
+                        if (cctor.Parameters.Count > 1)
+                            defaultValue = $", {ExpressionPrinter.VisitParameter(cctor.Parameters.Last())}";
+
                         Context.Before.WriteLine($"byte* __{Context.Parameter.Name}Memory = stackalloc byte[sizeof({nativeClass})];");
                         Context.Before.WriteLine($"__IntPtr __{Context.Parameter.Name}Ptr = (__IntPtr)__{Context.Parameter.Name}Memory;");
-                        Context.Before.WriteLine($"{nativeClass}.{cctorName}(__{Context.Parameter.Name}Ptr, {Context.Parameter.Name}.__Instance);");
+                        Context.Before.WriteLine($"{nativeClass}.{cctorName}(__{Context.Parameter.Name}Ptr, {Context.Parameter.Name}.__Instance{defaultValue});");
                         Context.Return.Write($"__{Context.Parameter.Name}Ptr");
 
                         if (Context.Context.ParserOptions.IsItaniumLikeAbi && @class.HasNonTrivialDestructor)