What's the difference between "bool" and "bool?"?

The ? symbol after a type is only a shortcut to the Nullable type, bool? is equivalent to Nullable<bool>.

bool is a value type, this means that it cannot be null, so the Nullable type basically allows you to wrap value types, and being able to assign null to them.

bool? can contain three different values: true, false and null.

Also, there are no short-circuiting operators (&& ||) defined for bool?

Only the logical AND, inclusive OR, operators are defined and they behave like this:

x        y      x & y   x | y   
true    true    true    true
true    false   false   true
true    null    null    true
false   true    false   true
false   false   false   false
false   null    false   null
null    true    null    true
null    false   false   null
null    null    null    null

The Nullable type is basically a generic struct, that has the following public properties:

public struct Nullable<T> where T: struct
{
    public bool HasValue { get; }
    public T Value { get; }
}

The HasValue property indicates whether the current object has a value, and the Value property will get the current value of the object, or if HasValue is false, it will throw an InvalidOperationException.

Now you must be wondering something, Nullable is a struct, a value type that cannot be null, so why the following statement is valid?

int? a = null;

That example will compile into this:

.locals init (valuetype [mscorlib]System.Nullable`1<int32> V_0)
IL_0000:  ldloca.s   V_0
IL_0002:  initobj    valuetype [mscorlib]System.Nullable`1<int32>

A call to initobj, which initializes each field of the value type at a specified address to a null reference or a 0 of the appropriate primitive type.

That's it, what's happening here is the default struct initialization.

int? a = null;

Is equivalent to:

Nullable<int> a = new Nullable<int>();

bool? is nullable while bool is not.

bool? first;
bool second;

In the above code, first will be null while second will be false.

One typical use is if you want to know whether there has been an assignment made to the variable. Since bool is a value type (just as int, long, double, DateTime and some other types), it will always be initialized to a default value (false in the case of a bool, 0 in the case of an int). This means that you can not easily know whether it's false because some code assigned false to it, or if it is false because it has not yet been assigned. In that case bool? comes in handy.

Whenever you see the ? character following a type name, it's shorthand for Nullable<TypeName>. Nullable is a special type that allows value types to act like a null value. It's a way of explicitly expressing a value type can have a non-value value.

For bool it effectively turns the variable into a tri-state value

• With Value: True
• With Value: False
• Without Value