is / as / Pattern Matching

// Checks the type (returns true/false).
bool result = obj is Type;

// Checks the type and assigns to a variable (pattern variable).
if (obj is Type variable) { ... }

// Safe cast — returns null if the cast fails.
Type variable = obj as Type;

// Pattern matching with a switch expression.
string result = value switch {
	Type1 v => ...,
	Type2 v when condition => ...,
	_ => defaultValue
};

using System;

// Check the type with is.
object value = "Hello";
if (value is string text) {
	Console.WriteLine($"String: {text.ToUpper()}"); // HELLO
}

// Safely cast with as.
object number = 42;
string str = number as string; // No crash even if the cast fails
Console.WriteLine(str == null ? "Cast failed" : str); // Cast failed

// Pattern matching with a switch expression.
object[] items = { 42, "abc", 3.14, true, null };
foreach (object item in items) {
	string description = item switch {
		int n when n > 100 => $"Large integer: {n}",
		int n             => $"Integer: {n}",
		string s          => $"String: {s}",
		double d          => $"Double: {d}",
		bool b            => $"Boolean: {b}",
		null              => "null",
		_                 => "Unknown type"
	};
	Console.WriteLine(description);
}

Since C# 7, is can perform a type check and variable assignment at the same time, so the old pattern of checking with is and then casting separately is no longer needed. Writing if (obj is string s) on a single line is the modern style.

as can be used for reference type casts, but not for value types such as int or structs. If you use the result without checking for null, a null reference exception will be thrown later. Always perform a null check after using as.

For setting a default value when a result is null, see 'Null-coalescing operators ?? / ??='.