下拉列表值：从字符串转换为整数，然后再转换回字符串并显示在文本框中

Question

I have multiple dropdown lists. I'm trying to have dropdown list input values to be added together and that the answer be displayed in a textbox. I'm converting the values into integers for the arithmetic operations and then back into strings to display the answer in the textbox.

It builds without errors in Visual Studio and when loaded in the browser. But when you select one dropdownlist an error will occur: "Input string was not in the correct format." For the life of me I can't find the error with the code:

displayRed is the Textbox i want the answer to be displayed in Red1DD, Red2DD etc. are the dropdownlists.

displayRed.Text =
  (Convert.ToInt32(Red1DD.SelectedValue.ToString())
 + Convert.ToInt32(Red2DD.SelectedValue.ToString())
 + Convert.ToInt32(Red4DD.SelectedValue.ToString())
 + Convert.ToInt32(Red8DD.SelectedValue.ToString())
 + Convert.ToInt32(Red16DD.SelectedValue.ToString())
 + Convert.ToInt32(Red32DD.SelectedValue.ToString())
 + Convert.ToInt32(Red64DD.SelectedValue.ToString())
 + Convert.ToInt32(Red128DD.SelectedValue.ToString())).ToString();

this is what one of the dropdownlists look like:

<asp:DropDownList ID="Red1DD" runat="server" AutoPostBack="True" OnSelectedIndexChanged="red_SelectedIndexChanged">
        <asp:ListItem Selected="True" Value="" Text="--" />
        <asp:ListItem value="0" Text="0" />
        <asp:ListItem value="1" Text="1" />
</asp:DropDownList>

Answer 1

you should use Red1DD.SelectedItem.Text; instead of SelectedValue.ToString()

displayRed.Text =
  (Convert.ToInt32(Red1DD.SelectedItem.Text)
 + Convert.ToInt32(Red2DD.SelectedItem.Text)
 + Convert.ToInt32(Red4DD.SelectedItem.Text)
 + Convert.ToInt32(Red8DD.SelectedItem.Text)
 + Convert.ToInt32(Red16DD.SelectedItem.Text)
 + Convert.ToInt32(Red32DD.SelectedItem.Text)
 + Convert.ToInt32(Red64DD.SelectedItem.Text)
 + Convert.ToInt32(Red128DD.SelectedItem.Text)).ToString();

Answer 2

Maybe the problem is, that you define a default value of empty string for each of your dropdowns. <asp:ListItem Selected="True" Value="" Text="--" />

Now when you change the selection of one of the dropdowns to 0 or 1, then the values of the other dropdowns are still empty strings. Convert.ToInt32(..) does not like an empty string as argument an throws an Exception.

Therfore change your default value to 0/1 or you have to handle empty strings in your "convert logic". For example Convert.ToInt32(string.IsNullOrEmpty(Red1DD.SelectedValue.ToString()) ? "0" : Red1DD.SelectedValue.ToString()); and so on.

(You can create a method, which checks if the string is empty and returns the parsed value or zero if emtpy.)