NullReferenceExceeption when making async call, but not when making sync call

user_706346 0
Viewed 235

Blazor server-side app, .NET5, EF Core 5.0.7

I have a Blazor component that injects an application database context, and then tries to load some data when the component initialises. This code is in a .razor.cs partial class file...

    [Inject]
    private ApplicationDbContext Context { get; set; }

    private List<User> _feeEarners = new();

    protected override async Task OnInitializedAsync() =>
      _feeEarners = await Context.Users.ToListAsync();

I've done this loads and loads of times, and it's always worked fine, but for some reason, the line of code above throws a NRE.

I've put a breakpoint inside OnInitializedAsync and checked in the debugger, and Context is not null, and I can expand the Users property and see the results, so I can't see what could be null here.

If I change that line of code so it's not async...

_feeEarners = Context.Users.ToList();

...then it works fine.

Anyone any idea what's going on here? As I said, I have done this loads of times, and it's always worked before.

Thanks

P.S. As often happens with Blazor, the stack trace is extremely unhelpful...

MySite.Web.Areas.Cases.Pages.Note.BuildRenderTree(RenderTreeBuilder __builder)
Microsoft.AspNetCore.Components.Rendering.ComponentState.RenderIntoBatch(RenderBatchBuilder batchBuilder, RenderFragment renderFragment)
Microsoft.AspNetCore.Components.RenderTree.Renderer.RenderInExistingBatch(RenderQueueEntry renderQueueEntry)
Microsoft.AspNetCore.Components.RenderTree.Renderer.ProcessRenderQueue()
Microsoft.AspNetCore.Components.Rendering.HtmlRenderer.HandleException(Exception exception)
Microsoft.AspNetCore.Components.RenderTree.Renderer.ProcessRenderQueue()
Microsoft.AspNetCore.Components.RenderTree.Renderer.ProcessPendingRender()
Microsoft.AspNetCore.Components.RenderTree.Renderer.AddToRenderQueue(int componentId, RenderFragment renderFragment)
Microsoft.AspNetCore.Components.ComponentBase.StateHasChanged()
Microsoft.AspNetCore.Components.ComponentBase.CallOnParametersSetAsync()
Microsoft.AspNetCore.Components.ComponentBase.RunInitAndSetParametersAsync()
Microsoft.AspNetCore.Components.Rendering.HtmlRenderer.HandleException(Exception exception)
Microsoft.AspNetCore.Components.RenderTree.Renderer.RenderRootComponentAsync(int componentId, ParameterView initialParameters)
Microsoft.AspNetCore.Components.Rendering.HtmlRenderer.CreateInitialRenderAsync(Type componentType, ParameterView initialParameters)
Microsoft.AspNetCore.Components.Rendering.HtmlRenderer.RenderComponentAsync(Type componentType, ParameterView initialParameters)
Microsoft.AspNetCore.Components.Rendering.RendererSynchronizationContext+<>c__11<TResult>+<<InvokeAsync>b__11_0>d.MoveNext()
Microsoft.AspNetCore.Mvc.ViewFeatures.StaticComponentRenderer.PrerenderComponentAsync(ParameterView parameters, HttpContext httpContext, Type componentType)
Microsoft.AspNetCore.Mvc.ViewFeatures.ComponentRenderer.PrerenderedServerComponentAsync(HttpContext context, ServerComponentInvocationSequence invocationId, Type type, ParameterView parametersCollection)
Microsoft.AspNetCore.Mvc.ViewFeatures.ComponentRenderer.RenderComponentAsync(ViewContext viewContext, Type componentType, RenderMode renderMode, object parameters)
Microsoft.AspNetCore.Mvc.TagHelpers.ComponentTagHelper.ProcessAsync(TagHelperContext context, TagHelperOutput output)
Microsoft.AspNetCore.Razor.Runtime.TagHelpers.TagHelperRunner.<RunAsync>g__Awaited|0_0(Task task, TagHelperExecutionContext executionContext, int i, int count)
MySite.Web.Areas.General.Pages.Areas_General_Pages__Host.<ExecuteAsync>b__14_1() in _Host.cshtml
+
    <component type="typeof(App)" render-mode="ServerPrerendered" />
Microsoft.AspNetCore.Razor.Runtime.TagHelpers.TagHelperExecutionContext.SetOutputContentAsync()
MySite.Web.Areas.General.Pages.Areas_General_Pages__Host.ExecuteAsync() in _Host.cshtml
+
  Layout = null;
Microsoft.AspNetCore.Mvc.Razor.RazorView.RenderPageCoreAsync(IRazorPage page, ViewContext context)
Microsoft.AspNetCore.Mvc.Razor.RazorView.RenderPageAsync(IRazorPage page, ViewContext context, bool invokeViewStarts)
Microsoft.AspNetCore.Mvc.Razor.RazorView.RenderAsync(ViewContext context)
Microsoft.AspNetCore.Mvc.ViewFeatures.ViewExecutor.ExecuteAsync(ViewContext viewContext, string contentType, Nullable<int> statusCode)
Microsoft.AspNetCore.Mvc.ViewFeatures.ViewExecutor.ExecuteAsync(ViewContext viewContext, string contentType, Nullable<int> statusCode)
Microsoft.AspNetCore.Mvc.Infrastructure.ResourceInvoker.<InvokeNextResultFilterAsync>g__Awaited|29_0<TFilter, TFilterAsync>(ResourceInvoker invoker, Task lastTask, State next, Scope scope, object state, bool isCompleted)
Microsoft.AspNetCore.Mvc.Infrastructure.ResourceInvoker.Rethrow(ResultExecutedContextSealed context)
Microsoft.AspNetCore.Mvc.Infrastructure.ResourceInvoker.ResultNext<TFilter, TFilterAsync>(ref State next, ref Scope scope, ref object state, ref bool isCompleted)
Microsoft.AspNetCore.Mvc.Infrastructure.ResourceInvoker.InvokeResultFilters()
Microsoft.AspNetCore.Mvc.Infrastructure.ResourceInvoker.<InvokeNextResourceFilter>g__Awaited|24_0(ResourceInvoker invoker, Task lastTask, State next, Scope scope, object state, bool isCompleted)
Microsoft.AspNetCore.Mvc.Infrastructure.ResourceInvoker.Rethrow(ResourceExecutedContextSealed context)
Microsoft.AspNetCore.Mvc.Infrastructure.ResourceInvoker.Next(ref State next, ref Scope scope, ref object state, ref bool isCompleted)
Microsoft.AspNetCore.Mvc.Infrastructure.ResourceInvoker.InvokeFilterPipelineAsync()
Microsoft.AspNetCore.Mvc.Infrastructure.ResourceInvoker.<InvokeAsync>g__Awaited|17_0(ResourceInvoker invoker, Task task, IDisposable scope)
Microsoft.AspNetCore.Routing.EndpointMiddleware.<Invoke>g__AwaitRequestTask|6_0(Endpoint endpoint, Task requestTask, ILogger logger)
Microsoft.AspNetCore.Authorization.AuthorizationMiddleware.Invoke(HttpContext context)
Microsoft.AspNetCore.Authentication.AuthenticationMiddleware.Invoke(HttpContext context)
Microsoft.AspNetCore.Diagnostics.DeveloperExceptionPageMiddleware.Invoke(HttpContext context)
entity-framework-core blazor-server-side blazor c#
1 Answers

This is not really an answer, but there's no other place for a long comment with code.

The primary difference between:

protected override async Task OnInitializedAsync() 
   => _feeEarners = await Context.Users.ToListAsync();

and

protected override Task OnInitializedAsync() 
   {
_feeEarners = Context.Users.ToList();
return Task.CompletedTask;
}

Is that in the async version there is likely to be a yield before _feeEarners is actually populated, and thus an initial render of the component when _feeEarners is empty. The problem is therefore likely to be in the UI code - the code we don't see in the question.

Here's the relevant ComponentBase code for reference. OnInitializedAsync is called and the returned task is assigned to a local variable. If it yields, StateHasChanged is called and it's awaited. The Renderer gets thread time to run its queue and render the component.

        private async Task RunInitAndSetParametersAsync()
        {
           OnInitialized();
            var task = OnInitializedAsync();

            if (task.Status != TaskStatus.RanToCompletion && task.Status != TaskStatus.Canceled)
            {
                // Call state has changed here so that we render after the sync part of OnInitAsync has run
                // and wait for it to finish before we continue. If no async work has been done yet, we want
                // to defer calling StateHasChanged up until the first bit of async code happens or until
                // the end. Additionally, we want to avoid calling StateHasChanged if no
                // async work is to be performed.
                StateHasChanged();

                try
                {
                    await task;
                }
                catch // avoiding exception filters for AOT runtime support
                {
                    // Ignore exceptions from task cancellations.
                    // Awaiting a canceled task may produce either an OperationCanceledException (if produced as a consequence of
                    // CancellationToken.ThrowIfCancellationRequested()) or a TaskCanceledException (produced as a consequence of awaiting Task.FromCanceled).
                    // It's much easier to check the state of the Task (i.e. Task.IsCanceled) rather than catch two distinct exceptions.
                     if (!task.IsCanceled)
                     {
                        throw;
                     }
                }

                // Don't call StateHasChanged here. CallOnParametersSetAsync should handle that for us.
            }

            await CallOnParametersSetAsync();
        }
user_130657810
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