how to show the numbers in textbox in c# . it is show to me 11 does not show 2 why ??

Question

public partial class Form1 : Form
{
public Form1()
{
InitializeComponent();
}
double o1;
string one;
private void textBox1_TextChanged(object sender, EventArgs e)
{

}

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

private void button2_Click(object sender, EventArgs e)
{
double o1 = Convert.ToDouble(textBox1.Text);

textBox1.Text += "+";

}

private void button3_Click(object sender, EventArgs e)
{
string o2 = Convert.ToString(textBox1.Text);
textBox1.Clear();
String[] pr = new String[] { o2 };
if (pr[0].Contains("+")) {

pr[0] = pr[0].Replace("+", "");
double d = Convert.ToInt16(pr[0]);
double v = d + o1;
string x =Convert.ToString(v);

textBox1.Text += x;

}

Answer 1

Because it treats it as a string. 1 + 1 = 11. Hello + world = helloworld

Answer 2

i convert it to double but no work