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

  MSDN Community Support
  Please remember to click "Mark as Answer" the responses that resolved your issue.
  If you have any compliments or complaints to MSDN Support, feel free to contact MSDNFSF@microsoft.com.
  
• • 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.

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  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