Making a StructLayout over a class working as it works over a struct

Question

I would like to better understand the mapping of structs/classes when it comes to deal with unmanaged code.

I have defined the following struct:

   [StructLayout(LayoutKind.Sequential)]
   public struct ProfileInfo
   {
      public int dwSize;
      public int dwFlags;
      [MarshalAs(UnmanagedType.LPTStr)] public string lpUserName;
      [MarshalAs(UnmanagedType.LPTStr)] public string lpProfilePath;
      [MarshalAs(UnmanagedType.LPTStr)] public string lpDefaultPath;
      [MarshalAs(UnmanagedType.LPTStr)] public string lpServerName;
      [MarshalAs(UnmanagedType.LPTStr)] public string lpPolicyPath;
      public IntPtr hProfile;

      public ProfileInfo(string userName, string profilepath)
      {
         dwFlags = 1;    
         dwSize = Marshal.SizeOf<ProfileInfo>();
         lpUserName = userName;
         lpServerName = null;
         lpProfilePath = string.IsNullOrWhiteSpace(profilepath) ? null : profilepath;
         lpPolicyPath = null;
         lpDefaultPath = null;
         hProfile = IntPtr.Zero;
      }
   }

to be used with the folling method:

      [DllImport("userenv.dll", SetLastError = true, CharSet = CharSet.Unicode, EntryPoint = "LoadUserProfileW")]
      public static extern bool LoadUserProfile(IntPtr hToken, ref ProfileInfo lpProfileInfo);

While it works great as long as ProfileInfo is a struct, LoadUserProfile starts failing when I make ProfileInfo a class.

I just wondered why ?
To me the StructLayout was applied the same way over a class or struct.

What are the possible differences of memory representation of ProfileInfo that makes LoadUserProfile fails when I change it from struct to class ?

My crystal ball says that you forgot to remove the `ref` keyword from the argument declaration. Required because class objects are always passed by reference. Smartest way to ask a question at SO is by posting the code that *doesn't* work. — Hans Passant, Aug 17 '15 at 15:43

score -1 · Answer 1 · answered Aug 17 '15 at 15:47

In C++, classes contain a hidden 'this' pointer. I am guessing C# does the same thing.

In the image below, pi is a struct, p2 is a class. You can see from the memory dumps (pi on the left, p2 on the right) that p2 has some extra bytes which is the hidden pointer.

Making a StructLayout over a class working as it works over a struct

1 Answers1