typedef void (__stdcall *fnSavedCB)(int savedLen);

__declspec(dllexport)
void __stdcall testSaveData(fnSavedCB fn)
{
    for (int i = 0; i < 10; ++i) {
        fn((i+1)*1024);
    }
}

[UnmanagedFunctionPointer(CallingConvention.StdCall)]
public delegate void fnSaveCB(int savedLen);

[DllImport("testDLL", CallingConvention = CallingConvention.StdCall, EntryPoint = "testSaveData")]
public static extern void testSaveData(fnSaveCB);

//调用测试
public void SaveCB(int saveLen)
{
    System.Diagnostics.Trace.WriteLine("Saved:" + savedLen.ToString());
}

public void TestDll()
{
    testSaveData(SaveCB);
}