Pass string from C# to C DLL

Question

I am trying to pass a string from C# to a C DLL. From what I have read .NET should do the conversion from string to char* for me, however I get "error CS1503: Argument '1': cannot convert from 'string' to 'char*'" Can someone advise me of where I have went wrong? Thanks.

C# code

[DllImport("Source.dll", ExactSpelling = true, CallingConvention = CallingConvention.Cdecl), SuppressUnmanagedCodeSecurity]
public static unsafe extern bool StreamReceiveInitialise(char* filepath);

const string test = "test";
// This method that will be called when the thread is started
public void Stream()
{
    if (StreamReceiveInitialise(test))
    {


    }
}

C DLL

extern "C"
{
    __declspec(dllexport) bool __cdecl StreamReceiveInitialise(char* filepath);
}

Why declare StreamReceiveInitialise as unsafe? Now you still need to pass a char* instead of a string. — diggingforfire, Jan 17 '12 at 14:14
I have now removed unsafe and am using as shown in answer 1. — integra753, Jan 17 '12 at 14:29

score 3 · Accepted Answer · answered Jan 17 '12 at 14:17

3

Declare your external method as:

public static extern bool StreamReceiveInitialise(string filepath);

answered Jan 17 '12 at 14:17

Rich O'Kelly

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Rasmus Faber · Answer 2 · 2012-01-17T16:26:50.413

1

Do it like this:

[DllImport("Source.dll", CallingConvention = CallingConvention.Cdecl, CharSet=CharSet.ANSI)]
static extern bool StreamReceiveInitialise([MarshalAs(UnmanagedType.LPStr)] string filepath);

(Marshalling as UnmanagedType.LPStr is the default, but I like being explicit).

edited Jan 17 '12 at 16:26

answered Jan 17 '12 at 14:20

Rasmus Faber

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1

That [MarshalAs] attribute is unnecessary, it is already the default marshaling for the string in this case. – Hans Passant Jan 17 '12 at 14:24
1

But specifying the `CharSet` on the `DllImport` attribute wouldn't be a bad idea. – Ben Voigt Jan 17 '12 at 14:35
I should have mentioned I do not need modify filepath within the C code. Does this change your comments? Apologies I am new to mixing C/C#. – integra753 Jan 17 '12 at 14:36
CharSet.ANSI is default, but I agree that it should be specified to be explicit. – Rasmus Faber Jan 17 '12 at 16:27
I have specfified CharSet.ANSI. – integra753 Jan 17 '12 at 16:53

score 1 · Answer 3 · edited May 23 '17 at 12:00

1

Use a StringBuilder in place of char*. See this

[DllImport("Source.dll")]
public static extern bool StreamReceiveInitialise(StringBuilder filepath);

edited May 23 '17 at 12:00

Community

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1

answered Jan 17 '12 at 14:22

Eren Ersönmez

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Pass string from C# to C DLL

3 Answers3