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 pointers:

extern 'C' {    struct Info    {       int id;       char* szName;    };     Info* LookupInfo( int id ); } 

In C#, how can I declare the structure layout, declare the Interop call, and (assuming a non-null value is returned) utilize the string value? In other words, how do I translate the following into C#?

#include 'InfoLookup.h' void foo() {    Info* info = LookupInfo( 0 );    if( info != 0 && info->szName != 0 )       DoSomethingWith( info->szName );    // NOTE: no cleanup here, the DLL is caching the lookup table internally } 

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));     }  }