Error: Toolkit requires ASP.NET Ajax 4.0 scripts. Ensure the correct version of the scripts are referenced. If you are using an ASP.NET ScriptManager, switch to the ToolkitScriptManager in AjaxControlToolkit.dll

Recently we have migrated one project from .NET 3.5  to 4.0 then the application which is using Ajax started throwing the error "Error: Toolkit requires ASP.NET Ajax 4.0 scripts. Ensure the correct version of the scripts are referenced. If you are using an ASP.NET ScriptManager, switch to the ToolkitScriptManager in AjaxControlToolkit.dll".

This was really frustrating one as we were using Ajax controls in many pages. Google search on this error has given me many solutions because many people were getting this error and solution was different for many people.

Here I am going to tell you how I was able to resolve my issue. It may or may not work for you but this is one of the solution. So it would be nice to check this option first.

Normal code you will be having in your 3.5 application will be like below,

<asp:ScriptManager ID="ScriptManager1" runat="server"></asp:ScriptManager>

You may change above code like below,

<asp:ToolkitScriptManager ID="ToolkitScriptManager" runat="server">
</asp:ToolkitScriptManager>


Let me know your feed back after trying this option.