Virtualize component's Items property should be IReadOnlyCollection

Background and Motivation

The <Virtualize TItem="T"> component should allow users to pass a IReadOnlyCollection<T> to the Items property/parameter instead of requiring an ICollection.

There should be no need for the Virtualize component to change the collection passed to it, so changing the parameter to an IReadOnlyCollection<T> should be straight forward, that allows users to pass a wider range of collection types (including custom ones) to the Virtualize component.

Proposed API change to Virtualize.cs

/// <summary>
/// Gets or sets the fixed item source.
/// </summary>
[Parameter]
-public ICollection<TItem>? Items { get; set; }
+public IReadOnlyCollection<TItem>? Items { get; set; }

Usage Examples

N/A

Alternative Designs

N/A

Risks

Since ICollection does not implements IReadOnlyCollection this is a breaking change if users are using custom collection types that implements ICollection but not IReadOnlyCollection. Changing that for the end user will be trivial though.

Updated: changed to reflect this is a breaking change. Op

[javiercn]

@egil unfortunately we don’t do breaking changes like this unless we have a really good justification for them.

It’s unfortunate that we chose ICollection instead of IReadOnlyCollection but we prefer to avoid breaking existing customers (no matter how few they might be) over improving the correctness of the interface.

With changes like these, is not just about breaking an existing app that would need to recompile to work, its potentially breaking existing libraries that build on top and that would need to be updated.

That, we think is a much bigger risk compared to the benefit of being more “correct/explicit” about the type used.

I understand that this might not align with your preferences, but at least I hope the arguments explain our rationale.

[campersau]

ICollection implements IReadOnlyCollection

I don’t think that this is currently the case: https://github.com/dotnet/runtime/blob/a14d27ac7ff3a58e39d889db72f7ce682c7492fd/src/libraries/System.Private.CoreLib/src/System/Collections/Generic/ICollection.cs#L8

See also https://github.com/dotnet/runtime/issues/31001