Nullable<T> / Nullable Types

// Declare a nullable int.
int? variable = null;
Nullable<int> variable = null; // Same as above.

// Get the value if it is not null.
int value = variable ?? defaultValue;
int value = variable.GetValueOrDefault(defaultValue);

// Check whether the value is null.
bool result = variable.HasValue;

// Retrieve the value (throws an exception if null).
int value = variable.Value;

using System;

// Use int? to handle a nullable integer.
int? score = null;
Console.WriteLine(score.HasValue);              // False
Console.WriteLine(score.GetValueOrDefault(-1)); // -1

score = 85;
Console.WriteLine(score.HasValue);  // True
Console.WriteLine(score.Value);     // 85
Console.WriteLine(score ?? 0);      // 85

// You can perform arithmetic directly on nullable types.
int? a = 10;
int? b = null;
int? total = a + b; // If either operand is null, the result is also null.
Console.WriteLine(total.HasValue); // False

// Use DateTime? to represent an unset date.
DateTime? birthDate = null;
if (!birthDate.HasValue) {
	Console.WriteLine("Birth date is not set.");
}

birthDate = new DateTime(1990, 4, 1);
Console.WriteLine($"Birth date: {birthDate?.ToString("yyyy/MM/dd")}"); // Use ?. for safe access

// Comparing nullable types.
int? x = 5;
int? y = null;
Console.WriteLine(x > 3);  // True
Console.WriteLine(y > 3);  // False (comparisons with null always return false)
Console.WriteLine(y == null); // True

T? (Nullable<T>) is useful for representing database NULL values or the "not entered" state. For example, you can use null for "no answer" in a survey or "not set" in a date field.

To convert a nullable value to a regular value type, either specify an explicit default with ?? 0 or use .GetValueOrDefault(). Calling .Value without checking first throws an InvalidOperationException.

For null-safe access, also see null-coalescing operators ?? / ??=.