Simplifying Conditional Expressions with C# is Operator Pattern Matching (and/or/not)

2025年12月10日

By using the enhanced pattern matching introduced in C# 9.0, you can use the is operator to write value range checks, null checks, and logical operations (AND, OR) in a very intuitive and concise way.

It allows you to reduce repetitive variable descriptions that were previously done using && and ||, making conditions read like English sentences.

In this article, I will explain how to utilize the new is operator, using string classification and numeric range determination as examples.

Implementation Example: Logical and Relational Patterns
Explanation
Summary

Implementation Example: Logical and Relational Patterns

The following code checks if an input character (string) meets specific conditions (letters or delimiters) and performs a null check on an object.

using System;

namespace PatternMatchingExample
{
    class Program
    {
        static void Main(string[] args)
        {
            // 1. String pattern matching
            // Range comparison in dictionary order (>= "a" and <= "z") or checking specific values
            string input = "Q";

            // Traditional way:
            // if ((input >= "a" && input <= "z") || (input >= "A" && input <= "Z") || input == "." || input == ",")
            
            // Way using pattern matching:
            bool isValidChar = input is (>= "a" and <= "z") 
                                     or (>= "A" and <= "Z") 
                                     or "." 
                                     or ",";

            Console.WriteLine($"Is character '{input}' a valid pattern?: {isValidChar}");


            // 2. Numeric range check
            // Define range without repeating variables
            int temperature = 25;
            
            if (temperature is >= 20 and < 30)
            {
                Console.WriteLine("The temperature is comfortable (20 to under 30 degrees).");
            }


            // 3. Null check (not pattern)
            // Can be used instead of "!= null". It is readable and safe as it is unaffected by operator overloading.
            var position = new { X = 10, Y = 20 };

            if (position is not null)
            {
                Console.WriteLine($"Position data exists: X={position.X}, Y={position.Y}");
            }
        }
    }
}

Explanation

1. Logical Patterns

You can combine patterns using the keywords and, or, and not.

and: Matches if both patterns are true (Logical AND).
or: Matches if either pattern is true (Logical OR).
not: Matches if the pattern is false (Negation).

2. Relational Patterns

Comparison operators like <, >, <=, and >= can be used as patterns. This allows range checks, previously written as variable >= min && variable <= max, to be concisely written as variable is >= min and <= max.

3. Null Check Best Practice

is not null is one of the recommended ways to check for null in C# 9.0 and later. Unlike the traditional != null, it purely checks if the reference is not null, even if the != operator is overloaded in the target class, making it safer and more reliable.