New LookupOrFallbackDefaultValueProvider base class for custom default value providers

[stakx]

This adds a new base class LookupOrFallbackDefaultValueProvider, which builds on top of DefaultValueProvider, to Moq's public API.

• This type becomes the new base class for both EmptyDefaultValueProvider and MockDefaultValueProvider.

• It has built-in support for task types Task, Task<TResult>, and ValueTask<TResult>.

• It also provides enough customizability for folks to easily create their own default value providers that work like the empty default value provider. Here's an example of a provider that will produce empty collections for IReadOnlyList<T>:

  sealed class MyEmptyDefaultValueProvider : LookupOrFallbackDefaultValueProvider
  {
      public MyEmptyDefaultValueProvider()
      {
          base.Register(typeof(IReadOnlyList<>), CreateReadOnlyListOf);
      }
  
      private static CreateReadOnlyListOf(Type type, Mock _)
      {
          var listType = typeof(List<>).MakeGenericType(type.GetGenericArguments());
          return Activator.CreateInstance(listType);
      }
  }

For the rationale behind this new type, and the reasoning why it doesn't come preconfigured as an empty default value provider supporting all standard collection types from the .NET BCL, see #173.

Perhaps we can add a customizable or expanded empty default value provider at a later time if there's still widespread demand / need for it.

[Serg046]

But why is it sealed?
protected internal sealed override object GetDefaultValue(Type type, Mock mock)

I would like to override it to customize batch of types instead of Register(...) call for each type. Something like this:

protected override object GetDefaultValue(Type type, Mock mock)
{
    var enumerableType = typeof(IEnumerable<>).MakeGenericType(type.GetGenericArguments());
    if (enumerableType.IsAssignableFrom(type))
    {
        var listType = typeof(List<>).MakeGenericType(type.GetGenericArguments());
        return Activator.CreateInstance(listType);
    }
    base.GetDefaultValue(type, mock);
}

[stakx]

But why is it sealed?

In order to guarantee a certain behavior with generic and array types.

(And because I believe that composition is generally better for composing behavior than inheritance, so I tend to define methods as either abstract, or sealed... but not virtual.)

If that's not what you want, you can always subclass the base class DefaultValueProvider.

[Serg046]

Yes, DefaultValueProvider is good way but how to get empty values for other types that are not covered my GetDefaultValue implementation? I can get it from LookupOrFallbackDefaultValueProvider's GetDefaultValue method if it is public (by composition) or not sealed (by inheritance). It contains important logic like values for Task etc.

[stakx]

Sorry I forgot to say, instead of trying to override GetDefaultValue, you can override GetFallbackDefaultValue which is what'll get invoked when there's no registered factory for a particular type.

Also, you can always use composition with / delegation to another default value provider (e.g. DefaultValueProvider.Empty) to cover those cases that your own provider does not handle itself.

[Serg046]

I cannot use DefaultValueProvider.Empty because GetDefaultValue is protected internal.
But GetFallbackDefaultValue works for me, thank you!

[stakx]

I cannot use DefaultValueProvider.Empty because GetDefaultValue is protected internal.

(You cannot override, but you can use composition.)

GetFallbackDefaultValue works for me, thank you!

Glad to hear. You're welcome!

[Serg046]

Do you mean DefaultValueProvider.Empty.GetDefaultValue(type, mock) call?

[stakx]

Sorry, you're right of course... I seem to have mis-remembered those methods' accessibility. Composition indeed doesn't seem possible in this case (which is probably a design flaw).

[krizex]

Sorry, you're right of course... I seem to have mis-remembered those methods' accessibility. Composition indeed doesn't seem possible in this case (which is probably a design flaw).

If so, how can I make a customized value provider which falls back to the Mock object if I did not register the type?