Is-keyword

This C# cast returns true if it succeeds. It returns false if the cast fails. This operator can be used to capture a cast variable.

This operator is useful when checking types in if-statements and expressions. We can use "is" for pattern-matching—it can capture the cast value.

Example

The is-operator is used to create an expression. It returns a boolean result of true or false. This is compatible with the if-statement, which requires a boolean context.

Info The is-operator evaluates the entire class derivation chain when it is applied. The string is both an object and string.

Here The first 2 if-statement bodies are reached. Their expressions are evaluated to the true value: the casts succeed.

Then The third if-statement body is not fully evaluated. The string is not a StringBuilder type.

using System;
using System.Text;

// Create a string variable and cast it to an object.
string value1 = "Example";
object value2 = value1;

// Apply the is-operator with 3 different parameters.
if (value2 is object)
{
    Console.WriteLine("is object");
}
if (value2 is string)
{
    Console.WriteLine("is string");
}
if (value2 is StringBuilder)
{
    Console.WriteLine("is StringBuilder"); // Not reached.
}is object
is string

Is, pattern matching

Here we see a separate syntax form for the "is" operator. We have an object, and we want to cast it to a string. We use pattern matching for this.

Here The is-cast tests to see if the variable "wrapper" is a string. If it is a string, a string local called temp2 is introduced.

Important This use of the is-cast combines the as-cast with the is-cast. This is probably the best way to cast C# variables.

string temp = "ancient ship";
object wrapper = temp;

// Use pattern matching to cast the object to a variable.
// ... Wrapper is an object.
// ... Temp2 is a string reference.
if (wrapper is string temp2)
{
    System.Console.WriteLine($"STRING = {temp2}, LENGTH = {temp2.Length}");
}
STRING = ancient ship, LENGTH = 12

Is, list matching

In newer versions of C#, we can use the is-keyword to perform pattern matching. So we can test the entire contents of a List with an is-statement.

Tip This is not casting, but is instead checking the entire contents of a List. It uses the same "is" operator.

using System;
using System.Collections.Generic;

List<int> values = [100, 200, 900];

// The is-keyword matches this list.
if (values is [100, 200, 900])
{
    Console.WriteLine("SAME LIST");
}

// This check evaluates to false.
if (values is [-100, -1000])
{
    Console.WriteLine("???");
}
SAME LIST

Summary

The is-operator is exception-neutral. It is elegantly used in an if-statement expression—it is evaluated to a boolean value. We examined the instruction-level code.