Why Doesn't C# 4.0 Support Generic Variance for Classes?

C# 4.0's Restrictions on Generic Variance in Classes

Unlike interfaces, C# 4.0 doesn't allow generic variance for classes. This limitation prompts the question: why the restriction? What are the potential downsides of enabling generic variance for classes?

Challenges with Covariant Classes

Imagine a covariant class, C<T>. To implement covariance, T would need to be strictly an output type. This means the class couldn't have methods or properties that take T as input. Similarly, fields of type T would be disallowed, as they essentially function like property setters.

Limited Applicability: Immutable Structures

The only realistic application of covariant classes would be immutable structures like lists or stacks. While immutable classes offer advantages, this limited use case doesn't justify the considerable overhead of natively supporting covariance for immutable class types.

Illustrative Example: Potential Advantages

A covariant Stack<T> could, theoretically, allow code like this:

<code class="language-csharp">Stack<string> strings = null;
strings = strings.Push("hello");
strings = strings.Push("goodbye");
Stack<object> objects = strings;
objects = objects.Push(123);</code>

Here, adding an integer to a string stack would be type-safe because immutable structures prevent operations that compromise type safety.

Summary

While generic variance for classes might offer some benefits, the constraints—the inability to use class type parameters as method or property inputs—outweigh the advantages. This explains its absence in C# 4.0 and later versions.

The above is the detailed content of Why Doesn't C# 4.0 Support Generic Variance for Classes?. For more information, please follow other related articles on the PHP Chinese website!