Aspnetcore: Async errors and warnings in Razor Pages @functions blocks

[SoftCircuits]

Because the @helper directive is no longer supported in ASP.NET Core Razor Pages, I've been using the @functions directive instead.

@functions
{
    void RenderTask(Models.Task task)
    {
        <tr>
            <td class="@Model.CssClass">
                <p class="compact">
                    <span class="font-weight-bold">@task.Title</span>
                    @if (!string.IsNullOrWhiteSpace(task.Description))
                    {
                        <br />@task.Description
                    }
                </p>
            </td>
            <td class="@Model.CssClass">
                <img src="~/images/Edit.png" class="edit-area button-img" data-id="@task.Id" title="Edit" />
                <img src="~/images/Delete.png" class="delete-area button-img" data-id="@task.Id" title="Delete" />
            </td>
        </tr>
    }
}

However, this gives me an error:

Error MVC1006: The method contains a TagHelper and therefore must be async and return a Task. For instance, usage of ~/ typically results in a TagHelper and requires an async Task returning parent method.

So I changed this function to be async, and I used the await keyword every place it is called.

@functions
{
    async System.Threading.Tasks.Task RenderTask(Models.Task task)
    {
        <tr>
            <td class="@Model.CssClass">
                <p class="compact">
                    <span class="font-weight-bold">@task.Title</span>
                    @if (!string.IsNullOrWhiteSpace(task.Description))
                    {
                        <br />@task.Description
                    }
                </p>
            </td>
            <td class="@Model.CssClass">
                <img src="~/images/Edit.png" class="edit-area button-img" data-id="@task.Id" title="Edit" />
                <img src="~/images/Delete.png" class="delete-area button-img" data-id="@task.Id" title="Delete" />
            </td>
        </tr>
    }
}

This actually works. But I get several warnings:

...\Razor\Pages\Tasks\Index.cshtml.g.cs(286,200,286,202): warning CS1998: This async method lacks 'await' operators and will run synchronously. Consider using the 'await' operator to await non-blocking API calls, or 'await Task.Run(...)' to do CPU-bound work on a background thread.
...\Razor\Pages\Tasks\Index.cshtml.g.cs(312,200,312,202): warning CS1998: This async method lacks 'await' operators and will run synchronously. Consider using the 'await' operator to await non-blocking API calls, or 'await Task.Run(...)' to do CPU-bound work on a background thread.

Index.cshtml.g.cs appears to be some sort of intermediary file. But I don't know what the numbers are that follow it, and double clicking these warnings does not take me to the offending line.

At this point, I'm not sure what the problem is. I've Googled extensively but haven't found a good example of this that shows what I'm supposed to be doing. Unless there is some special wave of the hands needed here, this appears to be a bug.

Note: I submitted a similar issue previously, but it was moved and closed and I didn't get the notifications at the new location.

[mkArtakMSFT]

Thanks for contacting us.
@NTaylorMullen can you please look into this? Thanks!

[NTaylorMullen]

Ah nice report @SoftCircuits. I was able to reproduce this.

Reason that this is causing issues is because we treat the img tag in the above example as a TagHelper because it has a ~/ attribute however everything in the @functions block doesn't get treated the same as something in the body. For instance in the body of the Razor page we have these nice little pragma warning disable/restores:

C# #pragma warning disable 1998 public async override global::System.Threading.Tasks.Task ExecuteAsync() { ...... } #pragma warning restore 1998

We need to add these to @code / @functions blocks.

[SoftCircuits]

Thanks @NTaylorMullen . Glad to know it wasn't just me.

[MartinZikmund]

In my case I can't avoid this error:

@page "{code?}"
@using Microsoft.Net.Http.Headers
@model ErrorModel
@{
    ViewData["Title"] = LocalizedStrings.Error;
}

@{
    switch (Model.Code)
    {
        case 404:
            {
                await NotFoundHandlerAsync();
                break;
            }
        default:
            {
                await DefaultHandlerAsync();
                break;
            }
    }
}

@functions{
    async Task NotFoundHandlerAsync()
    {
        <section class="error-page text-center">
            <h1 class="display-4">@LocalizedStrings.Error404_Title</h1>
            <p>@LocalizedStrings.Error404_Description</p>

            @{
                await NextStepsAsync();
            }
        </section>
    }

    async Task DefaultHandlerAsync()
    {
        <section class="error-page text-center">
            <h1 class="display-4">@LocalizedStrings.Error500_Title</h1>
            <p>@LocalizedStrings.Error500_Description</p>

            @{
                await NextStepsAsync();
            }
        </section>
    }

    async Task NextStepsAsync()
    {
        <a onclick="window.history.back()" href="#" class="btn btn-lg btn-primary"><span class="icon-back"></span> Go back</a>
        <a asp-page="Contact" class="btn btn-lg btn-info"><span class="icon-contacts"></span> Contact me</a>
    }
}

I get a compile-time error that NextStepsAsync contains a TagHelper and must be async and return a Task - which is the case. The code compiled fine before updating to ASP.NET Core 3.1.

[geoffappleford]

This problem is affecting me too.

I can reproduce in a brand new Asp.net core app (eg File, New Aspnet Core in VS) and enabling nullable reference types. eg csproj looks like:

<Project Sdk="Microsoft.NET.Sdk.Web">
  <PropertyGroup>
    <TargetFramework>netcoreapp3.1</TargetFramework>
    <Nullable>enable</Nullable>
  </PropertyGroup>
</Project>

Without enabling nullable, everything works as expected.

It also compiles fine (using dotnet build) when using sdk version 3.1.100 but fails in 3.1.200.

Visual Studio always compiles just fine but I don't know if it's using the sdk version specified in global.json.

[mkArtakMSFT]

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.

[spaasis]

I get the same error, but it persists even if I add the async/task definitions:

@functions{
    async Task RenderCompany(NotificationCompanyViewModel? vm) {
        <div>
            <dl>
                @if (vm != null) {
                   //snip
                    <dt><a href="~/Companies/CreateOrEdit/@vm.CompanyId">@vm.CompanyName</a></dt>
                }
            </dl>
        </div>
    }
}

Produces an error:
The method contains a TagHelper and therefore must be async and return a Task. For instance, usage of ~/ typically results in a TagHelper and requires an async Task returning parent method.

[PhilipF5]

This issue appears to still be present in 3.1.4. However, I was able to work around it fairly easily by adding #nullable disable and #nullable restore directives around my function definitions, like so:

@{
    #nullable disable
    async Task MyFunction()
    {
        // ...
    }
    #nullable restore
}

[spaasis]

@PhilipF5 good find, seems to fix my case as well