class SomeClass
    {
        public string? SomeNullableString { get; set; }
    }

    void Foo()
    {
        List<SomeClass> list = new List<SomeClass>();

        var query =
            from item in list
            where item.SomeNullableString != null
            select Bar(item.SomeNullableString);
    }

    string Bar(string someNonNullableString)
    {
        return someNonNullableString;
    }