Calculator in c#

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)
         {         
         }
     }

Output