Lỗi khi làm Windows app

Em click vào các button thì toàn bị như trong hình

namespace CTNC2
{
public partial class Form1 : Form
{
public System.Windows.Forms.TextBox tbtoantu1;
public System.Windows.Forms.TextBox tbtoantu2;
public System.Windows.Forms.TextBox tbketqua;

    public Form1()
    {
        InitializeComponent();
    }

    protected void Form1_Load(object sender, EventArgs e)
    {

    }

    protected void cong_Click(object sender, EventArgs e)
    {
        int toantu1 = Int32.Parse(tbtoantu1.Text);
        int toantu2 = Int32.Parse(tbtoantu1.Text);
        int ketqua = toantu1 + toantu2;
        tbketqua.Text = ketqua.ToString();
    }

    protected void tru_Click(object sender, EventArgs e)
    {
        int toantu1 = Int32.Parse(tbtoantu1.Text);    
        int toantu2 = Int32.Parse(tbtoantu2.Text);
        int ketqua = toantu1 - toantu2;
        tbketqua.Text = ketqua.ToString();
    }

    protected void nhan_Click(object sender, EventArgs e)
    {
        int toantu1 = Int32.Parse(tbtoantu1.Text);
        int toantu2 = Int32.Parse(tbtoantu1.Text);
        int ketqua = toantu1 * toantu2;
        tbketqua.Text = ketqua.ToString();
    }

    protected void chia_Click(object sender, EventArgs e)
    {
        int toantu1 = Int32.Parse(tbtoantu1.Text);
        int toantu2 = Int32.Parse(tbtoantu1.Text);
        if (toantu2 == 0)
        {
            tbketqua.Text = "ERROR";
        }
        else
        {
            double ketqua = (double)toantu1 / toantu2;
            tbketqua.Text = ketqua.ToString();
        }
        
    }

    private void thoat_Click(object sender, EventArgs e)
    {
        Application.Exit();
    }
}

}