@Olina-Zhang can your team please test?

Related StackOverflow post:
https://stackoverflow.com/questions/8940675/c-sharp-combobox-selectedvalue-gets-reset-on-focus-lost/

@elachlan
The issue can reproduce on both .NET Framework from 4.6 - 4.8.1 and .NET from 6.0 - 9.0.

@Tanya-Solyanik The core of the problem lies in the immutable nature of ImmutableArray. WinForms data binding mechanisms require collections that support change notifications to properly propagate changes between the UI and the data source. ImmutableArray does not support these notifications, leading to the observed behavior.

Also, defining Value1 as IReadOnlyList, as it is done in the minimal reproducible example provided by the author, might give the illusion of immutability, but due to its access level being set to public, it can be modified from outside the class, which is not the case with ImmutableArray.

Workaround:

For data binding scenarios in WinForms, it is advisable to use ObservableCollection or BindingList. These collections are mutable and, therefore, support the two-way data binding required by WinForms.

Here is a modified snippet of MainModel.cs, from the sample code, demonstrating how to adapt the existing code to use ObservableCollection<int> instead of ImmutableArray for Values2:

public ObservableCollection<int> Values2 { get; }

public MainModel()
{
    var values = new[] { 1, 2, 3 };
    Values1 = values.ToImmutableArray();
    Values2 = new ObservableCollection<int>(values);
}

This approach ensures that the ComboBox can properly interact with the data source and maintain its state across UI interactions.