C# can not write to file ("being used by another process")?

Question

I was hoping someone could explain to me why this is happening :)

When I use the code below it gives me the error "The process cannot access the file 'C:\test.txt' because it is being used by another process."

I'm very new to C# so I'm not sure what is going on, thanks in advance!

String fileNameBefore = @"C:\\test.txt";

public void output(String hex)
{
    using (StreamWriter writer = new StreamWriter(fileNameBefore, true))
    {
        writer.Write(hex);
        writer.Close();
    }
}

Answer 1

Close the text file before you write to it. (Not in code...physically close the text file)