The way you have written it, the char* buffers are allocated on the managed side. But that's the wrong place. The allocation happens on the unmanaged side. Declare the struct in C# like this:

[StructLayout(LayoutKind.Sequential)] public struct UserProfileData { int userProfileRevision; public IntPtr firstName; public IntPtr lastName; public IntPtr memberids; public IntPtr emailAddress; } 

Then call getUserProfileData, passing the struct by ref or out, whichever is correct (I can't tell from here).

Your DllImport will look like this (with the correct calling convention specified):

[DllImport(@"mydll.dll", CallingConvention=CallingConvention.???)] private static extern int getUserProfileData(ref UserProfileData userProfile); 

Then convert the returned pointers to strings like this:

string firstName = Marshal.PtrToStringAnsi(userProfile.firstName); 

and so on for the other fields.

Presumably the unmanaged code also exposes a function that deallocates the memory returned in the struct. Call that once you are done with the struct.

The way you have written it, the char* buffers are allocated on the managed side. But that's the wrong place. The allocation happens on the unmanaged side. Declare the struct in C# like this:

[StructLayout(LayoutKind.Sequential)] public struct UserProfileData { int userProfileRevision; public IntPtr firstName; public IntPtr lastName; public IntPtr memberids; public IntPtr emailAddress; } 

Then call getUserProfileData, passing the struct as an out parameter. Or possibly a ref parameter. I can't tell from here which it should be.

Your DllImport will look like this (with the correct calling convention specified):

[DllImport(@"mydll.dll", CallingConvention=CallingConvention.???)] private static extern int getUserProfileData(out UserProfileData userProfile); 

Then convert the returned pointers to strings like this:

string firstName = Marshal.PtrToStringAnsi(userProfile.firstName); 

and so on for the other fields.

Presumably the unmanaged code also exposes a function that deallocates the memory returned in the struct. Call that once you are done with the struct.