bool? is nullable while bool is not.

bool? first;
bool second;

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

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

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

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

 

REF: https://stackoverflow.com/questions/1181491/whats-the-difference-between-bool-and-bool

C# bool? 的意思