Nullable Types and Pattern Matching

Q: Can int? and int be used in arithmetic directly?

No. You need to convert to a non-null value using .Value or ?? before operating.

Q: What is the difference between string? and string ?

string? allows assigning null, while string will produce a compiler warning when assigned null with nullable enabled.

Nullable Value Types

Value types cannot be null by default. The Nullable<T> struct (shorthand T?) allows value types to carry null semantics.

CSHARP

int? a = 42;
int? b = null;
bool? flag = null;
double? price = 9.99;

Console.WriteLine(a.HasValue);
Console.WriteLine(b.HasValue);
Console.WriteLine(flag.GetValueOrDefault());
Console.WriteLine(price.GetValueOrDefault(0.0));

TEXT

True
False
False
9.99

Example

CSHARP

int? score = null;

Console.WriteLine(score.HasValue);
Console.WriteLine(score.GetValueOrDefault());
Console.WriteLine(score.GetValueOrDefault(60));

score = 88;
Console.WriteLine(score.Value);

▶ Try it Yourself

TEXT

False
0
60
88

Null Coalescing and Null Conditional Operators

?? provides a default value, ?. safely accesses members, and ! tells the compiler "this is not null here."

CSHARP

int? x = null;
int y = x ?? 0;

string? name = null;
int? len = name?.Length;

string s = null!;
Console.WriteLine(s is null);

TEXT

True

Example

CSHARP

string? input = null;

string display = input ?? "(empty)";
Console.WriteLine(display);

int? length = input?.Length;
Console.WriteLine(length ?? -1);

string? city = "Beijing";
Console.WriteLine(city?.Length ?? 0);

▶ Try it Yourself

TEXT

(empty)
-1
7

Nullable Reference Types

Starting with C# 8, you can use #nullable enable to turn on nullable reference type warnings. string? allows null, string does not.

CSHARP

#nullable enable
string? maybeNull = null;
string notNull = "hello";

maybeNull = null;
notNull = null!;

Example

CSHARP

#nullable enable
string Greet(string? name)
{
    return name is null ? "Hello, stranger" : $"Hello, {name}";
}

Console.WriteLine(Greet(null));
Console.WriteLine(Greet("Alice"));

▶ Try it Yourself

TEXT

Hello, stranger
Hello, Alice

Null Check Strategies

Strategy	Usage	Scenario
Explicit check	`if (x is null)`	Branch logic needed
Null coalescing	`x ?? defaultValue`	Providing a default value
Null conditional	`x?.Member`	Safe member access

Example

CSHARP

string? GetName() => null;

string? name = GetName();

if (name is null)
{
    Console.WriteLine("Name is null");
}

string upper = name?.ToUpper() ?? "DEFAULT";
Console.WriteLine(upper);

▶ Try it Yourself

TEXT

Name is null
DEFAULT

Introduction to Pattern Matching

Pattern matching combines type checking and variable extraction into one, commonly used with is and switch.

is Pattern

CSHARP

object obj = 42;

if (obj is int i)
{
    Console.WriteLine($"Is integer: {i}");
}

TEXT

Is integer: 42

Example

CSHARP

object value = "hello";

if (value is int n)
{
    Console.WriteLine(n);
}
else if (value is string s)
{
    Console.WriteLine($"String length: {s.Length}");
}

▶ Try it Yourself

TEXT

String length: 5

Type Pattern and switch

Branch by type in switch and extract variables.

Example

CSHARP

string Describe(object obj)
{
    return obj switch
    {
        int i => $"Integer {i}",
        double d => $"Double {d}",
        string s => $"String \"{s}\"",
        bool b => $"Boolean {b}",
        null => "null",
        _ => "Unknown type"
    };
}

Console.WriteLine(Describe(10));
Console.WriteLine(Describe(3.14));
Console.WriteLine(Describe("hi"));
Console.WriteLine(Describe(true));

▶ Try it Yourself

TEXT

Integer 10
Double 3.14
String "hi"
Boolean True

Property Pattern

Property patterns match by object properties without manually extracting values for comparison.

Example

CSHARP

var person = new { Name = "Alice", Age = 20 };

if (person is { Age: > 18 })
{
    Console.WriteLine("Adult");
}

string Category(object p) => p switch
{
    { Age: < 12 } => "Child",
    { Age: >= 12 and < 18 } => "Teenager",
    { Age: >= 18 } => "Adult",
    _ => "Unknown"
};

Console.WriteLine(Category(person));

▶ Try it Yourself

TEXT

Adult
Adult

Record Type

C# 9 introduced record, which provides value-based equality comparison and immutability by default.

CSHARP

record Person(string Name, int Age);

Example

CSHARP

record Person(string Name, int Age);

var p1 = new Person("Alice", 18);
var p2 = new Person("Alice", 18);

Console.WriteLine(p1 == p2);
Console.WriteLine(ReferenceEquals(p1, p2));
Console.WriteLine(p1);

▶ Try it Yourself

TEXT

True
False
Person { Name = Alice, Age = 18 }

with Expression

with creates a copy of an existing record with some properties modified, leaving the original instance unchanged.

Example

CSHARP

record Student(string Name, int Age, string Grade);

var s1 = new Student("Bob", 20, "A");
var s2 = s1 with { Age = 21, Grade = "A+" };

Console.WriteLine(s1);
Console.WriteLine(s2);

▶ Try it Yourself

TEXT

Student { Name = Bob, Age = 20, Grade = A }
Student { Name = Bob, Age = 21, Grade = A+ }

Tuple

Tuples combine multiple values into a lightweight structure, supporting named elements and deconstruction.

Example

CSHARP

(int Id, string Name) person = (1, "Charlie");
Console.WriteLine(person.Id);
Console.WriteLine(person.Name);

var coords = (X: 3.0, Y: 4.0);
double distance = Math.Sqrt(coords.X * coords.X + coords.Y * coords.Y);
Console.WriteLine(distance);

(int x, int y) = (10, 20);
Console.WriteLine($"x={x}, y={y}");

▶ Try it Yourself

TEXT

1
Charlie
5
x=10, y=20

Tuple Deconstruction and Pattern Matching

Combining tuples with pattern matching elegantly handles multi-condition branches.

Example

CSHARP

string Classify(int score, bool hasBonus) => (score, hasBonus) switch
{
    (>= 90, true) => "Excellent+Bonus",
    (>= 90, false) => "Excellent",
    (>= 60, true) => "Pass+Bonus",
    (>= 60, false) => "Pass",
    _ => "Fail"
};

Console.WriteLine(Classify(95, true));
Console.WriteLine(Classify(75, false));
Console.WriteLine(Classify(50, true));

▶ Try it Yourself

TEXT

Excellent+Bonus
Pass
Fail

❓ FAQ

Q Can int? and int be used in arithmetic directly?

A No. You need to convert to a non-null value using .Value or ?? before operating.

Q What is the difference between string? and string?

A string? allows assigning null, while string will produce a compiler warning when assigned null with nullable enabled.

Q What is the core difference between record and class?

A record uses value-based equality (== compares content), while class uses reference-based equality (== compares memory address).

Q Can the with expression be used with class?

A No. with only works with record types or structs with init-only properties.

Q Is there a limit on the number of tuple elements?

A ValueTuple supports up to 8 type parameters; beyond that, nested TRest is used for extension.

📖 Summary

T? is Nullable<T>, allowing value types to be null
HasValue/Value/GetValueOrDefault() access nullable values
??, ?., ! handle null scenarios
#nullable enable turns on nullable reference type checking
is pattern, type pattern, and property pattern simplify type checking
switch expressions with pattern matching concisely express multi-branch logic
record provides value equality and immutability
with creates record copies with some properties modified
Tuples lightweightly combine multiple values, supporting naming and deconstruction

📝 Exercises

Declare double? price = null;, use GetValueOrDefault(99.9) to get the default value and output it
Write a method that accepts a string? parameter, uses ?. and ?? to return the string length or -1
Use the is pattern to check if object obj = 3.14; is double d, and output d * 2
Define record Book(string Title, double Price);, create an instance, and use with to modify the price
Use a tuple switch expression to check (int month, int day) for whether it is "New Year's Day" or "National Day"

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Nullable Types and Pattern Matching

Nullable Value Types

Example

Null Coalescing and Null Conditional Operators

Example

Nullable Reference Types

Example

Null Check Strategies

Example

Introduction to Pattern Matching

is Pattern

Example

Type Pattern and switch

Example

Property Pattern

Example

Record Type

Example

with Expression

Example

Tuple

Example

Tuple Deconstruction and Pattern Matching

Example

❓ FAQ

📖 Summary

📝 Exercises

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