Blazor Two Way Binding Error

Describe the bug

A component with a parameter called say Value manages 2 way binding with an accompanying ValueChanged event handler. If (i) this event handler is called from within the setter for Value and (ii) the component attaches to a cascading value, the component’s ability to set the value is neutralized by the value bouncing back to its original state.

For what it’s worth I believe that I have seen repetitive 2 way binding bounce in the past but cannot reproduce that.

To Reproduce

Please fork https://github.com/simonziegler/TwoWayBindingDemo! The demonstration is in the index page. You will find two components each of which is referenced both within and outside a cascading value. The one that fails has its result shown in red. At the bottom of the page you’ll see a list of the bound value changes captured by the parent - you’ll see a bounce on the offending version.

Exceptions (if any)

n/a

Further technical details

• ASP.NET Core version: both .NET 3.1 and .NET 5
• Include the output of dotnet --info

.NET SDK (reflecting any global.json):
 Version:   5.0.100-preview.7.20366.6
 Commit:    0684df3a5b

Runtime Environment:
 OS Name:     Windows
 OS Version:  10.0.19041
 OS Platform: Windows
 RID:         win10-x64
 Base Path:   C:\Program Files\dotnet\sdk\5.0.100-preview.7.20366.6\

Host (useful for support):
  Version: 5.0.0-preview.7.20364.11
  Commit:  53976d38b1

.NET SDKs installed:
  3.1.201 [C:\Program Files\dotnet\sdk]
  3.1.302 [C:\Program Files\dotnet\sdk]
  3.1.400-preview-015203 [C:\Program Files\dotnet\sdk]
  5.0.100-preview.7.20366.6 [C:\Program Files\dotnet\sdk]

.NET runtimes installed:
  Microsoft.AspNetCore.All 2.1.16 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.All]
  Microsoft.AspNetCore.All 2.1.20 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.All]
  Microsoft.AspNetCore.App 2.1.16 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App]
  Microsoft.AspNetCore.App 2.1.20 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App]
  Microsoft.AspNetCore.App 3.1.2 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App]
  Microsoft.AspNetCore.App 3.1.3 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App]
  Microsoft.AspNetCore.App 3.1.6 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App]
  Microsoft.AspNetCore.App 5.0.0-preview.7.20365.19 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App]
  Microsoft.NETCore.App 2.1.16 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
  Microsoft.NETCore.App 2.1.20 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
  Microsoft.NETCore.App 3.1.2 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
  Microsoft.NETCore.App 3.1.3 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
  Microsoft.NETCore.App 3.1.6 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
  Microsoft.NETCore.App 5.0.0-preview.7.20364.11 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
  Microsoft.WindowsDesktop.App 3.1.2 [C:\Program Files\dotnet\shared\Microsoft.WindowsDesktop.App]
  Microsoft.WindowsDesktop.App 3.1.3 [C:\Program Files\dotnet\shared\Microsoft.WindowsDesktop.App]
  Microsoft.WindowsDesktop.App 3.1.6 [C:\Program Files\dotnet\shared\Microsoft.WindowsDesktop.App]
  Microsoft.WindowsDesktop.App 5.0.0-preview.7.20366.1 [C:\Program Files\dotnet\shared\Microsoft.WindowsDesktop.App]

To install additional .NET runtimes or SDKs:
  https://aka.ms/dotnet-download

• The IDE (VS / VS Code/ VS4Mac) you’re running on, and it’s version

[simonziegler]

Watching the community stand up just now @danroth27 said to point out what needs attention in issues. This is one!

[stefanloerwald]

I’ve played around with the code and added some Console.WriteLine. This reveals the following outputs:

For Component1 without cascading value:

Child: Value set invoked with value = 0
Child: OnParametersSet
Child: OnAfterRender
Child: Value set invoked with value = 1
Child: Value setting. Old value = 0
Child: Value set. New value = 1
Parent: Value updated to 1
Child: ValueChanged invoked
Child: Value changed to 1
Child: Value set invoked with value = 1
Child: OnParametersSet
Child: OnAfterRender

And with cascading value it becomes

Child: Value set invoked with value = 0
Child: OnParametersSet
Child: OnAfterRender
Child: Value set invoked with value = 1
Child: Value setting. Old value = 0
Child: Value set. New value = 1
Parent: Value updated to 1
Child: ValueChanged invoked
Child: Value changed to 1
Child: Value set invoked with value = 0
Child: Value setting. Old value = 1
Child: Value set. New value = 0
Parent: Value updated to 0
Child: ValueChanged invoked
Child: OnParametersSet
Child: Value set invoked with value = 0
Child: OnParametersSet
Child: Value set invoked with value = 0
Child: OnParametersSet
Child: Value set invoked with value = 0
Child: OnParametersSet
Child: OnAfterRender
Child: OnAfterRender
Child: OnAfterRender

The diff is therefore:

Child: Value set invoked with value = 0
Child: OnParametersSet
Child: OnAfterRender
Child: Value set invoked with value = 1
Child: Value setting. Old value = 0
Child: Value set. New value = 1
Parent: Value updated to 1
Child: ValueChanged invoked
Child: Value changed to 1
- Child: Value set invoked with value = 1
+ Child: Value set invoked with value = 0
+ Child: Value setting. Old value = 1
+ Child: Value set. New value = 0
+ Parent: Value updated to 0
+ Child: ValueChanged invoked
Child: OnParametersSet
+ Child: Value set invoked with value = 0
+ Child: OnParametersSet
+ Child: Value set invoked with value = 0
+ Child: OnParametersSet
+ Child: Value set invoked with value = 0
+ Child: OnParametersSet
Child: OnAfterRender
+ Child: OnAfterRender
+ Child: OnAfterRender

This to me shows the bug quite clearly: The parent receives the correct value, yet the child component is subsequently updated to the original value again.