Skip to content

Fix incorrect statement about generic interface inheritance requiring covariance #48862

New issue

Have a question about this project? Sign up for a free GitHub account to open an issue and contact its maintainers and the community.

Sign up for GitHub

By clicking “Sign up for GitHub”, you agree to our terms of service and privacy statement. We’ll occasionally send you account related emails.

Already on GitHub? Sign in to your account

Open
wants to merge 3 commits into
base: main
Choose a base branch
from
Open

Fix incorrect statement about generic interface inheritance requiring covariance #48862

Changes from all commits
Commits
Show all changes
3 commits
Select commit Hold shift + click to select a range
d7e3e42
Initial plan
Copilot Oct 1, 2025
0530114
Fix incorrect statement about generic interface inheritance
Copilot Oct 1, 2025
9663fc8
Update docs/csharp/programming-guide/generics/generic-interfaces.md
BillWagner Oct 2, 2025
File filter

Filter by extension

Filter by extension
Conversations
Failed to load comments.
Loading
Jump to
Jump to file
Failed to load files.
Loading
Diff view
Diff view
2 changes: 1 addition & 1 deletion docs/csharp/programming-guide/generics/generic-interfaces.md
View file
Open in desktop
Original file line number Diff line number Diff line change
Expand Up @@ -26,7 +26,7 @@ The rules of inheritance that apply to classes also apply to interfaces:

:::code language="csharp" source="./snippets/GenericInterfaces.cs" id="Months":::

Generic interfaces can inherit from non-generic interfaces if the generic interface is covariant, which means it only uses its type parameter as a return value. In the .NET class library, <xref:System.Collections.Generic.IEnumerable%601> inherits from <xref:System.Collections.IEnumerable> because <xref:System.Collections.Generic.IEnumerable%601> only uses `T` in the return value of <xref:System.Collections.Generic.IEnumerable%601.GetEnumerator%2A> and in the <xref:System.Collections.Generic.IEnumerator%601.Current%2A> property getter.
Generic interfaces can inherit from non-generic interfaces. In the .NET class library, <xref:System.Collections.Generic.IEnumerable%601> inherits from <xref:System.Collections.IEnumerable>. When a generic interface inherits from a non-generic interface, the type parameter typically replaces `object` in the overridden members. For example, <xref:System.Collections.Generic.IEnumerable%601> uses `T` in place of `object` in the return value of <xref:System.Collections.Generic.IEnumerable%601.GetEnumerator%2A> and in the <xref:System.Collections.Generic.IEnumerator%601.Current%2A> property getter. Because `T` is used only in output positions in these members, <xref:System.Collections.Generic.IEnumerable%601> can be marked as covariant. If `T` were used in an input position in an overridden member, the interface couldn't be covariant, and the compiler would generate an error.

Concrete classes can implement closed constructed interfaces, as follows:

Expand Down