Save string from RichTextBox

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C#

Hello, I have a Notepad made by me and it is like the real Notepad. If I save a text(with Save As) then I modify it and I press Save, an error appears: "The process cannot access the file because it is used by another process". The code from the Save button is this:

StreamWriter sw = new StreamWriter(loc);
     sw.Write(richTextBox1.Text);
     sw.Close();

Where loc is the text location.
What is wrong? Can you tell me the correct code?