Best way to check for nullable bool in a condition expression (if ...)

You may not like it, but personally I find

if (x.HasValue && x.Value)

the most readable. It makes it clear you are working with a nullable type and it makes it clear you are first checking whether the nullable type has a value before acting on it conditionally.

If you take your version and replace the variable with x also it reads:

if (x ?? false)

Is that as clear? Is it obvious x is a nullable type? I'll let you decide.

How about using GetValueOrDefault, which is pretty self-explaining and allows to use whatever default you want:

if (nullableBool.GetValueOrDefault(false)) {
}

I think a lot of people concentrate on the fact that this value is nullable, and don't think about what they actually want :)

bool? nullableBool = true;
if (nullableBool == true) { ... } // true
else { ... } // false or null

Or if you want more options...

bool? nullableBool = true;
if (nullableBool == true) { ... } // true
else if (nullableBool == false) { ... } // false
else { ... } // null

(nullableBool == true) will never return true if the bool? is null :P