Cant use asp-for tag helpers with nullable model

[G1Vh]

Is there an existing issue for this?

• I have searched the existing issues

Describe the bug

After #37510 we can mark model as nullable like @model MyModel?, but then using asp-for tag helpers
<input asp-for="Id" type="hidden">
<label asp-for="Name" class="d-flex w-50"></label>
shows "Dereference of a possibly null reference" during view compilation.

When model not marked nullable and real passed model is null - inputs renders correctly:

<input type="hidden" data-val="true" data-val-required="The Id field is required." id="Id" name="Id" value="">
<label class="d-flex w-50" for="Name">TestName</label>

Probably problem is LabelTagHelper.ModelExpression and InputTagHelper.ModelExpression is not nullable but can really accept null (#5680 shows that Microsoft.AspNetCore.Mvc.TagHelpers not annotated)

Expected Behavior

View compiled without errors and renders correct inputs

Steps To Reproduce

1. Mark page model as nullable
2. Create input or label asp-for for any model field

Exceptions (if any)

Dereference of a possibly null reference

.NET Version

6.0.200

Anything else?

No response

[iamzm]

@G1Vh is there compile or runtime error,

[G1Vh]

Compile

[pranavkm]

This error makes sense - asp-for="Id" is equivalent to asp-for="Model.Id" where Model is nullable like you declared. Mvc handles traversing the tree in a null-safe way (or simply catches null-ref if it's too hard). Unfortunately because Model.Id is an ExpressionTree, you cannot use a null-coalescing operator e.g. Model?.Id since it's unsupported within expression trees. Using the null-forgiveness operator does work though, so consider using it: Model!.Id

[G1Vh]

This error makes sense - asp-for="Id" is equivalent to asp-for="Model.Id" where Model is nullable like you declared. Mvc handles traversing the tree in a null-safe way (or simply catches null-ref if it's too hard). Unfortunately because Model.Id is an ExpressionTree, you cannot use a null-coalescing operator e.g. Model?.Id since it's unsupported within expression trees. Using the null-forgiveness operator does work though, so consider using it: Model!.Id

As temporal workaround it works, yes.

But it still be wrong behavior, compiler should allow writing asp-for="Id" in nullable context

Thanks for contacting us.

We're moving this issue to the .NET 7 Planning milestone for future evaluation / consideration. We would like to keep this around to collect more feedback, which can help us with prioritizing this work. We will re-evaluate this issue, during our next planning meeting(s).
If we later determine, that the issue has no community involvement, or it's very rare and low-impact issue, we will close it - so that the team can focus on more important and high impact issues.
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]

cc @mkArtakMSFT @pranavkm for consideration for .NET 7