Create an new windwos forms project. Design Size. Add panel. design panel color,size,style. Add button. calculartor button and oper button. Coding button function. Declaring variables double fstNum,secNum; string oper; Identify button function btnCE private void btnCE_Click(object sender, EventArgs e) { textDisplay.Text = "0"; string f, s; f = Convert.ToString(fstNum); s = Convert.ToString(secNum); f = ""; s = ""; } backspace private void backspace_Click(object sender, EventArgs e) { if (textDisplay.Text.Length > 0) { textDisplay.Text = textDisplay.Text.Remove(textDisplay.Text.Length - 1, 1); } if (textDisplay.Text == "") { textDisplay.Text = "0"; } } btn9 private void btn9_Click(object sender, EventArgs e) { if (textDisplay.Text == "0") { textDisplay.Text = "9"; } else { textDisplay.Text = textDisplay.Text + "9"; } } btnAdd private void btnAdd_Click(object sender, EventArgs e) { fstNum = double.Parse(textDisplay.Text); oper = "+"; textDisplay.Text = ""; } btnPM private void btnPM_Click(object sender, EventArgs e) { double q = Convert.ToDouble(textDisplay.Text); textDisplay.Text = Convert.ToString(-1 * q); } btnC private void btnC_Click(object sender, EventArgs e) { textDisplay.Text = "0"; } benDec private void benDec_Click(object sender, EventArgs e) { if (benDec.Text == ".") { if (!textDisplay.Text.Contains(".")) { textDisplay.Text = textDisplay.Text + benDec.Text; } } } use switch statement out put equal private void btnEuq_Click(object sender, EventArgs e) { secNum = double.Parse(textDisplay.Text); switch (oper) { case "+": textDisplay.Text = (fstNum + secNum).ToString(); break; case "-": textDisplay.Text = (fstNum - secNum).ToString(); break; case "*": textDisplay.Text = (fstNum * secNum).ToString(); break; case "/": textDisplay.Text = (fstNum / secNum).ToString(); break; default: break; } } Code download：caluclator.zip Reference：