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

Question

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?

Does this answer your question? [In c# what does 'where T : class' mean?](https://stackoverflow.com/questions/3786774/in-c-sharp-what-does-where-t-class-mean) — 0 x 5 4 4 D, Mar 06 '20 at 03:27
The `?` addresses nullable reference types. I do not think this should have been closed as a duplicate. — eouw0o83hf, Mar 06 '20 at 03:30

eouw0o83hf · Accepted Answer · 2020-03-06T06:22:39.900

4

? 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).

edited Mar 06 '20 at 06:22

answered Mar 06 '20 at 03:35

eouw0o83hf

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score 1 · Answer 2 · answered Mar 06 '20 at 03:35

1

This is valid syntax for C# 8. The syntax constrains T to be a nullable reference type

answered Mar 06 '20 at 03:35

Jonesopolis

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score 0 · Answer 3 · answered Mar 06 '20 at 03:47

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.

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

3 Answers3