namespace MyNameSpace
{
   public class MyClass
   {
       private string mFileName = String.Empty;
       private System.Text.StringBuilder mContents = null;

       public MyClass() : base()
       {
           mContents = new System.Text.StringBuilder();
       }

       public string FileName
       {
           get {
               return mFileName;
           }
           set {
               mFileName = value;
           }
       }

       public string Contents
       {
           get {
               return mContents.ToString();
           }
           set {
               mContents = new System.Text.StringBuilder(value);
           }
       }

       public void SaveFile()
       {
           StreamWriter sw = System.IO.File.CreateText(this.FileName);

           try {
               sw.Write(this.Contents);
           } finally {
               // Cleanup.
               sw.Close();
           }
       }

       public overloads void AddText(string Text)
       {
           mContents.Append(Text);
       }

       public overloads void AddText(string Format, params object[] Args)
       {
           mContents.AppendFormat(Format, Args);
       }

       public void AddSignature()
       {
           this.AddText("Dave Colliver");
       }
   }
}

To use:

MyClass mc = new MyClass();
mc.FileName = "C:\\dave.txt";
mc.AddText("Learning to write a class file.");
mc.AddSignature();
mc.SaveFile();

Using the above code (off top of head), the file itself is not opened until
the SaveFile method is called.  This prevents the file from being locked or
having an open handle when you don't really need it (not that it would be
locked as the file it is being "Created", but you should get the idea).

HTH,
Mythran