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

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  kevinfarrell...

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

  Jul 03, 2018 08:08 AM|kevinfarrell2k|LINK

  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 

• • Reply
  Brando ZWZ

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

  Jul 04, 2018 03:21 AM|Brando ZWZ|LINK

  Hi kevinfarrell2k,

  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

• • Reply
  kevinfarrell...

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

  Jul 04, 2018 08:16 AM|kevinfarrell2k|LINK

  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.

• • Reply
  mudassarkhan

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

  Jul 04, 2018 06:16 PM|mudassarkhan|LINK

  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;
          }

  .Net Consultant | MVP[2009-2013] | MCC[2012]
  Blog: ASPSnippets | Forum: ASPForums | Company: Excelasoft