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C# 11–Generic Attributes

C# 11 introduces a large list of new language features. One of the smaller features is the support for generic attributes.

Before C# 11, when you had to pass type information to an attribute, you had to use a type parameter.

Let’s use an example that always has annoyed me; the [ServiceFilterAttribute]. If you have ever used an ASP.NET MVC filter that needed dependencies, then you should know this attribute.

Here is an example I copied from the Microsoft documentation:

You see in the example above is that the type of the actual filter we want to invoke is passed as a parameter to the ServiceFilterAttribute.

The constructor looks something like this:

By not being able to use generics, I could pass anything I want as a parameter to the ServiceFilter no matter if it is actually a filter or not. The compiler will not help me to avoid passing an incorrect argument.

A type safe ServiceFilter

With the release of C# 11 and the introduction of Generic Attributes, we can create a type safe variant of the [ServiceFilterAttribute] as an example:

Now the compiler will warn me when I try to pass a type that isn’t a filter:

Remark: There are multiple interfaces that can be used to implement a filter. In the example above I only used the IActionFilter. So this is not a production ready solution for all use cases.

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