Resolving Namespace Issues After Upgrading to UI for ASP.NET AJAX 2026.3.812

Updated on Aug 27, 2026

Environment

Product	UI for ASP.NET AJAX
Version	2026.3.812

Description

After upgrading to UI for ASP.NET AJAX version 2026.3.812, namespace errors may occur in projects that reference Telerik Document Processing Libraries. This issue is caused by a breaking change introduced in version 2026.3.805, where the Windows prefix was removed from namespaces to provide a platform-agnostic convention.

For example:

• Telerik.Windows.Documents.Spreadsheet.Model changed to Telerik.Documents.Spreadsheet.Model.
• Telerik.Windows.Documents.Spreadsheet.FormatProviders.OpenXml.Xlsx changed to Telerik.Documents.Spreadsheet.FormatProviders.OpenXml.Xlsx.
• Telerik.Windows.Zip changed to Telerik.Zip.

The assembly and NuGet package names remain unchanged. For example, the assembly is still named Telerik.Windows.Documents.Spreadsheet.dll. This is why upgrade tools do not report errors, while existing using directives and type references in source code fail to resolve.

This knowledge base article also answers the following questions:

• Why am I seeing "namespace not found" errors after upgrading UI for ASP.NET AJAX?
• How do I update namespaces in my project after upgrading Telerik references?
• What changed in Telerik Document Processing Libraries namespaces?

Solution

To resolve namespace errors, update your source code as follows:

1. Open Find and Replace in Visual Studio by pressing Ctrl+H.
2. Set the scope to your entire solution.
3. Search for Telerik.Windows.Documents. and replace it with Telerik.Documents..
4. If applicable, search for Telerik.Windows.Zip and replace it with Telerik.Zip.
5. Rebuild the solution to confirm that the errors are resolved.

Example of updated using directives:

C#

using Telerik.Documents.Spreadsheet.FormatProviders;
using Telerik.Documents.Spreadsheet.FormatProviders.OpenXml.Xlsx;
using Telerik.Documents.Spreadsheet.Model;
using xlsx = Telerik.Documents.Spreadsheet.Model;

Also, check for the following references:

• Fully qualified type names, such as Telerik.Windows.Documents.Fixed.Model.RadFixedDocument, elsewhere in the code.
• String-based type references in XML configurations or reflection logic.

Refer to the Migrating to Platform-Agnostic Namespaces article for guidance on projects that use both .NET Framework and .NET Standard package variants simultaneously.

For examples of updated namespace usage, see the code-behind in the RadSpreadProcessing - Convert Documents demo.

See Also

• Telerik Document Processing Libraries Release Notes for Q3 2026
• Telerik Document Processing Libraries - Migrating to Platform-Agnostic Namespaces