New Features in C# 2.0—Nullable Types: What just happened?

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New Features in C# 2.0 Type-Safe List Generic Collection Collection Interfaces Generic Iterators Implement GetEnumerator Anonymous Methods Partial Types Create Static Classes Nullable Types How do I do that? What just happened? What about… Where can I learn more? Global Namespace Limit Properties Access Co- and Contravariance

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What just happened?

Let’s focus on the Main method. Five nullable types are created:

int? myNullableInt = 25;
double? myNullableDouble = 3.14159;
bool? myNullableBool = null; // neither yes nor no
 
// string? myNullableString = "Hello";
// Dog? myNullableDog = new Dog(3); 

The first three are perfectly valid, but you cannot create a nullable string or a nullable user-defined type (class), and thus they should be commented out.

However, structs can be user defined, and it’s OK to use them as nullables.

We check whether each nullable type is null (or, equivalently, whether the HasValue property is true). If so, we print their value (or equivalently, we access their Value property).

After this the value null is assigned to myNullableInt:

myNullableInt = null;

The next line would like to declare an integer and initialize it with the value in myNullableInt, but this is not legal; there is no implicit conversion from a nullable int to a normal int. You can solve this in two ways. The first is with a cast:

b = (int)myNullableInt;

This will compile, but it will throw an exception at runtime if myNullableInt is null (which is why we’ve enclosed it in a try/catch block).

The second way to assign a nullable int to an int is to provide a default value to be used in case the nullable int is null:

int c = myNullableInt ?? -1;

This line reads as follows: "initialize int c with the value in myNullableInt unless myNullableInt is null, in which case initialize c to -1."

Comparison operators always return false if one value is null!

It turns out that all the comparison operators (>, <, <=, etc.) return false if either value is null. Thus, a true value can be trusted:

if (myNullableInt >= c)
{
Console.WriteLine("myNullableInt is greater than or equal to c");
}

W A R N I N G

Note, however, that == will return true if both arguments are null.

If the statement "myNullableInt is greater than or equal to c" displays, you know that myNullableInt is not null, nor is c, and that myNullableInt is greater than c. However, a false value cannot be trusted in the normal fashion:

else
{
  Console.WriteLine("Is myNullableInt less than c?");
}

This else clause can be reached if myNullableInt is less than c, but it can also be reached if either myNullableInt or c is null.