#1,205 – C# 6.0 – Using the Null-Conditional when Invoking a Delegate

When you fire an event (by invoking a delegate), you typically need to check for null before invocation.  This avoids a null reference exception if there are no subscribers to the event.

    public class Dog
    {
        public Dog(string name, int age)
        {
            Name = name;
            Age = age;
        }

        public event EventHandler<string> NameChange;

        private string name;
        public string Name
        {
            get { return name; }
            set
            {
                if (value != name)
                {
                    name = value;

                    // Check for null before invocation
                    if (NameChange != null)
                    {
                        NameChange(this, name);
                    }
                }
            }
        }

        public int Age { get; set; }
    }

(The above pattern is not thread-safe).

An alternative pattern is to declare the event with a null handler, so that there is always at least one handler on the invocation list. (This is thread-safe).

In C# 6.0, we can use the null-conditional to invoke a delegate.

                    NameChange?.Invoke(this, name);

#1,204 – C# 6.0 – Using Null-Conditional with Indexer

The new null-conditional operator in C# 6.0 allows checking for null and de-referencing a variable in a single step.

You can use the null-conditional operator with dot (.) notation, to access an object’s properties or to invoke a method.  You can also use the null-conditional with an indexer, as shown below.  The expression returns the value returned by the indexer if the variable is non-null, or returns null if the variable is null.

            string sTest = "Howdy";

            char? thirdChar = sTest?[2];   // 'w'

            sTest = null;
            thirdChar = sTest?[2];   // null

            sTest = "Ho";
            thirdChar = sTest?[2];   // throws IndexOutOfRangeException

Notice that the null-conditional operator protects you from de-referencing a null pointer, but does not protect you from using an index that is longer than the string length.

#1,203 – C# 6.0 – Using the Null-Conditional with Value Types

The new null-conditional allows checking for null and de-referencing a reference-typed variable in a single step.

            string sLeft = sTest?.Substring(0, 1);

If the variable being checked for null (e.g. sTest) is null, the result of the expression is null.  This works as expected if the result of the expression is being assigned to a reference-typed variable (e.g. sLeft).  If the method or property being invoked, however, is a value type, then the result of the expression must be a nullable type.

For example, we might do the following in C# 5.0:

            int sLen;
            if (sTest != null)
                sLen = sTest.Length;

If sTest is null, the assignment isn’t done and sLen retains its value.

In C# 6.0, you can do this assignment using the null-conditional.  The variable being assigned to must be a nullable type whose base type matches the type of the expression.

            int? sLen = sTest?.Length;