Example

         StringBuilder sb = new StringBuilder();
        //creating new string builder onject.
        private void button1_Click(object sender, EventArgs e)
        {
         sb.Append(textBox1.Text);
         //adding value to string builder
            textBox1.Text = "";
        } 
        private void button2_Click(object sender, EventArgs e)
        {
        MessageBox.Show("Capacity: " + sb.Capacity.ToString() + "\nValue: " + sb.ToString() + "\nLength: " + sb.Length.ToString());      
        //displaying capacity, length and value of stringbuilder     
     }

Screen shot