List<T> is non nullable?

Recently, Microsoft enabled nullable reference types by default on new projects.

You have five ways to make this warning go away:

1. You can respect nullable reference types, and change the type of your parameter:

public static void test(List<String>? list = null) { }

2. You can ignore the null assignment using the ! operator. (I like to call this the "I know better" operator because I will typically use it when I know what I'm assigning is not null but the compiler thinks it could be.)

public static void test(List<String> list = null!) { }

3. You can disable nullable reference types for just this piece of the code:

#nullable disable
public static void test(List<String> list = null) { }
#nullable restore

4. You can disable nullable reference types for the whole file by putting #nullable disable at the beginning of it (with your namespace declaration)
5. You can disable nullable reference types for the project. Right click on the project in Solution Explorer -> Properties -> Build -> General -> Nullable.

Go into your project file and delete the line that says <Nullable>Enable</Nullable>

Once Nullable reference types are enabled, List<T> is not nullable by default.

You have to mark it as nullable explicitly, by appending a ? character.

public static void test(List<String>? list = null)
{ }