# V3123. Perhaps the '??' operator works in a different way than it was expected. Its priority is lower than priority of other operators in its left part.

The analyzer detected a code fragment that is very likely to contain a logic error. The code uses an expression with the operator '??' or '?:' that may be evaluated differently from what the programmer intended.

The '??' and '?:' operators have lower precedence than the operators ||, &&, |, ^, &, !=, ==, +, -, %, /, *. Programmers sometimes forget about this and write faulty code like in the following example:

    public bool Equals(Edit<TNode> other)
    {
        return _kind == other._kind
            && (_node == null) ? other._node == null :
                                 node.Equals(other._node);
    }

Since the '&&' operator's precedence is higher than that of '?:', the '_kind == other._kind && (_node == null)' expression will be evaluated in the first place. To avoid errors like that, make sure to enclose the whole expression with the '?:' operator in parentheses:

    public bool Equals(Edit<TNode> other)
    {
        return _kind == other._kind
            && ((_node == null) ? other._node == null :
                                  node.Equals(other._node));
    }

The next example of incorrect code uses the '??' operator:

    public override int GetHashCode()
    {
        return ValueTypes.Aggregate(...)
             ^ IndexMap?.Aggregate(...) ?? 0;
    }

The '^' operator's precedence is higher than that of '??', so if 'IndexMap' is found to be null, the left operand of the '??' operator will also have the value of "null", which means that the function will always return 0 regardless of the contents of the 'ValueTypes' collection.

Like in the case with the '?:' operator, it is recommended that you enclose expressions with the '??' operator in parentheses:

    public override int GetHashCode()
    {
        return ValueTypes.Aggregate(...)
             ^ (IndexMap?.Aggregate(...) ?? 0);
    }

From now on, the 'GetHashCode()' function will return different values depending on the contents of the 'ValueTypes' collection even when 'IndexMap' is equal to 'null'.

This diagnostic is classified as:

CWE-783

You can look at examples of errors detected by the V3123 diagnostic.