I change code like:

decimal rahn2 = decimal.Parse (Txtrahn2.Text);

but again error happen...

nedash says:

Txtrahn2.Text

 What the value you are passing in Txtrahn2.Text

dharmendr says:

 What the value you are passing in Txtrahn2.Text

 nothing below are codes:

<asp:UpdatePanel ID="UpdatePanel7" runat="server">
                                                <ContentTemplate>
                            <div id="AmFoD12" runat="server" >
 <asp:TextBox ID="Txtrahn2" runat="server" CssClass="vadi1"></asp:TextBox>
                                 <asp:TextBox ID="Txtrahn1" runat="server" CssClass="vadi2"></asp:TextBox>

                            </div>
                                                      </ContentTemplate>
                                   </asp:UpdatePanel>

Best regards

neda

nedash says:

if (!IsPostBack)

{ int rahn2 = int.Parse(Txtrahn2.Text);

    int rahn1 = int.Parse(Txtrahn1.Text);

}

Change above with the below code.

if (!IsPostBack)
{
    int rahn2 = int.Parse(string.IsNullOrEmpty(Txtrahn2.Text) ? "0" : Txtrahn2.Text);
    int rahn1 = int.Parse(string.IsNullOrEmpty(Txtrahn1.Text) ? "0" : Txtrahn1.Text);
}

OR

if (!IsPostBack)
{
    decimal rahn2 = decimal.Parse(string.IsNullOrEmpty(Txtrahn2.Text) ? "0" : Txtrahn2.Text);
    decimal rahn1 = decimal.Parse(string.IsNullOrEmpty(Txtrahn1.Text) ? "0" : Txtrahn1.Text);
}