C# Overwriting file with StreamWriter created from FileStream

Question

I need to manipulate the contents of a file:

 FileStream fs = new FileStream(filePath, FileMode.OpenOrCreate, FileAccess.ReadWrite, FileShare.None);               
 StreamReader sr = new StreamReader(fs);
 StreamWriter sw = new StreamWriter(fs);
 newString = someStringTransformation(sr.ReadToEnd());
 sw.Write(newString);
 fs.flush();
 fs.Close();

However the above appends the newString instead of overwriting the file with the new changes. It needs to be done so that no other application can access the file in between reading a writing which is why I'm creating the reader and writer from a FileStream object.

I know that you can create a StreanWriter with the second parameter set to false as described here. However when creating the StreamWriter as above that does not seem to be one of the parameters.

Answer 1

The problem you are having is that reading from the stream advances to the end of the file. Further writes will then append.

This will achieve a full overwrite.

using(FileStream fs = new FileStream(filePath, FileMode.OpenOrCreate, FileAccess.ReadWrite, FileShare.None))
{
    StreamReader sr = new StreamReader(fs);
    using (StreamWriter sw = new StreamWriter(fs))
    {
        newString = someStringTransformation(sr.ReadToEnd());

        // discard the contents of the file by setting the length to 0
        fs.SetLength(0); 

        // write the new content
        sw.Write(newString);
    }
}

Why use SetLength? Your new content might be shorter than the existing string! The last thing you want is the old content at the end of your file.