C# get string/characters value from c++ function which return char pointer

Question

I've a DLL written in C++. A function of this DLL is like the following code:

C++ code:

     char _H *GetPalette() {

            -------Functions body

            -------Functions body

            return pPaletteString;

      }

Now I want to get the Pallet String from that GetPalette() function in C# code.

How could I get string from that function? I've tried this in C# code. But could not get the correct result.

C# code:

    [DllImport("cl.dll", EntryPoint = "GetPalette@0", CallingConvention = CallingConvention.StdCall)]

    private static extern IntPtr libGetPalette();

    public IntPtr GetPalette()
    {
        return libGetPalette();
    }

Finally I want to get string like this

            IntPtr result;
            result = imgProcess.GetPallet();

            string pallet;
            pallet = Marshal.PtrToStringAnsi(result);
            MessageBox.Show(pallet);

This code does not work properly. Can some body plz help me, How could I get the string value from my C++ DLL function?

Thanks

Shahriar

It is good practice to define what error or behavior you are getting. How can we know what does "This code does not work properly" mean? — Marino Šimić, May 02 '11 at 10:17
I've got '-->' characters from that code. But it should return as like '473757.473767.574767.............' string. — Shahriar Hasan Sayeed, May 02 '11 at 10:28

score 1 · Answer 1 · answered May 02 '11 at 10:16

1

You've told C# that the calling convention is __stdcall but there's no evidence of __stdcall marking on the function itself. In addition, char* could be UTF-8.

answered May 02 '11 at 10:16

Puppy

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Actually, I'm new in .net Interoperability/Marshalling . I've used this stdcall convention code from other sources. May be I'm wrong. So what can I do now to get string from that C++ DLL function? Thanks – Shahriar Hasan Sayeed May 02 '11 at 10:36
1

Changing your C# code to match the calling convention of the C++ code would be a start. – Security Hound May 02 '11 at 12:03

score 1 · Accepted Answer · answered May 02 '11 at 15:01

1

You can define your C++ function in C# code with string return type.

[DllImport("cl.dll")]
private static extern string GetPalette();

And than simply call it from in your C# code.

string palette = GetPalette();

Within DllImport attribute you might need to set correct calling convention CallingConvention and character encoding CharSet

answered May 02 '11 at 15:01

Lukas Kabrt

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I think you might want to explain the onwership semantics of the returned buffers (IIRC .NET will free the memory with free()?) – sehe May 02 '11 at 17:12
Thanks Lukas Kabrt. Your answer is 100% correct. I've used CharSet=CharSet.Ansi and changed my functions return type as string. And now I'm getting the correct results. – Shahriar Hasan Sayeed May 03 '11 at 05:01

C# get string/characters value from c++ function which return char pointer

2 Answers2