textBox1 is the textbox on which we should use backspace funtionality.
ar is an arrarlist. 

First declare
Arrayalist ar = new ArrayList();

In backspace button click add this piece of code

 if (textBox1.Text != "0.")
            {
               
                if (ar.Count>1)
                {
                    ar.RemoveAt(ar.Count - 1);

                    string strText = "";
                    for (int i = 0; i < ar.Count; i++)
                    {
                        strText = strText + ar[i].ToString();
                    }
                    textBox1.Text = strText;
                    return;
                }
                else
                {
                    ar.Clear();
                   textBox1.Text= "0.";
                }
                return;
                
            }
            else
            {
                ar.Clear();
                textBox1.Text = "0.";
                return;
            }