pass char * to C DLL from C# string

Question

I need to use a library of C functions in a DLL from a C# application. I am having trouble calling DLL functions with char * arguments:

The C DLL:

extern "C" __declspec(dllexport) int CopyFunc(char *, char *);
int CopyFunc(char *dest, char *src)
{
    strcpy(dest, src);
    return(strlen(src));
}

the C# app needs to look something like this:

[DllImport("dork.dll")]
public static extern int CopyFunc(string dst, string src);

int GetFuncVal(string source, string dest)
{
    return(CopyFunc(dest,source));
}

I've seen examples using string or StringBuilder, or IntPtr as replacements for the char * required by the DLL function prototype, but I've not been able to get any of them to work. The most common exception I get is that PInvoke is unbalancing the stack because the function call does not match the prototype.

Is there a simple solution to this?

A char * is a pointer to a 1-byte character type. A C# string is Unicode. You need to straighten that out first. — PaulMcKenzie
– PaulMcKenzie, Commented Oct 11, 2016 at 22:02

BugsFree · Accepted Answer · 2016-10-11 22:23:09Z

10

Update your P/Invoke declaration of your external function as such:

[DllImport("dork.dll")]
public static extern int CopyFunc([MarshalAs( UnmanagedType.LPStr )]string a, [MarshalAs( UnmanagedType.LPStr )] string b);

int GetFuncVal(string src, string dest)
{
    return(CopyFunc(dest,src));
}

answered Oct 11, 2016 at 22:23

BugsFree

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3 Comments

buzzard51 Over a year ago

That did work; however the DLL function modifies one of the char * parameters; the string 'dest' in the caller remains unmodified. How do I get this change to return to the C# caller? Most of the functions in the DLL modify passed buffers. What I will actually need to do is create a byte array with binary data, pass it to the DLL expecting a char *, and receive the modified byte array back from the DLL.

BugsFree Over a year ago

Try adding out keyword to the string dest

public static extern int CopyFunc([MarshalAs( UnmanagedType.LPStr )]string a, [MarshalAs( UnmanagedType.LPStr )] out string b);

or

public static extern int CopyFunc([MarshalAs( UnmanagedType.LPStr )]string a, [Out, MarshalAs( UnmanagedType.LPStr )] string b);

buzzard51 Over a year ago

Thanks for your answer! I ended up tinkering with this and got it all to work by using byte arrays instead of strings. I also had to change the UnmanagedType to LPArray. Now I have the bidirectional arrays functioning as expected.

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