.net - Access violation when calling Delphi DLL from C# in a multi-threaded environment -

i calling dll function written in delphi xe2 c# using p/invoke. appears working when calls made sequentially single thread. however, when multiple threads calling function, c# host application throws system.accessviolationexception seemingly @ random.

why code below trigger access violation , how fix this?

minimum delphi library code reproducing problem:

library pinvokeproblem;  {$r *.res}  uses windows, sysutils;  procedure test(const testbyte: byte); stdcall; begin   outputdebugstring(pwidechar(inttostr(testbyte))); end;  exports test;  end. 

minimum c# host application code reproduce problem:

[dllimport(     "pinvokeproblem.dll",     callingconvention = callingconvention.stdcall,     entrypoint = "test")] private static extern void test(byte testbyte);  public static void main(string[] args) {     (int = 1; <= 1000; i++) // more iterations = better chance fail     {         int threadcount = 10;         parallel.for(1, threadcount, new paralleloptions { maxdegreeofparallelism = threadcount }, test =>         {             byte byteargument = 42;             test(byteargument);             console.writeline(string.format("iteration {0}: {1}", test, byteargument));         });      } } 

additional information:

• platform x64 windows 7. c# host application built x86 in .net 4.0, delphi dll compiled 32-bit.

• the library appears working ok when used in multi-threaded delphi host application.

• an msvc version of dll function signature extern __declspec(dllexport) void __stdcall test(char testbyte) works fine c# host (which suggests somehow specific delphi).

• the code not fail if library function has no return value (void) , arguments.

• changing calling convention in both code cdecl did not help.

any ideas appreciated.

all have set ismultithread true (as first line in dll's main begin..end block) switch memory manager thread-safe mode:

ismultithread := true;