Here i show you Button codes, variables, equal button codes and switch codes. Please see.

public partial class Form1 : Form
{
double a,b;
int i;
double ans;
public Form1()
{
InitializeComponent();
}

private void button1_Click(object sender, EventArgs e)
{
textBox1.Text = textBox1.Text + "1";
}

private void button2_Click(object sender, EventArgs e)
{
textBox1.Text = textBox1.Text + "2";
}

private void button3_Click(object sender, EventArgs e)
{
textBox1.Text = textBox1.Text + "3";
}

private void button4_Click(object sender, EventArgs e)
{
textBox1.Text = textBox1.Text + "4";
}

private void button5_Click(object sender, EventArgs e)
{
textBox1.Text = textBox1.Text + "5";
}

private void button6_Click(object sender, EventArgs e)
{
textBox1.Text = textBox1.Text + "6";
}

private void button7_Click(object sender, EventArgs e)
{
textBox1.Text = textBox1.Text + "7";
}

private void button8_Click(object sender, EventArgs e)
{
textBox1.Text = textBox1.Text + "8";
}

private void button9_Click(object sender, EventArgs e)
{
textBox1.Text = textBox1.Text + "9";
}

private void button11_Click(object sender, EventArgs e)
{
textBox1.Text = textBox1.Text + "0";
}

private void button19_Click(object sender, EventArgs e)
{
textBox1.Text = textBox1.Text + ".";
}

private void button16_Click(object sender, EventArgs e)
{
a = Convert.ToDouble(textBox1.Text);
textBox1.Text = "";
i = 1;
}

private void button12_Click(object sender, EventArgs e)
{
a = Convert.ToDouble(textBox1.Text);
textBox1.Text = "";
i = 2;
}

private void button13_Click(object sender, EventArgs e)
{
a = Convert.ToDouble(textBox1.Text);
textBox1.Text = "";
i = 3;
}

private void button14_Click(object sender, EventArgs e)
{
a = Convert.ToDouble(textBox1.Text);
textBox1.Text = "";
i = 4;
}

private void button15_Click(object sender, EventArgs e)
{


b = Convert.ToDouble(textBox1.Text);


switch (i)
{
case 1:
ans = a + b;
textBox1.Text = (""+ans);
break;
case 2:
ans = a - b;
textBox1.Text = ("" + ans);
break;
case 3:
ans = a * b;
textBox1.Text = ("" + ans);
break;
case 4:
ans = a / b;
textBox1.Text = ("" + ans);
break;
default:
MessageBox.Show("Error");
break;

}

}

private void button10_Click(object sender, EventArgs e)
{
textBox1.Text = "";
}
private void button20_Click(object sender, EventArgs e)
{
this.Close();
}

private void Form1_Load(object sender, EventArgs e)
{

}


}




And here is output