How to insert(append) data to a textfile that has data

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C#

Hi guys, i am currently developing a GUI application and part of this application allows a user to insert a line of text in to a textfile that is located in the C drive that exists but it is supposed to append the new line, but with the code that i have come up with, it does not allow me to have access to the file but the file is not read only. i dont even know if the code that i have come up with does what it is intended to do because of this access issues. here is my code...

    string newcity = txtCity.Text;
    int newfigure = int.Parse(txtSalefigure.Text);
    string filename = txtFileName.Text;

    StreamWriter file = null;
    if (File.Exists("C:/"+filename+".txt") == false)
    {
        file = new StreamWriter("C:/"+filename+".txt", true);
    }
    else
    {
        file = new StreamWriter("C:/"+filename+".txt");
    }
    file.WriteLine(newcity +"   "+newfigure+ "\n");
    file.Flush();
    file.Close();
}