Selected Index Changed Event Problem in ASP.net

If you are not using AJAX Check that the DropDown List Should be enabled with postback=true for selected index changed event

Coming to me,
I had a problem of using Dropdown List With AJAX



After fighting for several hours.
I found that the following nodes are missing in web.config
check whether the following nodes are present in the web.config under <configuration> node

<runtime>
    <assemblyBinding xmlns="urn:schemas-microsoft-com:asm.v1">
      <dependentAssembly>
        <assemblyIdentity name="System.Web.Extensions" publicKeyToken="31bf3856ad364e35"/>
        <bindingRedirect oldVersion="1.0.0.0-1.1.0.0" newVersion="3.5.0.0"/>
      </dependentAssembly>
      <dependentAssembly>
        <assemblyIdentity name="System.Web.Extensions.Design" publicKeyToken="31bf3856ad364e35"/>
        <bindingRedirect oldVersion="1.0.0.0-1.1.0.0" newVersion="3.5.0.0"/>
      </dependentAssembly>
    </assemblyBinding>
  </runtime>

After adding these nodes it worked smoothly....
But it has killed 5-6 hours of time..