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-preview5-011568 and SDK 3.0.100-preview6-012264.

Also I have installed the Blazor extension but when I am building without any modification in any files, the build fails with the following error:

"The type or namespace "App" could not be found.

Any help will be highly appreciated. Thanks.

Answer 1

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 2

<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 3

For me, Blazor app was building locally, but not in Azure pipeline.

Adding App.razor.cs solved the build problem.

using Microsoft.AspNetCore.Components;

namespace BlazorDemo.Client
{
    public partial class App : ComponentBase
    {
    }
}

Answer 4

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. :)