Fix #2957375 (C#) - SWIGStringHelper and SWIGExceptionHelper not always being initialized before use in .NET 4 as the intermediary class was not marked as beforefieldinit.

git-svn-id: https://swig.svn.sourceforge.net/svnroot/swig/trunk@12044 626c5289-ae23-0410-ae9c-e8d60b6d4f22

Lib/csharp/csharphead.swg

@@ -319,6 +319,14 @@ SWIGEXPORT void SWIGSTDCALL SWIGRegisterStringCallback_$module(SWIG_CSharpString
 %}
 #endif // SWIG_CSHARP_NO_STRING_HELPER
 
+#if !defined(SWIG_CSHARP_NO_IMCLASS_STATIC_CONSTRUCTOR)
+// Ensure the class is not marked beforefieldinit
+%pragma(csharp) imclasscode=%{
+  static $imclassname() {
+  }
+%}
+#endif
+
 %insert(runtime) %{
 /* Contract support */