Using DLLImport with an output char[] containing null characters

Question

I am writing a c# app using .NET 2.0. I need to use an old library for proprietary compression. I don't have source code for the library, and the developers behind it are long gone.

My problem is that the resulting char[] contains nulls, and is being truncated. Here is the declaration for the function:

[DLLImport("foo.dll")]
public static extern bool CompressString(char[] inputValue, out char[] outputValue, uint inputLength, out uint outputLength);

How can I declare that the output char[] should be handled as a byte[], and not null terminated?

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More info:

I do have the header file. Here is the declaration:

BOOL CompressString(char *DecompBuff, char **RetBuff, unsigned long DecompLen, unsigned long *RetCompLen);

Answer 1

Have a look at the MSDN article for passing arrays in P/Invoke. I think that you may want to use the SizeParamIndex to tell the marshaller which argument holds the size of the array passed along.

Edit: SizeParamIndex is unfortunately not allowed on out and ref parameters. You could, however, copy it manually:

[DLLImport("foo.dll")]
public static extern bool CompressString([MarshalAs(UnmanagedType.LPArray, SizeParamIndex=2)] char[] inputValue, out IntPtr outputValue, uint inputLength, out uint outputLength);

public static bool CompressStringInvoke(char[] inputValue, out char[] outputValue, uint inputLength) {
 IntPtr outputPtr;
 uint outputLen;
 if (CompressString(inputValue, out outputPtr, inputLength, out outputLen)) {
  outputValue = new char[outputLen];
  Marshal.Copy(outputPtr, outputValue, 0, (int)outputLen);
  return true;
 }
              outputValue = new char[0];
 return false;
}

Answer 2

Pinvoke.Net should provide you some hints on declaring your method signature