C# nullable values, patterns, and switch expressions

The concept

A value type such as int cannot normally be null. int? is a nullable int that either holds a value or is null. HasValue tests which, and the null-coalescing operator ?? supplies a default. The null-conditional operator ?. stops and gives null if the object is null, so text?.Length is null for a null string instead of throwing.

Pattern matching tests the shape of a value. A switch expression returns a value from the first matching arm: type patterns like int n, constant patterns like null, and a when clause for extra conditions. The _ arm matches anything else. Arms are checked in order, so more specific arms must come first. Tuples let a method return several values, which the caller can unpack with deconstruction.

Code example

using System;
using System.Globalization;

class Program {
  static (int Min, int Max) MinMax(int[] values) {
    int min = values[0], max = values[0];
    foreach (int v in values) {
      if (v < min) min = v;
      if (v > max) max = v;
    }
    return (min, max);
  }

  static void Main() {
    int? maybe = null;
    Console.WriteLine(maybe.HasValue + " " + (maybe ?? -1));
    string text = null;
    Console.WriteLine(text?.Length ?? 0);

    object[] things = { 42, 7, "hello", 3.5, null, new[] { 1, 2, 3 } };
    foreach (object thing in things) {
      string description = thing switch {
        int n when n > 40 => "big int " + n,
        int n => "int " + n,
        string s => "string of length " + s.Length,
        double x => "double " + x.ToString(CultureInfo.InvariantCulture),
        null => "null",
        int[] array => "array with " + array.Length + " items",
        _ => "something else",
      };
      Console.WriteLine(description);
    }

    var (min, max) = MinMax(new[] { 4, 9, 2, 7 });
    Console.WriteLine(min + " " + max);
  }
}

False -1
0
big int 42
int 7
string of length 5
double 3.5
null
array with 3 items
2 9

Follow the code

Open the walkthrough to inspect the example one step at a time.

—

False -1

False -1
0

False -1
0

False -1
0

False -1
0
big int 42
int 7
string of length 5
double 3.5
null
array with 3 items

False -1
0
big int 42
int 7
string of length 5
double 3.5
null
array with 3 items

False -1
0
big int 42
int 7
string of length 5
double 3.5
null
array with 3 items
2 9

Behind the scenes

Cost and common mistakes

Each operation is O(1); MinMax is O(n).

• A switch expression with no matching arm throws a SwitchExpressionException at run time. Add _ unless the compiler proves the cases are complete.
• A nullable reference type annotation (string?) is a compile-time warning system. It does not change run-time behaviour.
• Reading maybe.Value when it has no value throws InvalidOperationException. Check HasValue or use ??.

Try it yourself

What does maybe.GetValueOrDefault() return when maybe is null?

Further reading

• C# documentation (external)
• .NET API browser (external)