FileStream and StreamWriter - How to truncate the remainder of the file after writing?

Question

var fs = new FileStream(filePath, FileMode.OpenOrCreate, FileAccess.ReadWrite);
using(var writer = new StreamWriter(fs))
    writer.Write(....);

If the file previously contained text and the newly-written text is shorter than what was already in the file, how do I make sure that the obsolete trailing content in the file is truncated?

Note that opening the file in truncate mode isn't an option in this case. The file is already open when I receive the FileStream object. The above code is just to illustrate the stream's properties.

EDIT

Expanding on the answer below, the solution is:

var fs = new FileStream(filePath, FileMode.OpenOrCreate, FileAccess.ReadWrite);
using(var writer = new StreamWriter(fs))
{
    writer.Write(....);
    writer.Flush();
    fs.SetLength(fs.Position);
}

Answer 1

Use SetLength to set the new length of the file - the file should get truncated.

See this answer to a related question.

Answer 2

you could try writer.BaseStream.SetLength(writer.BaseStream.Position) although I'm not sure how well that would work.

For a FileStream I think that should truncate the file to the current position.

Answer 3

This code will truncate part of the log file, if the file grows above 1 MB.

using (FileStream fs = File.Open("C:\\LogFile.txt", FileMode.OpenOrCreate))
        {
            int OneMB = 1000000;

            fs.Seek(0, SeekOrigin.Begin);
            if (fs.Length > OneMB)
            { 
                int fileByte = 1;
                fs.Position  = fs.Seek(fs.Length / 2, SeekOrigin.Begin);
                List<byte> bytes = new List<byte>();

                while (fileByte > 0)
                {
                   fileByte = fs.ReadByte();
                   bytes.Add((byte)fileByte);
                }

                fs.SetLength(0);
                fs.Position = 0;
                fs.Write(bytes.ToArray(), 0, bytes.Count());

                fs.Seek(0, SeekOrigin.End);
                var stringBytes =  UTF8Encoding.ASCII.GetBytes($"Test1" + Environment.NewLine);
                fs.Write(stringBytes, 0, stringBytes.Length);  
            }
            else
            {
                fs.Seek(0, SeekOrigin.End);
                var stringBytes = UTF8Encoding.ASCII.GetBytes($"Test2" + Environment.NewLine);
                fs.Write(stringBytes, 0, stringBytes.Length); 
            }

            fs.Flush();
        }
    }