String basics
Learn to store text with string and use escape sequences.
Nullable types
Sometimes a number variable needs to be empty. An int cannot hold null, but int? can. The question mark adds nullability to the type.
Example: int? count = null; The value of count is either a whole number or null. The HasValue property tells you if there is a value; Value returns it.
Nullable types are useful for representing 'no value' from a database or external source.
The ?? operator
The ?? operator (null coalescing) gives a default value when a reference or nullable value is null.
Example: int n = count ?? 0; If count is null, n becomes zero; otherwise the value of count is used. This turns a long if check into a short expression.
The right side is used only when the left side is null. It is a clean way to supply defaults when reading settings.
Exercise Print a default from a nullable
Assign null to an int? count variable. Write int total = count ?? 10; to get a default value. Then print total with Console.WriteLine.
Show example solution
int? count = null; int total = count ?? 10; Console.WriteLine(total);
Example solution:
using System;
class Program {
static void Main() {
int? count = null;
int total = count ?? 10;
Console.WriteLine(total);
}
}Test yourself
-
Assign null to an int?
int? count = ____;Show answer
int? count = null; -
What is the value of n when count is null?
int? count = null; int n = count ?? 5;- 0
- null
- throws
- 5
Show answer
5 — Correct! count is null, so ?? returns the right side (5).
-
Match each ?? expression to its result
- null ?? 5
- 5
- 3 ?? 5
- 3
- null ?? 0
- 0
- (int?)null ?? 7
- 7