Question

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AND operation cannot be applied between nullable bools.

I would expect similar behavior like + or *. So if any of the operand is null, it should return null, but the compile complains if it’s used in && or ||. Why?

To illustrate the point:

//this compiles
int? x = null;
int? y = null;
var z = x*y;

// this doesn't compile
bool? x = null;
bool? y = null;
if(x && y)
    ....

Answer 1

If it were to return null, then you invalidate the boolean requirement.

||
&&

Basically, it would allow for a value other than bool (specifically, null) within an if statement which would be undefined behavior since if statements require boolean expressions.

Interestingly, this is one of the areas where Java and C# differ. Java requires boolean values within the if statement as well, but they allow their version of bool? (Nullable<bool>, or just Boolean in Java) to be null. This leads to somewhat unexpected NullPointerExceptions at runtime, which C# is avoiding with this requirement.