UsingRegistery.cs

 private void Form1_FormClosing(object sender, FormClosingEventArgs e)
        {
            //initializing width, height,
            int width, height;

            //setting values to form's current width & height.
            width = this.Width;
            height = this.Height;

            //accessing & modifying registery at SOFTWARE
            //if registry exists it will modify it, if doesn't exist
            //then it will be created thats why w are using 'true' statement.
            RegistryKey software = Registry.LocalMachine.OpenSubKey("SOFTWARE", true);
            //creating & modifying subregistery at RegDotNet of SOFTWARE
            RegistryKey RegEx = software.CreateSubKey("RegDotNet");
            //creating & modifying subregistery at Size of RegDotNet
            RegistryKey SizeKey = RegEx.CreateSubKey("Size");

            //storing values of width & height into Registry.
            SizeKey.SetValue("width",width.ToString());
            SizeKey.SetValue("height", height.ToString());
        }
    }
}