Two-way databinding + CaptureUnmatchedValues + missing EventCallback

[szmalec]

Describe the bug

When using two-way databinding for a Prop1 property of a component that does NOT have the Prop1Changed property (of the EventCallback type), then when rendering this component an error is reported: Object of type 'XXX' does not have a property matching the name 'Prop1Changed' (problem probably somwhere here: ref). However, if the component has an additional parameter with the attribute CaptureUnmatchedValues then this error is NOT reported. Should this be treated as a bug?

To Reproduce

ChildComponent.razor:

<h1>Child component</h1>

@code {
    [Parameter(CaptureUnmatchedValues = true)]
    public Dictionary<string, object> Attributes { get; set; }

    [Parameter] public bool Prop1 { get; set; }
    //[Parameter] public EventCallback<bool> Prop1Changed { get; set; }
}

Index.razor:

<ChildComponent @bind-Prop1="_prop" />

@code {
    private bool _prop;
}

See also: blazorfiddle

Further technical details

• ASP.NET Core version: 3.2.0-preview1.20073.1
• Include the output of dotnet --info:

  
  dotnet --info
  .NET Core SDK (reflecting any global.json):
  Version:   3.1.200-preview-014977
  Commit:    ca5522cead

Runtime Environment: OS Name: Windows OS Version: 10.0.18362 OS Platform: Windows RID: win10-x64 Base Path: C:\Program Files\dotnet\sdk\3.1.200-preview-014977\

Host (useful for support): Version: 3.1.1 Commit: a1388f194c

.NET Core SDKs installed: 1.0.0-preview2-003131 [C:\Program Files\dotnet\sdk] 1.1.0 [C:\Program Files\dotnet\sdk] 2.1.202 [C:\Program Files\dotnet\sdk] 2.1.509 [C:\Program Files\dotnet\sdk] 2.1.802 [C:\Program Files\dotnet\sdk] 2.2.402 [C:\Program Files\dotnet\sdk] 3.0.100-preview9-014004 [C:\Program Files\dotnet\sdk] 3.0.100 [C:\Program Files\dotnet\sdk] 3.0.101 [C:\Program Files\dotnet\sdk] 3.1.100-preview1-014459 [C:\Program Files\dotnet\sdk] 3.1.100 [C:\Program Files\dotnet\sdk] 3.1.200-preview-014977 [C:\Program Files\dotnet\sdk]

.NET Core runtimes installed: Microsoft.AspNetCore.All 2.1.12 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.All] Microsoft.AspNetCore.All 2.1.13 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.All] Microsoft.AspNetCore.All 2.1.15 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.All] Microsoft.AspNetCore.All 2.2.7 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.All] Microsoft.AspNetCore.App 2.1.12 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App] Microsoft.AspNetCore.App 2.1.13 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App] Microsoft.AspNetCore.App 2.1.15 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App] Microsoft.AspNetCore.App 2.2.7 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App] Microsoft.AspNetCore.App 3.0.0-preview9.19424.4 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App] Microsoft.AspNetCore.App 3.0.1 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App] Microsoft.AspNetCore.App 3.1.0-preview1.19508.20 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App] Microsoft.AspNetCore.App 3.1.1 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App] Microsoft.NETCore.App 1.0.1 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App] Microsoft.NETCore.App 1.0.5 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App] Microsoft.NETCore.App 1.1.2 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App] Microsoft.NETCore.App 2.0.9 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App] Microsoft.NETCore.App 2.1.12 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App] Microsoft.NETCore.App 2.1.13 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App] Microsoft.NETCore.App 2.1.15 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App] Microsoft.NETCore.App 2.2.7 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App] Microsoft.NETCore.App 3.0.0-preview9-19423-09 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App] Microsoft.NETCore.App 3.0.1 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App] Microsoft.NETCore.App 3.1.0-preview1.19506.1 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App] Microsoft.NETCore.App 3.1.1 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App] Microsoft.WindowsDesktop.App 3.0.0-preview9-19423-09 [C:\Program Files\dotnet\shared\Microsoft.WindowsDesktop.App] Microsoft.WindowsDesktop.App 3.0.1 [C:\Program Files\dotnet\shared\Microsoft.WindowsDesktop.App] Microsoft.WindowsDesktop.App 3.1.0-preview1.19506.1 [C:\Program Files\dotnet\shared\Microsoft.WindowsDesktop.App] Microsoft.WindowsDesktop.App 3.1.1 [C:\Program Files\dotnet\shared\Microsoft.WindowsDesktop.App]

- The IDE (VS / VS Code/ VS4Mac) you're running on, and it's version: Version 16.5.0 Preview 4.0

[captainsafia]

Thanks for reporting this issue and taking the time to make this repro, @szmalec!

I'm labelling this as a bug. Although the ChildComponent will still support two-way binding, the user will have to know that they need to include a changed handler on the component. Our CaptureUnmatchedValues handlers usurps this logic and makes the DX for this super meh.

We might be able to improve the way we handle capturing required values when we add support for required parameters as part of the user-facing API. In particular, a changed handler is a required parameter on components with a bound value.

[mkArtakMSFT]

We've moved this issue to the Backlog milestone. This means that it is not going to happen for the coming release. We will reassess the backlog following the current release and consider this item at that time. However, keep in mind that there are many other high priority features with which it will be competing for resources.

[ghost]

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.