Aspnetcore: Blazor code-behind class inheriting OwningComponentBase won't build

[stoyandimov]

Describe the bug

When a code behind class of a Blazor component inherits from OwningComponentBase
a Blazor web assembly project will not build.

Microsoft (R) Build Engine version 16.5.0+d4cbfca49 for .NET Core
Copyright (C) Microsoft Corporation. All rights reserved.

  Restore completed in 32.95 ms for /tmp/Website/Website.csproj.
obj/Debug/netstandard2.1/RazorDeclaration/Pages/Index.razor.g.cs(73,26): error CS0263: Partial declarations of 'Index' must not specify different base classes [/tmp/Website/Website.csproj]
obj/Debug/netstandard2.1/RazorDeclaration/Pages/Index.razor.g.cs(76,33): error CS0115: 'Index.BuildRenderTree(RenderTreeBuilder)': no suitable method found to override [/tmp/Website/Website.csproj]

Build FAILED.

obj/Debug/netstandard2.1/RazorDeclaration/Pages/Index.razor.g.cs(73,26): error CS0263: Partial declarations of 'Index' must not specify different base classes [/tmp/Website/Website.csproj]
obj/Debug/netstandard2.1/RazorDeclaration/Pages/Index.razor.g.cs(76,33): error CS0115: 'Index.BuildRenderTree(RenderTreeBuilder)': no suitable method found to override [/tmp/Website/Website.csproj]
    0 Warning(s)
    2 Error(s)

To Reproduce

dotnet new blazorwasm --name Website
cd Website/Pages

```C#

Create a file /Pages/Index.razor.cs to host the partial code-behind class

using Microsoft.AspNetCore.Components;

namespace Website.Pages
{
// public partial class Index : ComponentBase { } // <-- works
public partial class Index : OwningComponentBase { } // <-- DOESN'T WORK
}

Building the project will fail with the errors mentioned in the `Describe the bug` section

### Further technical details
- ASP.NET Core version 3.1.3
- Include the output of `dotnet --info`

.NET Core SDK (reflecting any global.json):
Version: 3.1.201
Commit: b1768b4ae7

Runtime Environment:
OS Name: ubuntu
OS Version: 19.10
OS Platform: Linux
RID: ubuntu.19.10-x64
Base Path: /usr/share/dotnet/sdk/3.1.201/

Host (useful for support):
Version: 3.1.3
Commit: 4a9f85e9f8

.NET Core SDKs installed:
2.2.402 [/usr/share/dotnet/sdk]
3.0.103 [/usr/share/dotnet/sdk]
3.1.201 [/usr/share/dotnet/sdk]

.NET Core runtimes installed:
Microsoft.AspNetCore.All 2.2.8 [/usr/share/dotnet/shared/Microsoft.AspNetCore.All]
Microsoft.AspNetCore.App 2.2.8 [/usr/share/dotnet/shared/Microsoft.AspNetCore.App]
Microsoft.AspNetCore.App 3.0.3 [/usr/share/dotnet/shared/Microsoft.AspNetCore.App]
Microsoft.AspNetCore.App 3.1.3 [/usr/share/dotnet/shared/Microsoft.AspNetCore.App]
Microsoft.NETCore.App 2.2.8 [/usr/share/dotnet/shared/Microsoft.NETCore.App]
Microsoft.NETCore.App 3.0.3 [/usr/share/dotnet/shared/Microsoft.NETCore.App]
Microsoft.NETCore.App 3.1.3 [/usr/share/dotnet/shared/Microsoft.NETCore.App]
```

[javiercn]

@stoyandimov thanks for contacting us.

The definition of the generated class needs to match exactly the one in your partial. You might be missing @namespace or @inherits OwningComponentBase<YourComponent> in your page definition

[javiercn]

Closing for now since we have good coverage for this scenario and we know there are no functional issues in this area. If you get stuck with the given suggestions I would suggest you look at the generated code within the obj folder and work out the differences between the generated class and the partial class.