See the question and my original answer on StackOverflow

Few points when working with p/invoke:

  • don't add what's obvious for .NET (structs are structs, ...)
  • don't add pack if you're unsure, by default .NET should behave like C/C++ on that matter
  • don't add Ansi information if there's no string in the definition (only when there are strings or TStr, etc.). It doesn't cause problems but is useless
  • in general, don't add attributes if you don't know what they are used for
  • int and long in C/C++ are (in general) 32-bit. long is not 64-bit in C/C++

Here is a code that should be better:

  [DllImport(@".\IFDLL.dll", EntryPoint = "GetProgramFileList")]
  public static extern int GetProgramFileListTest(uint proNum, ref PROFILE_INFO pro);

  public struct PROFILE_INFO
      public PROINFO proInfo;            // WNo/name/type
      public long proSize;               // Program size
      public PROGRAM_DATE createDate;    // Program creating date
      public PROGRAM_DATE writeDate;     // Program updating date

  [StructLayout(LayoutKind.Sequential, CharSet = CharSet.Ansi)]
  public struct PROINFO
      [MarshalAs(UnmanagedType.ByValTStr, SizeConst = 33)] public string wno;         // WNo.
      [MarshalAs(UnmanagedType.ByValTStr, SizeConst = 7)] private string dummy;       // dummy
      [MarshalAs(UnmanagedType.ByValTStr, SizeConst = 49)] public string comment;     // program name
      [MarshalAs(UnmanagedType.ByValTStr, SizeConst = 7)] private string dummy2;      // dummy
      public char type;                                                               // program type
      [MarshalAs(UnmanagedType.ByValTStr, SizeConst = 7)] private string dummy3;      // dummy

  [StructLayout(LayoutKind.Sequential, CharSet = CharSet.Ansi)]
  public struct PROGRAM_DATE
      public short year;                 // Date (Year) 4-digit
      public char month;                 // Date (Month)
      public char day;                   // Date (Day)
      public char hour;                  // Date (Time)
      public char min;                   // Date (Minutes)
      [MarshalAs(UnmanagedType.ByValTStr, SizeConst = 2)] private string dummy; // Dummy