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

Try the following layout. Code automatically generated using the PInvoke Interop Assistant. Hand coded LookpInfoWrapper()

[System.Runtime.InteropServices.StructLayoutAttribute(System.Runtime.InteropServices.LayoutKind.Sequential)]
public struct Info {

    /// int
    public int id;

    /// char*
    [System.Runtime.InteropServices.MarshalAsAttribute(System.Runtime.InteropServices.UnmanagedType.LPStr)]
    public string szName;
}

public partial class NativeMethods {

    /// Return Type: Info*
    ///id: int
    [System.Runtime.InteropServices.DllImportAttribute("InfoLookup.dll", EntryPoint="LookupInfo")]
public static extern  System.IntPtr LookupInfo(int id) ;

    public static LoopInfoWrapper(int id) {
       IntPtr ptr = LookupInfo(id);
       return (Info)(Marshal.PtrToStructure(ptr, typeof(Info));
    }

}