How to access editmode listview control on radiobutton checked event?

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C#

ASP.NET

Hi Expert,

In listview(lstvwResultInquiry) EditTemplate there are many controls in edit Mode.

I took one radio button as follows with oncheckedchanged event

<asp:RadioButton ID="radioSource" runat="server" AutoPostBack="True"
                           GroupName="inquiryRadio" oncheckedchanged="radioSource_CheckedChanged" />

and there is one dropdownlist(lstEditSource) ,
eg.

<asp:DropDownList ID="lstEditSource" runat="server" Width="175px"  Height="23px">

I used following code on radioSource_CheckedChanged event to enable dropdownlist(<b>lstEditSource</b>)

if (lstvwResultInquiry.EditIndex > -1)
       {
           DropDownList ddlSource = (DropDownList)lstvwResultInquiry.EditItem.FindControl("lstEditSource");

           ddlSource.Enabled = true;
       }

but ddlSource shows null reference.

How to access these editmode listview control on radiobutton checked event?

Please help me.

Thanks in advance.