Sep 26, 2014 07:51 PM|A2H|LINK
Looks like your project is not having the reference of AjaxControlToolkit.dll . You can check the below mentioned steps to add the ajaxtoolkit controls to your solution.
You need to add AjaxControl Toolkit controls to your Visual Studio IDE Manually and then Drag and drop the controls to your page this will ensure that Ajax Controls are registerd Properly.
You can check the follow the below steps to Add the controls to Visual Studio IDE
For More Details you can get from this link. Take a look at the Tip no 3 in below link : Adding controls to VS IDE - Select the Show Me Option to see the
animated gif which shows all the steps
After this you have all toolkit controls available in your VSIDE Then just drag and drop any controls in your page.
If you havent installed the ajaxcontroltoolkit in your machine then you can use nugget to install the Ajax Control toolkit.
You should be able to find it by searching for "AJAX Control Toolkit" through NuGet, which may be the easier approach :
To install AjaxControl Toolkit through nugget you can check the below links