Get an array of structures from native dll to c# application

Question

I have a C# .NET 2.0 CF project where I need to invoke a method in a native C++ DLL. This native method returns an array of type TableEntry. At the time the native method is called, I do not know how large the array will be.

How can I get the table from the native DLL to the C# project? Below is effectively what I have now.

// in C# .NET 2.0 CF project
[StructLayout(LayoutKind.Sequential)]
public struct TableEntry
{
    [MarshalAs(UnmanagedType.LPWStr)] public string description;
    public int item;
    public int another_item;
    public IntPtr some_data;
}

[DllImport("MyDll.dll", 
    CallingConvention = CallingConvention.Winapi, 
    CharSet = CharSet.Auto)]
public static extern bool GetTable(ref TableEntry[] table);

SomeFunction()
{
    TableEntry[] table = null;
    bool success = GetTable( ref table );
    // at this point, the table is empty
}


// In Native C++ DLL
std::vector< TABLE_ENTRY > global_dll_table;
extern "C" __declspec(dllexport) bool GetTable( TABLE_ENTRY* table )
{
    table = &global_dll_table.front();
    return true;
}

Thanks, PaulH

Emerald

Shouldn't that be TABLE_ENTRY**, since you want to write a pointer? – Kulturkampf

@Kulturkampf - You are correct, it should be. – Emerald

Answer 1

When marshalling an array of unknown size from native to managed I find the best strategy is as follows

• Type the array to IntPtr in managed code
• Have the native code return both the array and a size parameter.
• Manually marshal the data from IntPtr to the custom struct on the managed side.

As such I would make the following changes to your code.

Native:

extern "C" __declspec(dllexport) bool GetTable( TABLE_ENTRY** table, __int32* pSize )
{
    *table = &global_dll_table.front();
    *pSize = static_cast<int32>(global_dll_table.size());
    return true;
}

Managed:

[DllImport("MyDll.dll", 
    CallingConvention = CallingConvention.Winapi, 
    CharSet = CharSet.Auto)]
[return: MarshalAs(UnmanagedType.I1)]
public static extern bool GetTable(out IntPtr arrayPtr, out int size);

public static List<TableEntry> GetTable() {
  var arrayValue = IntPtr.Zero;
  var size = 0;
  var list = new List<TableEntry>();

  if ( !GetTable(out arrayValue, out size)) {
    return list; 
  }

  var tableEntrySize = Marshal.SizeOf(typeof(TableEntry));
  for ( var i = 0; i < size; i++) {  
    var cur = (TableEntry)Marshal.PtrToStructure(arrayValue, typeof(TableEntry));
    list.Add(cur);
    arrayValue = new IntPtr(arrayValue.ToInt32() + tableEntrySize);
  }
  return list;
}

Hamhung

Good solution, unless the interface of the native DLL cannot be changed (which is, unfortunately, a common case when P/Invoking existing libraries). – Kulturkampf

Hi @JaredPar, I see that you're still active here. I have a new question that my code is modeled after your solution here. I was hoping you could take a look if you have time and see if you can point me in the right direction, here is the question #38648673 Thanks – Hither