CS8601 and MemberNotNull with ref parameter

[brinko99]

I have the need to apply MemberNotNull to a property setter and see a surprising result. I'm seeing:

"CS8601: Possible null reference assignment" on ref _name
"CS8774: Member '_name' must have a non-null value when exiting" on set

Set is a generic method defined in an external library in a nullable context.

private string _name;

[MemberNotNull(nameof(_name))]
set
{
    Set(ref _name, value);
}

CS8601 appears to be telling me that _name must be demonstrated as not-null going into Set. This is a surprise to me as _name is already guaranteed to be not-null, no? Verifying that _name is not-null before Set resolves CS8601. I assume Set is seen as a MaybeNull method with regard to its first parameter. Isn't passing in a null legal in this context anyway?

My question is, why does MemberNotNull produce CS8601 in this case at all?

The CS8774 can be resolved by checking the value of _name after the Set call as expected.

[Tinister]

The compiler doesn't know that Set doesn't assign null to ref _name before the call to Set returns.

If the external library author changes Set to add NotNullIfNotNull like below then your warnings go away.

static void Set<T>([NotNullIfNotNull("value")] ref T field, T value) { ... }

[brinko99]

@Tinister, indeed, that's what I assumed was going on.

However, assigning a value to _name before the call to Set clears CS8601. Even though Set could still assign null.

[MemberNotNull(nameof(_name))]
set
{
    _name = string.Empty;
    Set(ref _name, value);
}

[TahirAhmadov]

@brinko99 that's because the Set method takes a generic argument and when you set _name to a not-null value (which string.Empty is), the generic argument becomes string, as opposed string?. Therefore, the MemberNotNull is satisfied.

[brinko99]

I see... very subtle!

I remain surprised that setting _name before the call to Set also clears CS8774. As Set is in a nullable oblivious context, it can set _name to null'. It seems like a guard here should be required.

Thanks

[TahirAhmadov]

That would be potentially a warning inside of Set method itself, depending on how it's written.