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

I use the "bool" type for variables as I was used to in C++, and I try to put the values of functions or properties I expect to be boolean into my variable. However I often encounter cases where the result type is "bool?" and not "bool" and the implicit casting fails.

What is the difference between the two and when is each used? Also, should I use "bool?" as the type for my variable? Is this the best practice?

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

Adding ? makes the type null-able. Which means you can do this:

bool? x = null;

And it would be totally OK.

Another good place to use bool? is in a method to add null checking

public bool? IsTurkeyStillInFridge(Turkey turkey)
{
  if (turkey == null)
     return null;
  else if (fridge.Contains(turkey))
     return true;
  else
     return false;
}

bool? canStayAtDesk = IsTurkeyStillInFridge(turkey);

if (canStayAtDesk == null)
    MessageBox.Show("No turkey this year, check for ham.");
else if (canStayAtDesk == true)
    MessageBox.Show("Turkey for lunch. Stay at desk.");
else
    MessageBox.Show("Turkey's gone, go out to lunch.");

bool? means the boolean is nullable and is syntactic sugar for a stucture Nullable<bool>. Because a boolean is a value type, you cannot set it to null, but there are some cases where you'd want to like in a data access class because database fields can have null values.

bool can contain only true and false values while bool? can also have a null value.

bool means you can have values of true and false. bool? means you can have a value of true, false, and null.

It works for datetime and booleans.

Means that the variable declared with (double?) is nullable

double i1=0.2; //ok
double i2=null; //not ok

double? i3=0.1; //ok
double? i4=null; //ok