为什么C＃编译器会产生NULL和数字比较的结果？

Question

I have the below code snippet which made TaskA to run when the Employee object is NULL (expected that to run TaskA only when the Employee ID not equals to 1).

if (Employee?.Id != 1) <PerformTaskA>;

Here, why C# compiler compares NULL and 1? shouldn't it throw exception? Also, what is the better practice to avoid these kind of cases?

Answer 1

Why C# compiler compares NULL and 1?

Because Nullable<T> is capable of comparing with T (or actually int becomes int? in this case). And in your case, it ends up with null != 1 which is always true.

The easiest thing for you is to make this explicit:

if (Employee != null && Employee.Id != 1) <PerformTaskA>;

Answer 2

When you perform comparisons with nullable types, if the value of one of the nullable types is null and the other is not, all comparisons evaluate to false except for != (not equal) . It is important not to assume that because a particular comparison returns false, the opposite case returns true. In the following example, 10 is not greater than, less than, nor equal to null. Only num1 != num2 evaluates to true.

int? num1 = 10;
int? num2 = null;
if (num1 >= num2)
{
    Console.WriteLine("num1 is greater than or equal to num2");
}
else
{
    // This clause is selected, but num1 is not less than num2.
    Console.WriteLine("num1 >= num2 returned false (but num1 < num2 also is false)");
}

if (num1 < num2)
{
    Console.WriteLine("num1 is less than num2");
}
else
{
    // The else clause is selected again, but num1 is not greater than
    // or equal to num2.
    Console.WriteLine("num1 < num2 returned false (but num1 >= num2 also is false)");
}

if (num1 != num2)
{
    // This comparison is true, num1 and num2 are not equal.
    Console.WriteLine("Finally, num1 != num2 returns true!");
}

// Change the value of num1, so that both num1 and num2 are null.
num1 = null;
if (num1 == num2)
{
    // The equality comparison returns true when both operands are null.
    Console.WriteLine("num1 == num2 returns true when the value of each is null");
}

/* Output:

• num1 >= num2 returned false (but num1 < num2 also is false)

• num1 < num2 returned false (but num1 >= num2 also is false)

• Finally, num1 != num2 returns true!

• num1 == num2 returns true when the value of each is null

*/

Check this Microsoft doc .

Check the Live Fiddle here .

Answer 3

You state "I have the below code snippet which made TaskA to run when the Employee object is NULL ".

For that, you'd want:

if (employee == null) {
    PerformTaskA();
}

(NB: I've put employee rather than Employee on the assumption that employee refers to an object rather than its class / to be in line with naming standards ).

Were you just comparing with 1 because ID 1 means the employee is not null? That would also cause your task to run if the employee was not null but had any ID other than one; though I suspect you realised that?

Regarding comparison with a nullable type, see the documentation on Nullable.Equals , which explains this behaviour.

Answer 4

Employee?.Id evaluates to null if Employee is null.
?. and ?[] null-conditional Operators

Hover over this in Visual studio and you will get the answer

Debug.WriteLine(null != 1);

The expression will always be true because a value of type int will never equal null of type int?

It does not throw an exception. Why did you not test this?

use

Debug.WriteLine(sc != null && sc.ID != 1);