Using null conditional operator("?.") with "function that returns a bool value"

Question

using System;

public class A{
    public bool func(){
        return true;
    }
    

    public int func2(){
        return 10;
    }
}

public class HelloWorld
{
    public static void Main(string[] args)
    {
        A a = new A();
        if(a?.func()){
            Console.WriteLine("true"); // Error
        }
        
        if(a?.func2() == 10){
            Console.WriteLine("true"); // print: True
        }
    }
}

Like above case, I want to use null conditional operator with A function that returns a bool value. But, It throws error only when used with bool returning function.

Can I know why it works like that?

Ironically, It works well with the phrase

if(a?.func() == true){
    Console.WriteLine("true"); // print: true
}

Answer 1

Please, note that even if func returns bool

 a?.func()

returns nullable bool? (true, false and ... null):

 a        : a?.func()
 ------------------------------------------
 null     : null      
 not null : a.func() # either true or false

So you can put

 // if `a` is `null` then `a?.func() is null` 
 // since `null != true` the condition true if and only if
 // `a` is not null and `a.func()` returns true
 if (a?.func() == true)

here .net compares T? and T instances (T is bool in our case). It does it as follow

 // First compute the expression
 bool? result = a?.func()

 // Then .net checks if Value is not null
 // and if it is the Value equals to required value 
 if (result.HasValue && result.Value == true) ...

Another way is to use ?? to map null into true or false

 // in case of null - when `a` is null - assume false
 if (a?.func() ?? false)

The very same logic with

 if (a?.func2() == 10)

here a?.func2() returns nullable int? which you compare as == 10, i.e.

if a is not null and a.func2() returns 10 then...

Answer 2

The if statement requires a boolean expression. That is, whatever is inside your parenthesis must resolve to either true or false. When you use the null-conditional operator it says, "give me the bool result of func() unless a is null, then give me null". So by adding the ?. you change your expression to a bool?. But as we established, the if statement requires a bool. if (null) is invalid which is what you get when a is null.

So why does if (a?.func() == true) work? By adding == true, you are now changing your expression back to a bool result type by doing a comparison. When a is null, instead of being

if (null)

you now have

if (null == true)

which is a valid boolean expression that resolves to false. So incidentally, the inverse is also valid if (a?.func() == false).