In C#, how do I invoke a DLL function that returns an unmanaged structure containing a string pointer?

Question

I have been given a DLL (\"InfoLookup.dll\") that internally allocates structures and returns pointers to them from a lookup function. The structures contain string pointer

Answer 1

You need to implement the structure in C# as well, making sure to use the attributes in the Marshal class properly to ensure that the memory layout matches the unmanaged version.

So, some variation of this:

using System.Runtime.InteropServices;

[DllImport("mydll.dll")]
public static extern Info LookupInfo(int val);

[StructLayout(LayoutKind.Sequential)]
struct Info
{
   int id;
   String szName;
}

private void SomeFunction
{
   Info info = LookupInfo(0);
   //Note here that the returned struct cannot be null, so check the ID instead
   if (info.id != 0 && !String.IsNullOrEmpty(info.szName))
      DoSomethingWith(info.szName);
}