Using route constraints throws NullReferenceException when reloading multiple pages at the same time

[jaryn-kubik]

Describe the bug

I'm using custom reconnectionHandler in Blazor server-side to automatically reload a page when the connection is lost (like on server restart).

When I open two pages that have some route constraints (like @page "/{Id:int}") in two different tabs and do something that causes a server restart, both tabs reload at the same time and usually at least one fails with the following exception:

Error: System.NullReferenceException: Object reference not set to an instance of an object.
   at System.Collections.Generic.Dictionary`2.TryInsert(TKey key, TValue value, InsertionBehavior behavior)
   at System.Collections.Generic.Dictionary`2.set_Item(TKey key, TValue value)
   at Microsoft.AspNetCore.Components.Routing.RouteConstraint.Parse(String template, String segment, String constraint)
   at Microsoft.AspNetCore.Components.Routing.TemplateSegment.<>c__DisplayClass0_0.<.ctor>b__0(String token)
   at System.Linq.Enumerable.SelectListPartitionIterator`2.ToArray()
   at System.Linq.Enumerable.ToArray[TSource](IEnumerable`1 source)
   at Microsoft.AspNetCore.Components.Routing.TemplateSegment..ctor(String template, String segment, Boolean isParameter)
   at Microsoft.AspNetCore.Components.Routing.TemplateParser.ParseTemplate(String template)
   at Microsoft.AspNetCore.Components.RouteTableFactory.<>c.<Create>b__4_1(String v)
   at System.Linq.Enumerable.SelectArrayIterator`2.ToArray()
   at System.Linq.Enumerable.ToArray[TSource](IEnumerable`1 source)
   at Microsoft.AspNetCore.Components.RouteTableFactory.Create(Dictionary`2 templatesByHandler)
   at Microsoft.AspNetCore.Components.RouteTableFactory.Create(IEnumerable`1 componentTypes)
   at Microsoft.AspNetCore.Components.RouteTableFactory.Create(IEnumerable`1 assemblies)
   at Microsoft.AspNetCore.Components.Routing.Router.SetParametersAsync(ParameterView parameters)
   at Microsoft.AspNetCore.Components.Rendering.ComponentState.SetDirectParameters(ParameterView parameters)
   at Microsoft.AspNetCore.Components.RenderTree.RenderTreeDiffBuilder.InitializeNewComponentFrame(DiffContext& diffContext, Int32 frameIndex)
   at Microsoft.AspNetCore.Components.RenderTree.RenderTreeDiffBuilder.InitializeNewSubtree(DiffContext& diffContext, Int32 frameIndex)
   at Microsoft.AspNetCore.Components.RenderTree.RenderTreeDiffBuilder.InsertNewFrame(DiffContext& diffContext, Int32 newFrameIndex)
   at Microsoft.AspNetCore.Components.RenderTree.RenderTreeDiffBuilder.AppendDiffEntriesForRange(DiffContext& diffContext, Int32 oldStartIndex, Int32 oldEndIndexExcl, Int32 newStartIndex, Int32 newEndIndexExcl)
   at Microsoft.AspNetCore.Components.RenderTree.RenderTreeDiffBuilder.ComputeDiff(Renderer renderer, RenderBatchBuilder batchBuilder, Int32 componentId, ArrayRange`1 oldTree, ArrayRange`1 newTree)
   at Microsoft.AspNetCore.Components.Rendering.ComponentState.RenderIntoBatch(RenderBatchBuilder batchBuilder, RenderFragment renderFragment)
   at Microsoft.AspNetCore.Components.RenderTree.Renderer.RenderInExistingBatch(RenderQueueEntry renderQueueEntry)
   at Microsoft.AspNetCore.Components.RenderTree.Renderer.ProcessRenderQueue()

To Reproduce

1. download repo from https://github.com/jaryn-kubik/BlazorApp1 and launch the project
2. open up the two example links in two different tabs
3. do something that make VS rebuild the project (add something to a file and save it or whatever)
4. wait a few seconds till the tabs try to automatically reload
5. -> one of them should fail and throw an exception

There seems to be some race condition in the code handling constraints. If I remove the constraints and keep the route (@page "/{Id}" instead of @page "/{Id:int}") it works fine.

Further technical details

• ASP.NET Core 3.0
• Blazor server-side

[javiercn]

@jaryn-kubik thanks for contacting us and thanks for the repro project.

We'll take a look at it and get back to you.

[javiercn]

I can repro this.

The issue is likely that this needs to be a concurrent dictionary: https://github.com/aspnet/AspNetCore/blob/master/src/Components/Components/src/Routing/RouteConstraint.cs#L12

@mkArtakMSFT the fix should be simple, we should likely patch this.

[pranavkm]

@rynowak all yours