Blazor 'System.Reflection.AmbiguousMatchException' on .NET 8

[thanuja89]

Is there an existing issue for this?

• I have searched the existing issues

Describe the bug

When we update our blazor webassembly app, we noticed that we are getting "System.Reflection.AmbiguousMatchException: Ambiguous match found for" for properties if the code blocks (code-behind) contained two properties with the similar names which differs in case only (for eg: Model and model).

Expected Behavior

No error thould be thrown and the correct property should be used.

Steps To Reproduce

This is a demo of the bug. Go to counter page and you'll get an error.

https://github.com/thanuja89/BlazorBugRepo

Exceptions (if any)

System.Reflection.AmbiguousMatchException: Ambiguous match found for 'BlazorAppBug.Client.Pages.Counter ModelModel Model'.
at System.RuntimeType.GetField(String name, BindingFlags bindingAttr)
at System.Linq.Expressions.Expression.PropertyOrField(Expression expression, String propertyOrFieldName)
at Microsoft.AspNetCore.Components.Forms.FieldIdentifier.g__CreateAccessor|14_0(ValueTuple2 arg) at System.Collections.Concurrent.ConcurrentDictionary2[[System.ValueTuple2[[System.Type, System.Private.CoreLib, Version=8.0.0.0, Culture=neutral, PublicKeyToken=7cec85d7bea7798e],[System.String, System.Private.CoreLib, Version=8.0.0.0, Culture=neutral, PublicKeyToken=7cec85d7bea7798e]], System.Private.CoreLib, Version=8.0.0.0, Culture=neutral, PublicKeyToken=7cec85d7bea7798e],[System.Func2[[System.Object, System.Private.CoreLib, Version=8.0.0.0, Culture=neutral, PublicKeyToken=7cec85d7bea7798e],[System.Object, System.Private.CoreLib, Version=8.0.0.0, Culture=neutral, PublicKeyToken=7cec85d7bea7798e]], System.Private.CoreLib, Version=8.0.0.0, Culture=neutral, PublicKeyToken=7cec85d7bea7798e]].GetOrAdd(ValueTuple2 key, Func2 valueFactory)
at Microsoft.AspNetCore.Components.Forms.FieldIdentifier.GetModelFromMemberAccess(MemberExpression member, ConcurrentDictionary2 cache) at Microsoft.AspNetCore.Components.Forms.FieldIdentifier.ParseAccessor[String](Expression1 accessor, Object& model, String& fieldName)
at Microsoft.AspNetCore.Components.Forms.FieldIdentifier.Create[String](Expression1 accessor) at Microsoft.AspNetCore.Components.Forms.InputBase1[[System.String, System.Private.CoreLib, Version=8.0.0.0, Culture=neutral, PublicKeyToken=7cec85d7bea7798e]].SetParametersAsync(ParameterView parameters)
at Microsoft.AspNetCore.Components.Rendering.ComponentState.SupplyCombinedParameters(ParameterView directAndCascadingParameters)
crit: Microsoft.AspNetCore.Components.WebAssembly.Rendering.WebAssemblyRenderer[100]
Unhandled exception rendering component: Ambiguous match found for 'BlazorAppBug.Client.Pages.Counter ModelModel Model'.
System.Reflection.AmbiguousMatchException: Ambiguous match found for 'BlazorAppBug.Client.Pages.Counter ModelModel Model'.
at System.RuntimeType.GetField(String name, BindingFlags bindingAttr)
at System.Linq.Expressions.Expression.PropertyOrField(Expression expression, String propertyOrFieldName)
at Microsoft.AspNetCore.Components.Forms.FieldIdentifier.g__CreateAccessor|14_0(ValueTuple2 arg) at System.Collections.Concurrent.ConcurrentDictionary2[[System.ValueTuple2[[System.Type, System.Private.CoreLib, Version=8.0.0.0, Culture=neutral, PublicKeyToken=7cec85d7bea7798e],[System.String, System.Private.CoreLib, Version=8.0.0.0, Culture=neutral, PublicKeyToken=7cec85d7bea7798e]], System.Private.CoreLib, Version=8.0.0.0, Culture=neutral, PublicKeyToken=7cec85d7bea7798e],[System.Func2[[System.Object, System.Private.CoreLib, Version=8.0.0.0, Culture=neutral, PublicKeyToken=7cec85d7bea7798e],[System.Object, System.Private.CoreLib, Version=8.0.0.0, Culture=neutral, PublicKeyToken=7cec85d7bea7798e]], System.Private.CoreLib, Version=8.0.0.0, Culture=neutral, PublicKeyToken=7cec85d7bea7798e]].GetOrAdd(ValueTuple2 key, Func2 valueFactory)
at Microsoft.AspNetCore.Components.Forms.FieldIdentifier.GetModelFromMemberAccess(MemberExpression member, ConcurrentDictionary2 cache) at Microsoft.AspNetCore.Components.Forms.FieldIdentifier.ParseAccessor[String](Expression1 accessor, Object& model, String& fieldName)
at Microsoft.AspNetCore.Components.Forms.FieldIdentifier.Create[String](Expression1 accessor) at Microsoft.AspNetCore.Components.Forms.ValidationMessage1[[System.String, System.Private.CoreLib, Version=8.0.0.0, Culture=neutral, PublicKeyToken=7cec85d7bea7798e]].OnParametersSet()
at Microsoft.AspNetCore.Components.ComponentBase.CallOnParametersSetAsync()
at Microsoft.AspNetCore.Components.ComponentBase.RunInitAndSetParametersAsync()

.NET Version

.NET 8

Anything else?

No response

[javiercn]

@thanuja89 thanks for the additional details.

It seems to do with the fact that you have two model fields declared:

    protected ModelModel Model;
    protected ModelModel model = new();

If I comment out model which is not used, the issue goes away.

Seems like the root of the exception comes from the call to Expression.PropertyOrField which seems to do a case-insensitive search, detects both, and throws an AmbiguousMatchException.

Relevant code is here

[springy76]

@javiercn
I have a related problem to the same Expression.PropertyOrField case-insensitive search but in my case one member is a field and the other member is a property. So I don't get an AmbiguousMatchException at that exact position but an ArgumentNullException some cpu cycles later within ctor of FieldIdentifier:

System.ArgumentNullException: Value cannot be null. (Parameter 'model')
   at System.ArgumentNullException.Throw(String paramName)
   at System.ArgumentNullException.ThrowIfNull(Object argument, String paramName)
   at Microsoft.AspNetCore.Components.Forms.FieldIdentifier..ctor(Object model, String fieldName)
   at Microsoft.AspNetCore.Components.Forms.FieldIdentifier.Create[TField](Expression`1 accessor)
   at Microsoft.AspNetCore.Components.Forms.InputBase`1.SetParametersAsync(ParameterView parameters)
   at Microsoft.AspNetCore.Components.Rendering.ComponentState.SupplyCombinedParameters(ParameterView directAndCascadingParameters)

That's because I have a field which is never null:
private Model myModel

and an optional parameter (sometimes used, then no circuit crash):
[Parameter] public Model? MyModel { get; set; }

and the usage of this now crashes the circuit:
<InputTextArea @bind-Value="@this.myModel.Memo" />

because it silently changed @bind-Value="@this.myModel.Memo" into @bind-Value="@this.MyModel.Memo" and then crashes because this.MyModel is null and so https://github.com/dotnet/aspnetcore/blob/v8.0.0/src/Components/Forms/src/FieldIdentifier.cs#L179 returns null.

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.

[springy76]

interesting way of handling a regression - any comments @mkArtakMSFT ? - is this a well documented breaking change?

[springy76]

@javiercn What's next? A soundex match maybe? I have a strong typed precompiled C# managed expression to existing code. I neither expect nor accept any blurry "maybe" here.

[AbstractionsAs]

This is definetly a bug with critical impact.
It has broken half my deployed applications, and it's not listed under breaking changes for net8