How do I prevent the ViewState of a dropdown throwing an error if no option is selected?

Question

User1158330421 posted

I tried put in try catch :

string userMeasurement = ViewState["DropDown"].ToString();

try

{

if (userMeasurement == null && userMeasurement == "")
{
showError("Please select a measurement type");
return;
}

}

catch(Exception)

{

showError(Please enter valid measurement;

return;

}

The first line with ViewState keeps throwing NullReferenceException even when I tried to wrap the String userMeasurement line in the try 

Answer 1

User283571144 posted

Hi kevinfarrell2k,

The first line with ViewState keeps throwing NullReferenceException even when I tried to wrap the String userMeasurement line in the try 

According to your description, I suggest you try to use as keywrod to aviod throwing the exception.

From MSDN article.

The as operator is like a cast operation. However, if the conversion isn't possible, asreturns null instead of raising an exception.

More details, you could refer to below demo codes:

ASPX:

        <div>
            <asp:DropDownList ID="DropDown" runat="server">
                <asp:ListItem Value="aaa" Text="aaa"></asp:ListItem>
                <asp:ListItem Value="bbb" Text="bbb"></asp:ListItem>
            </asp:DropDownList>
            <asp:Button ID="Button1" runat="server" Text="Button" OnClick="Button1_Click" />
         </div>

Code-behind:

using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;
using System.Web.UI;
using System.Web.UI.WebControls;

namespace AspNetNormalIssue.Webform
{
    public partial class DropDownListString : System.Web.UI.Page
    {
        protected void Page_Load(object sender, EventArgs e)
        {

        }

        protected void Button1_Click(object sender, EventArgs e)
        {
            try
            {
                string userMeasurement = ViewState["DropDown"] as string;
                if (userMeasurement == null && userMeasurement == "")
                {
                     return;
                }
            }
            catch (Exception)
            {
                return;

            }
        }
    }
}

Result:

Best Regards,

Brando

Answer 2

User1158330421 posted

Thanks Brando, but I get an error because the userMeasurement appears in a switch statement after and if I use this solution then userMeasurement isn't recognised because it doesn't exist in the current context.

Answer 3

User-1171043462 posted

You don't need any Try Catch. Just use Convert.ToString() instead of ToString() function

Convert.ToString() will not throw error even if the object is NULL.

        string userMeasurement = Convert.ToString(ViewState["DropDown"]);
        if (!string.IsNullOrEmpty(userMeasurement))
        {
            showError("Please select a measurement type");
            return;
        }