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I have a problem with calling a C DLL fom C#

The C function is (I don't have a c header or a good spec for this :( )

int knr12_read ( char *kn12, char *ik9, char *wok, char *wlc,
char *plz, char *ort, char *woz );

kn12 is a ref parameter

This is what I've tried in C#

 [return: MarshalAs(UnmanagedType.U4)]
 [DllImport("Knr12.dll", CallingConvention = CallingConvention.StdCall, EntryPoint = "knr12_read", CharSet = CharSet.Ansi)] 
unsafe public static extern int knr12_read(out IntPtr buffer, string ik9, string wok, string wlc, string plz, string ort, string woz);

int knr = knr12_read(out pBuffer, knrTemp, "11111", "", "98529", "Suhl", "1");

string data = Marshal.PtrToStringAnsi(pBuffer);

The returning int is always right, how it should be, but I have problems with the ref parameter pBuffer...

Also the sting type for the other variables is working...

When I use a ref,I always get an AccessViolation error knr12_read().In case I use out I get a pointer,but the String is always empty which can't be.I even tried out String as ref for char* but I get an AccessViolation error on knr12_read().

Please guide.

Aayush
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Mike
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2 Answers2

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StringBuilder is often a good type to use when P/Invoking to functions with string returning parameters:

static extern int knr12_read(StringBuilder kn12, ...)

You'll need to initialise the string builder before you call the function, something like:

StringBuilder outString = new StringBuilder(100);

You shouldn't need the 'unsafe', and unless the 'C' code holds onto the pointers for longer than the duration of the call, you shouldn't need to worry about pinning - the framework is doing that for you.

Here's a SO question which should help: Marshal "char *" in C#

Community
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Will Dean
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Probably you have not pinned the buffer. here is the example of how to pin the buffer data.

GCHandle pinnedRawData = GCHandle.Alloc(rawData,
        GCHandleType.Pinned);

Pinning the object makes sure that the pointer is valid cause .Net runtime can always reallocate the memory as and when it thinks fit.

Try it out and let me know if it helps you.

Murtuza Kabul
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  • It's probably worth adding that this is only necessary if the 'C' code is going to keep the pointers after it returns. – Will Dean Oct 19 '12 at 13:46