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

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.

<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'.

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
    {
    }
}

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. :)

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.

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

Restart Visual Studio worked for me

In my case the issue was the namespace definition in _Host.cshtml. Renaming it to the correct namespace solved the issue.

For me adding the namespace with a using statement worked.

@using PieShopHrmClientWebAssembly.App.Shared

Here PieShopHrmClientWebAssembly.App is my Blazor project name.

In addition to @Jonno answer, if you copy razor files you need to change Build Action to Content in File Properties

Renaming my project from "App" to "GCL-App" worked for me, as it is throwing "circular dependency issue inclusive of the above error.

I had a similar error, and restarting Visual Studio 2019 fixed it for me.

This seems to be a generic message that the compilation has failed. In my case the Error List was not useful, but Build Output had more information (search for "error") but it only had a partial hint:

warning CS8785: Generator 'RazorSourceGenerator' failed to generate source. It will not contribute to the output and compilation errors may occur as a result. Exception was of type 'KeyNotFoundException'

It was still not enough, so I right-clicked the solution and chose "open in terminal" and there I executed this command manually:

dotnet build

And this one gave me some more information about the actual key not being found ("TableItem" in my case). This led me to some trial and error and I had some conflicting libraries (custom copy of BlazorTable and a new nuget version of MudBlazor) with similar component names.

I renamed <Table> to <BlazorTable>, and later to <BlaTable> 
(and ended up using <BlazorTable.BlaTable> to avoid repeating the namespace in class name)

Since I was still getting errors, I also removed all "@using BlazorTable" and instead included namespace in all tags, including attribute values in enum cases:

<BlazorTable.BlaTable TableItem="BillPlanListModel"
    Items=Rows 
    SelectionType=BlazorTable.SelectionType.Single>
        <BlazorTable.Column TableItem="BillPlanListModel" />
</BlazorTable.BlaTable>

This was a bit painful but it helped in my case after a few hours of work, and now I could upgrade MudBlazor to a new version. I was using .net 6 in VS2022, and got this error when updating MudBlazor from 6.02 to 6.03 (and later I was able to update MudBlazor to 6.0.10).