Why variance specifier can only be declare on Interface or delegate

Asked May 06 '14 at 08:36

Active May 06 '14 at 08:36

Viewed 52 times

I have a class that have a type parameter that I would like to assign to a more generic type parameter:

public StrategyContext<T> : where T : IStrategy {}

StrategyContext is a class to help designing unit test. Using xUnit and the Theroy attribute I can assert some common behavior to all my strategy. For example:

[Theory]
[InlineData(typeof(LeftStrategy))]
[InlineData(typeof(RightStrategy))]
public void HasAName(Type contextType)
{
    Type strategy = typeof(StrategyContext<>);
    var context = (StrategyContext<IStrategy>)Activator.CreateInstance(strategy.MakeGenericType(contextType));
}

Unfortunatly the runtime can not convert from a StrategyContext<LeftStrategy> to StrategyContext<IStrategy>

So I thought that covariance is what I needed but it is not possible to declare the class as:

public StrategyContext<out T> : where T : IStrategy {}

Any other solution?

asked May 06 '14 at 08:36

mathk

7,973
6
45
74

4

I think it's an artificial limitation, because in Scala you can do this – Display Name May 06 '14 at 08:41
@SargeBorsch I have the same impression – mathk May 06 '14 at 08:42
1

btw, there are also many other things that can't be done in C# but can be done in Scala or Haskell. So if you are not forced by someone else to use C#, it may be a good idea to switch the language until it's not too late... – Display Name May 06 '14 at 08:46
1

@SargeBorsch I would love to but that is not my decision ;) – mathk May 06 '14 at 08:48

Why variance specifier can only be declare on Interface or delegate

0 Answers0