Passing an array of strings from managed C# to unmanaged function using P-Invoke

Question

Is it possible to pass a string array from managed C# to an unmanaged function using P-Invoke?

This works fine:

[DllImport("LibraryName.dll")]
private static extern void Function_Name(string message);

While this:

[DllImport("LibraryName.dll")]
private static extern void Function_Name(string[] message);

fails with

Unhandled exception: System.NotSupportedException: NotSupportedException

I have tried using MarshalAs with no luck ([MarshalAs(UnmanagedType.LPArray, ArraySubType=UnmanagedType.LPWStr)] String[] dataToLoadArr)

Is it possible to pass string arrays this way?

What's the message on the exception? Also, what is the declaration of the unmanaged function? — casperOne
– casperOne, Commented Sep 27, 2011 at 13:52
Thanks, mtijn, that helped. Solved by using IntPtr structures representing the strings to be marshaled. — Rendijs S
– Rendijs S, Commented Sep 27, 2011 at 15:01
This works fine when I try it, no manual marshaling required. There is a nasty flaw in the approach though, the native code has no way to tell how large the array is. An extra argument is required. — Hans Passant
– Hans Passant, Commented Sep 27, 2011 at 15:27
What version of Windows are you using? Is this Windows CE or something like that? — Laurion Burchall
– Laurion Burchall, Commented Sep 27, 2011 at 16:14

Ani · Accepted Answer · 2012-02-13 19:24:59Z

[DllImport(Library)]
private static extern IntPtr clCreateProgramWithSource(Context context,
                                                       cl_uint count,
                                                       [In] [MarshalAs(UnmanagedType.LPArray, ArraySubType = UnmanagedType.LPStr, SizeParamIndex = 1)] string[] strings,
                                                       [In] [MarshalAs(UnmanagedType.LPArray, ArraySubType = UnmanagedType.SysUInt, SizeParamIndex = 1)] IntPtr[] lengths,
                                                       out ErrorCode errcodeRet);
public static Program CreateProgramWithSource(Context context,
                                                 cl_uint count,
                                                 string[] strings,
                                                 IntPtr[] lengths,
                                                 out ErrorCode errcodeRet)

This works fine in my OpenCL library, OpenCL.NET (http://openclnet.codeplex.com/SourceControl/changeset/view/94246#1251571). Note that I am passing in the count using SizeParamIndex as well.

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