Need help to work on dropdownlist when pagepostback

Question

I have created simple webform with two dropdownlists which get loaded with numbers on page load event. I have add, sub, mul, divide buttons to operate calculation and a label to display result.

When i select two numbers from dropdownlist and hit the buttons, I got 0 as result.

 

My Code:

protected void Page_Load(object sender, EventArgs e)

{

MyCalculationClient mc = new MyCalculationClient();

DropDownList1.DataSource = mc.DisplayNumbers();

DropDownList1.DataTextField = "Number";

DropDownList1.DataBind();

DropDownList2.DataSource = mc.DisplayNumbers();

DropDownList2.DataTextField = "Number";

DropDownList2.DataBind();

}

protected void Button1_Click(object sender, EventArgs e)

{

int a = Convert.ToInt32(DropDownList1.Text);

int b = Convert.ToInt32(DropDownList2.Text);

MyCalculationClient add = new MyCalculationClient();

Label1.Text = add.Add(a, b).ToString();

}