.NET 5 Razor build error with templated (generic) component with abstract generic base class.

[majorimi]

I'm developing some components which have lots of common code. I removed code duplication by created abstract base classes which can be inherited in Blazor code behind files. It works fine with non generic components. Code looks like this:

Abstract base class in MyComponentBase.cs:

public abstract class MyComponentBase : ComponentBase
{ ... }

Razor component Code behind file Component1.razor.cs:

public partial class Component1 : MyComponentBase
{ ... }

Razor component file Component1.razor:

@inherits MyComponentBase

However there is a build error with generic @typeparam components. But both class (generated and codebehind) has the same generic base types:

Severity	Code	Description	Project	File	Line	Suppression State
Error	CS0263	Partial declarations of 'TypeaheadInput<TItem>' must not specify different base classes	Blazor.Components.Typeahead	D:\Projects\GitHub\blazor-components\src\Blazor.Components.Typeahead\obj\Debug\net5.0\RazorDeclaration\TypeaheadInput.razor.g.cs	78	Active

Generic code is very similar to the above code:
Abstract generic base class in MyComponentBase.cs :

public abstract class MyComponentBase<TItem>  : ComponentBase
{ ... }

Razor component Code behind file Component1.razor.cs:

public partial class Component1<TItem> : MyComponentBase<TItem>
{ ... }

Razor component file Component1.razor`:

@typeparam TItem
@inherits MyComponentBase<TItem>

[mkArtakMSFT]

Thank you for filing this issue. In order for us to investigate this issue, please provide a minimalistic repro project (ideally a GitHub repo) that illustrates the problem.

[majorimi]

@mkArtakMSFT Ok no problem. I have created a repo with basically the code above. It was very suppressing. NO Build error... For a moment I went crazy but by comparing what can be the difference I have found 1 in the .csproj files.

<nullable>enable</nullable>

And BUILD ERROR. It seems nullable messing with generic base types in Blazor components. Here is the repo. It has only 2 commits first commit will build the second breaks it with Nullable.

[ghost]

Thanks for contacting us.
We're moving this issue to the Next sprint planning milestone for future evaluation / consideration. We will evaluate the request when we are planning the work for the next milestone. To learn more about what to expect next and how this issue will be handled you can read more about our triage process here.

[SteveSandersonMS]

Seems like this might be a duplicate of #26971

[hesuruio]

This issue has been plaguing our project for months. It means most of our solution can have nullable enabled but for a particular Blazor project which can't build because of this issue.

[Codinkat]

@hesuruio I have the exact same issue. The current workaround is to NOT specify the base class in the code-behind file:

public partial class Component1<TItem> // : MyComponentBase<TItem>
{ ... }

Specifying the base class in the code-behind file is not technically needed (because the inheritance is defined by @inherits in the razor file), but I used to specify it at both places for more clarity. So until this issue is fixed, I just comment it.

[ghost]

We've moved this issue to the Backlog milestone. This means that it is not going to be worked on for the coming release. We will reassess the backlog following the current release and consider this item at that time. To learn more about our issue management process and to have better expectation regarding different types of issues you can read our Triage Process.

[ghost]

We've moved this issue to the Backlog milestone. This means that it is not going to be worked on for the coming release. We will reassess the backlog following the current release and consider this item at that time. To learn more about our issue management process and to have better expectation regarding different types of issues you can read our Triage Process.

[jjonescz]

Cannot reproduce this in .NET 6 or newer, so I think it's fixed. (Tried that to see if it's fixed by dotnet/razor#8348).