#1,207 – C# 6.0 – Auto-Property Initializers for Read-Only Properties

Prior to C# 6.0, if you wanted a read-only (immutable) property, you’d typically use a read-only backing field that is initialized in the constructor, as shown below.

    public class Dog 
    {
        public string Name { get; set; }

        // DogCreationTime is immutable
        private readonly DateTime creTime;
        public DateTime DogCreationTime 
        {
            get { return creTime; }
        }

        public Dog(string name)
        {
            Name = name;
            creTime = DateTime.Now;
        }
    }

In C# 6.0, you can use auto-implemented properties to implement a read-only property.  You do this by using an auto-property initializer.  The result is much cleaner than the above example, where we had to explicitly declare a backing field.

    public class Dog
    {
        public string Name { get; set; }

        // DogCreationTime is immutable
        public DateTime DogCreationTime { get; } = DateTime.Now;

        public Dog(string name)
        {
            Name = name;
        }
    }

#1,206 – C# 6.0 – Auto-Property Initializers

C# 3.0 introduced auto-implemented properties.

Older syntax:

private string name = "default";
public string Name
{
    get { return name; }
    set { name = value; }
}

Improved syntax, using auto-implemented properties:

public string Name { get; set; }

The new syntax provided no mechanism for specifying a default property value. To specify a default value, you either needed to go back to using an explicit backing variable or to set the default value from within a constructor.

C# 6.0 supports specifying default values for auto-implemented properties, as shown below.

        public string Name { get; set; } = "default";

#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;

#1,202 – C# 6.0 – Null-Conditional Operator

One of the new features coming in C# 6.0 is the null-conditional operator.

If you have a reference-typed variable, it can have a null value or can refer to instance of the appropriate type.  You’ll often see the following pattern, checking for null before invoking a method on the object, to avoid a NullReferenceException.

            string sTest = DateTime.Now.Hour == 5 ? "Five" : null;

            // OLD: Check for null to avoid NullReferenceException
            string sLeft;
            if (sTest != null)
                sLeft = sTest.Substring(0, 1);

The null-conditional operator allows us to do the same check for null, but in a more concise manner.

            // NEW: Null-Conditional operator
            sLeft = sTest?.Substring(0, 1);

If sTest is non-null, the Substring method is called and the result is assigned to sLeft.  If sTest is null, the expression returns null, so a null value is assigned to sLeft.

#1,101 – Mathematical Constants

For your convenience, the System.Math class provides two useful mathematical constants, as fields.

  • Math.E – base of the natural logarithm, or Euler’s Number e, with value of approximately 2.71828
  • Math.PI – ratio of circle’s circumference to its diameter π, with value of approximately 3.14159

If you need to use either of these constants in your code, you can use the corresponding field in the Math class.

            double radius = 5.0;
            double circumference = 2.0 * Math.PI * radius;

            Console.Write("Radius={0}, Circumference={1}", radius, circumference);

1101-001

#984 – The Birthday Problem

To celebrate today’s Thanksgiving holiday in the United States, the post today is just a fun little code fragment for exploring the birthday problem.  The idea of the birthday problem is to look at the probability of having at least one shared birthday in a group of n people.

Instead of calculating the actual probability of a collision, the code below empirically creates a group of people, assigns them birthdays, and then looks for shared birthdays.  It does this 100,000 times and then reports what percentage of time there actually was a match.

        static void Main(string[] args)
        {
            while (true)
            {
                Console.Write("Enter # people: ");
                int numPeople = int.Parse(Console.ReadLine());

                const double NumTrials = 100000.0;

                int numBirthdayMatches = 0;
                double numUniqueBirthdaySum = 0;

                List<int> personBirthdays;
                Random rnd = new Random();

                for (int trialNum = 1; trialNum <= NumTrials; trialNum++)
                {
                    // Generate birthdays
                    personBirthdays = new List<int>(numPeople);
                    for (int personNum = 1; personNum <= numPeople; personNum++)
                        personBirthdays.Add(rnd.Next(1, 366));

                    if (personBirthdays.Count != personBirthdays.Distinct().Count())
                        numBirthdayMatches++;

                    numUniqueBirthdaySum += personBirthdays.Distinct().Count();
                }

                double percentMatched =
                    numBirthdayMatches / NumTrials * 100.0;
                double numUniquePerTrial = numUniqueBirthdaySum / NumTrials;
                Console.WriteLine(string.Format("For {0} people, there was at least one matching birthday {1}% of the time.  Avg # unique birthdays={2}",
                    numPeople,
                    percentMatched,
                    numUniquePerTrial));
            }

            Console.ReadLine();
        }

984-001

#947 – Specifying Lazy Instantiation Using a Lambda Expression

When you are declaring an object that will be lazily instantiated, you pass a delegate instance to the Lazy<T> constructor, where the delegate instance indicates a method to be called that will instantiate the underlying object.

For example, you could do the following:

        private static Lazy<Dog> dog1 = new Lazy<Dog>(CreateDefaultDog);
        private static Dog CreateDefaultDog()
        {
            return new Dog("Lassie");
        }

Very often, however, you’ll want to pass some parameter value to the delegate or just invoke one of the normal constructors for the object, passing one or more parameters to the constructor.  You can do this by just using a lambda expression, as follows:

        private static Lazy<Dog> dog2 = new Lazy<Dog>(() => new Dog("Lassie"));

#946 – Specifying Whether Lazy Instantiated Object Should Be Thread-Safe

You can use the Lazy<T> class to declare an object that will be lazily instantiated–that is, created just before it is used.

In the example below, we indicate that the listOfAllFamousDogs object should only be created (by calling GenerateBigListOfFamousDogs) just before it is referenced.  The GenerateBigListOfFamousDogs method returns an instance of List<Dog>.

private static Lazy<List<Dog>> listOfAllFamousDogs = new Lazy<List<Dog>> (GenerateBigListOfFamousDogs);

In the Lazy<T> constructor, we specify a delegate that will be called to initialize our object.  By default, this results in a thread-safe implementation that lazily instantiates the specified object.

If we do not need thread-safety, we can pass a second parameter to the Lazy<T> constructor, with a value of false.

        // No need for thread safety
        private static Lazy<List<Dog>> listOfAllFamousDogs = new Lazy<List<Dog>> (GenerateBigListOfFamousDogs, false);