What does the bool? return type mean?

Question

I saw a method return bool?, does anyone know the meaning of it?

Answer 1

Any (no-nonsense¹) value type suffixed with ? makes it a nullable type:

• int? is a nullable integer
• bool? is nullable boolean which makes is actually a triple state boolean (true, false and null) - think of it when you need to implement some triple state behaviour.
• ...

In other words object type doesn't support nullable definition as object? because it's reference type and can have a value of null already.

Real-life usage

Nullable types are most often used against database, because they make it really easy to transform nullable int/bit/datetime/etc. columns to CLR types. Like in this example:

create table Test
(
    Id int identity not null primary key,
    Days int null,
    ExactDate datetime null,
    Selected bit null
)

This can easily be translated to POCO:

public class Test
{
    public int Id { get; set; }
    public int? Days { get; set; }
    public DateTime? ExactDate { get; set; }
    public bool? Selected { get; set; }
}

¹ no-nonsense refers to all value types except Nullable which is a value type per se, but it wouldn't make any sense in making it nullable once more... Avoid such stupidity... It won't compile anyway because Nullable> i = null; can be interpreted in two ways:

• i.HasValue == false
• i.HasValue == true && i.Value.HasValue == false