Invalid variance: The type parameter 'T' must be invariantly valid on 'xxx.IItem.GetList()'. 'T' is covariant

Question

Why the following code get the error?

Invalid variance: The type parameter 'T' must be invariantly valid on 'UserQuery.IItem<T>.GetList()'. 'T' is covariant.

public interface IFoo {}
public interface IBar<T> where T : IFoo {}

public interface IItem<out T> where T: IFoo
{
    IEnumerable<IBar<T>> GetList();
}

Why did you mark `IItem` as covariant (`out T`) ? – Bartosz Sep 18 '12 at 20:20 — Bartosz, Sep 18 '12 at 20:20
@Bartosz other part of code need it be covariant – ca9163d9 Sep 18 '12 at 20:23 — ca9163d9, Sep 18 '12 at 20:23

Bartosz · Accepted Answer · 2012-09-19T06:52:32.077

9

The interfaces IBar and IItem do not agree on variance: in your IBar declaration, the T is not covariant, as there is no out keyword, whereas in IITem the T is covariant.

edited Sep 19 '12 at 06:52

answered Sep 18 '12 at 20:18

Bartosz

3,318
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31

score 1 · Answer 2 · answered Sep 18 '12 at 20:24

1

The following code will get rid of the error.

public interface IFoo {}
public interface IBar<out T> where T : IFoo {}

public interface IItem<out T> where T: IFoo
{
    IEnumerable<IBar<T>> GetList();
}

answered Sep 18 '12 at 20:24

ca9163d9

27,283
64
210
413

Invalid variance: The type parameter 'T' must be invariantly valid on 'xxx.IItem.GetList()'. 'T' is covariant

2 Answers2

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