Nullable operators problems in classes containing nullable objects of classes with enum fields

Question

I've a problem with using??= operator for setting a value of enum field in class that is a nullable field of other class. Simplified example below:

public class Program {
    public static void Main() {
        var en = En.val2;
        var b = new B(new A());
        b.a?.en ??= en;
    }
}

public class A {
    public En en;
    
    public A() {
        en = En.val1;
    }
}

public class B {
    public A? a;
    
    public B(A a = null) {
        this.a = a;
    }
}

public enum En {
    val1,
    val2
}

The above gives an error: "The left-hand side of an assignment must be a variable, property or indexer".

I can still do it with simply checking if ba is null, like this:

if(b.a != null){
    b.a.en = en;
}

Is there anyway to do what I want using nullable operators?

Answer 1

Before you try to access en in ba , You can use ?? operator to set the default value to ba .

static void Main(string[] args)
{
  var en = En.val2;
  var b = new B(new A());

  b.a = b.a ?? new A();
  b.a.en = en;
}

Answer 2

No, there is no way, because if ba is null, what will you assign to it? You only specify what you assign to en , which is not sufficient information for the operation you want to do.

Anyways, the ba?.en??= en; operation and the piece of code below do NOT have the same meaning:

if (b.a != null) {
    b.a.en = en;
}

The null coalescing assignment translates to this:

if (b.a?.en == null) {
   b.a.en = en;
}

Which is clearly different. So I don't understand what you mean and what your code should do? So the correct answer depends on what you mean.

If you want to assign a value only if ba is null (assign a default value), the solution would be:

if (b.a is null) {
   b.a = new A();
   b.a.en = en;
}

Or

b.a = b.a ?? new A();
b.a.en = en;

If you want to assign a value only if ba is not null, then you should go with the if solution that you provided yourself.

Answer 3

In C#, when using ? operator means check this value, if this value is not null , it gets the value of this value's property, otherwise gets the default value of value's property default. ?? operator means that if value is null , assign the value of member at the right side of ??operator.

But at the left side of equality, if value is null , you can't assign a value for the property before create a new instance. That's why, you should check this condition using if keyword before assignment.

For your example:

if(b.a != null){
    b.a.en = en;
}

is valid one.

ba?.en??= en; this is incorrect syntax. as i can see, you especially want to use a property as nullable. Imo, In this situation, the best way is control a property with if before assignment operation of en property.