The easiest way to get the toolkit is via NuGet.org . You can install specific older versions, such as 4.1.51116 , which was the primary release for .NET 4.0 and Visual Studio 2010.

For very specific historical builds (like the original 4.0 preview), you can find archived releases on GitHub via the CodePlex archive. How to Install and Reference the DLL

Finding a reliable is essential for developers maintaining legacy ASP.NET Web Forms applications. While the toolkit has evolved significantly over the years, specific versions targeting .NET Framework 4.0 remain necessary for older server environments. Official Download Options

It is highly recommended to use official repositories to ensure the security and integrity of the DLL file.