Visual Studio 2026 Web App

For Wisej 3.5 you need to target net48 to get the designer to work, However, that is no longer necessary in Wisej 4.0.
You can use ..NET 8 9 or 10 in visual studio 2026. Your TargetFrameworks in your .csproj should look like this: net9.0-windows;net9.0 The -windows target is necessary for the designer to work.

For resolving the “WisejHandler not found” error, try to create a brand new Wisej 4 .NET 8, 9 or 10 project and compare its Program.cs and csproj to your old project. If that doesn’t work, attach a small test case the reproduces the issue and we can take a look.

Hope this helps,
Julie