In c# what does 'where T : class?' mean

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In C# what does where T : class? mean (note the question mark on the end of the class constraint)

For instance

public IList<T> DoThis<T>() where T : class?
syntax c#
3 Answers

? attached to a type in C# (supported from version 8) means it is a nullable reference type. class? means that the calling code has to pass in a type parameter which is a class, and may be nullable.

For instance, DoThis<string?>(someNullableString) is valid. It may also be called with the non-nullable version, e.g. DoThis<string>(someNonNullableString).

user_5701900
user_17864280

I haven't seen some things mentioned. And it's a bit counter-intuitive at first since we all know reference types can be null. This feature really answers should they be null.

"Nullable reference type" basically states your intent that null is a valid value, both to those reading the code and to the compiler.

It helps to protect you from doing things like string shouldntBeNull = null;.

Which you can spot if you enable this feature using <Nullable>enable</Nullable> in the project settings.

In contrast, this shows intent more clearly string? couldBeNull = null;

This is an opt-in feature.

user_39803310
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