Question

I’m working with a 3rd party component that returns an IStream object (System.Runtime.InteropServices.ComTypes.IStream). I need to take the data in that IStream and write it to a file. I’ve managed to get that done, but I’m not really happy with the code.

With “strm” being my IStream, here’s my test code…

// access the structure containing statistical info about the stream
System.Runtime.InteropServices.ComTypes.STATSTG stat;
strm.Stat(out stat, 0);
System.IntPtr myPtr = (IntPtr)0;

// get the "cbSize" member from the stat structure
// this is the size (in bytes) of our stream.
int strmSize = (int)stat.cbSize; // *** DANGEROUS *** (long to int cast)
byte[] strmInfo = new byte[strmSize];
strm.Read(strmInfo, strmSize, myPtr);

string outFile = @"c:\test.db3";
File.WriteAllBytes(outFile, strmInfo);

At the very least, I don’t like the long to int cast as commented above, but I wonder if there’s not a better way to get the original stream length than the above? I’m somewhat new to C#, so thanks for any pointers.

Answer 1

You don’t need to do that cast, as you can read data from IStream source in chunks.

// ...
System.IntPtr myPtr = (IntPtr)-1;
using (FileStream fs = new FileStream(@"c:\test.db3", FileMode.OpenOrCreate))
{
    byte[] buffer = new byte[8192];
    while (myPtr.ToInt32() > 0)
    {
        strm.Read(buffer, buffer.Length, myPtr);
        fs.Write(buffer, 0, myPtr.ToInt32());
    }
}

This way (if works) is more memory efficient, as it just uses a small memory block to transfer data between that streams.