Blazor the type or namespace name 'App' could not be found (are you missing a using directive or an assembly reference?)

Question

I have setup the basic application in Blazor in Microsoft Visual Studio Community 2019 Version 16.1.3 and I have tried both of the versions of .NET Core SDK 3.0.100-preview5

Answer 1

<component type="typeof(App)" render-mode="ServerPrerendered" />

The error I got (using Visual Studio 2019 v16.8.2 & .NET 5 in a Blazor Server App) was pointing to the "App" part of the line above (in the file _Host.cshtml). This happened after copying in some code from another project, and encountering issues with a different (application) namespace, I got this error (and no others) when trying to build. @SᴇM's answer helped me solve the problem - I closed and re-opened my solution, and then got namespace errors when building - which I could resolve... after fixing that it worked.

I would have commented to @SᴇM's solution, but I don't have enough 'reputation'.

Answer 2

In my case the problem was that I had the following package reference in my csproj file:

<PackageReference Include="Microsoft.AspNetCore.Mvc" Version="2.2.0" />

Removing this fixed the issue.

The following output in my build.log file helped me track this down

1>/usr/local/share/dotnet/sdk/3.1.402/Sdks/Microsoft.NET.Sdk.Razor/build/netstandard2.0/Sdk.Razor.CurrentVersion.targets(404,5): warning RAZORSDK1006: Detected Razor language version downgrade. This is typically caused by a reference to the Microsoft.AspNetCore.Razor.Design package. Consider removing this package reference.

Answer 3

Addition to @SᴇM answer, please make sure you are using latest version of Visual studio. For my case, I followed the same steps mentioned by @SᴇM but still option Blazor App option not shown.

After that, I updated visual studio to 16.3.7 and now option is visible while creating new project. :)

Answer 4

To use Blazor you need to have VS2019 preview edition or enable preview features on VS2019 by checking

Tools -> Options -> Environment -> Preview Features -> Use previews of the .NET Core SDK

or on the older versions of VS2019

Tools -> Options -> Projects and Solutions -> .NET Core -> Use previews of the .NET Core SDK

checkbox, then reload the solution and build.

Answer 5

After migrating my .NET Core 3.1 application to .NET 5.0 I experienced the same problem. My application was working just fine but since I opened my App.razor file errors showed.

"the type or namespace name 'pageName' could not be found" and the pages show normally.

Fixed the problem with Tools -> Options -> Environment -> Preview Features -> Use previews of the .NET Core SDK. Restart of Visual Studio and reloading project was also needed.

Which is kind of weird since I just moved to .NET 5.0