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<!DOCTYPE html><html><head><title>Kotlin is like C#</title><meta charset="utf-8"><meta name="description" content="Kotlin vs C#. Compare the syntax of Kotlin and C# through short code examples."><link rel="icon" href="https://kotlinlang.org/assets/images/favicon.ico" type="image/x-icon"><link rel="stylesheet" href="https://cdnjs.cloudflare.com/ajax/libs/highlight.js/9.11.0/styles/github.min.css"><style>body {
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</style><script src="https://cdnjs.cloudflare.com/ajax/libs/highlight.js/9.11.0/highlight.min.js"></script><script src="https://cdnjs.cloudflare.com/ajax/libs/highlight.js/9.11.0/languages/kotlin.min.js"></script><script src="https://cdnjs.cloudflare.com/ajax/libs/highlight.js/9.11.0/languages/cs.min.js"></script><script>hljs.initHighlightingOnLoad();</script></head><body><a target="_blank" href="https://github.com/ttu/kotlin-is-like-csharp"><img id="fork-me" src="https://github.blog/wp-content/uploads/2008/12/forkme_right_red_aa0000.png?resize=149%2C149"></a><div id="note">Kotlin is like C#</div><div class="disclaimer">Compare the syntax of Kotlin vs C#. Don't take language likeness comparison too seriously.</div><div class="disclaimer">Fixes, improvents and additions are welcome. Open an issue or a pull request.</div><div class="section"><div class="title">BASICS</div><div class="case"><div class="name">Hello World</div><div class="pair"><div class="card"><div class="lang">Kotlin</div><pre class="code"><code>println("Hello, world!")
</code></pre></div><div class="card"><div class="lang">C#</div><pre class="code"><code>public void Main()
{
Console.WriteLine("Hello, world!");
}
// C# 9 supports top-level statements
Console.WriteLine("Hello, world!");</code></pre></div></div></div><div class="case"><div class="name">Variables and Constants</div><div class="pair"><div class="card"><div class="lang">Kotlin</div><pre class="code"><code>var myVariable = 42
myVariable = 50
val myConstant = 42
</code></pre></div><div class="card"><div class="lang">C#</div><pre class="code"><code>var myVariable = 42;
myVariable = 50;
// C# doesn't have local runtime constants
</code></pre></div></div></div><div class="case"><div class="name">Explicit Types</div><div class="pair"><div class="card"><div class="lang">Kotlin</div><pre class="code"><code>val explicitDouble: Double = 70.0
</code></pre></div><div class="card"><div class="lang">C#</div><pre class="code"><code>double explicitDouble = 70.0;
</code></pre></div></div></div><div class="case"><div class="name">Type Coercion</div><div class="pair"><div class="card"><div class="lang">Kotlin</div><pre class="code"><code>val label = "The width is "
val width = 94
val widthLabel = label + width
// The width is 94</code></pre></div><div class="card"><div class="lang">C#</div><pre class="code"><code>var label = "The width is ";
var width = 94;
var widthLabel = label + width;
// The width is 94</code></pre></div></div></div><div class="case"><div class="name">Compile Time Constants</div><div class="pair"><div class="card"><div class="lang">Kotlin</div><pre class="code"><code>const val SYSTEM_DEPRECATED: String = "System is deprecated"</code></pre></div><div class="card"><div class="lang">C#</div><pre class="code"><code>const string SYSTEM_DEPRECATED = "System is deprecated";</code></pre></div></div></div><div class="case"><div class="name">String Interpolation</div><div class="pair"><div class="card"><div class="lang">Kotlin</div><pre class="code"><code>val apples = 3
val oranges = 5
val fruitSummary = "I have ${apples + oranges} " +
"pieces of fruit."
</code></pre></div><div class="card"><div class="lang">C#</div><pre class="code"><code>var apples = 3;
var oranges = 5;
var fruitSummary = $"I have {apples + oranges} " +
"pieces of fruit.";
</code></pre></div></div></div><div class="case"><div class="name">If Expression / Statement</div><div class="pair"><div class="card"><div class="lang">Kotlin</div><pre class="code"><code>val age = 42
if (age < 10) {
println("You're too young to watch this movie")
} else if (age < 13) {
println("You can watch this movie with a parent")
} else {
println("You can watch this movie")
}</code></pre></div><div class="card"><div class="lang">C#</div><pre class="code"><code>var age = 42;
if (age < 10)
{
Console.WriteLine("You're too young to watch this movie");
}
else if (age < 13)
{
Console.WriteLine("You can watch this movie with a parent");
}
else
{
Console.WriteLine("You can watch this movie");
}</code></pre></div></div></div><div class="case"><div class="name">Conditionals</div><div class="pair"><div class="card"><div class="lang">Kotlin</div><pre class="code"><code>// if is an expression, so ternary operation not needed
val loaded = true
val status = if (loaded) "Ready" else "Loading..."
// "Ready</code></pre></div><div class="card"><div class="lang">C#</div><pre class="code"><code>var loaded = true;
var status = loaded ? "Ready" : "Loading...";
// "Ready</code></pre></div></div></div></div><div class="section"><div class="title">FUNCTIONS</div><div class="case"><div class="name">Functions</div><div class="pair"><div class="card"><div class="lang">Kotlin</div><pre class="code"><code>fun greet(name: String, day: String): String {
return "Hello $name, today is $day."
}
val text = greet("Bob", "Tuesday")
// Hello Bob, today is Tuesday.</code></pre></div><div class="card"><div class="lang">C#</div><pre class="code"><code>string Greet(string name, string day)
{
return $"Hello {name}, today is {day}.";
}
var text = Greet("Bob", "Tuesday");
// Hello Bob, today is Tuesday</code></pre></div></div></div><div class="case"><div class="name">Single Expression Functions</div><div class="pair"><div class="card"><div class="lang">Kotlin</div><pre class="code"><code>// Single expression functions can be without braces and return type
fun double(value: Int) = value * 2
val result = double(4)
// 8</code></pre></div><div class="card"><div class="lang">C#</div><pre class="code"><code>// Single expression functions can be without braces
int Double(int value) => value * 2;
var result = Double(4);
// 8</code></pre></div></div></div><div class="case"><div class="name">Named Arguments</div><div class="pair"><div class="card"><div class="lang">Kotlin</div><pre class="code"><code>fun area(width: Int, height: Int) = width * height
var result = area(width = 2, height = 3)
// This is also possible with named arguments
result = area(2, height = 2)
result = area(height = 3, width = 2)
</code></pre></div><div class="card"><div class="lang">C#</div><pre class="code"><code>int Area(int width, int height) => width * height;
var result = Area(width: 2, height: 3);
// This is also possible with named arguments
result = Area(2, height: 2);
result = Area(height: 3, width: 2);</code></pre></div></div></div><div class="case"><div class="name">Default Arguments</div><div class="pair"><div class="card"><div class="lang">Kotlin</div><pre class="code"><code>fun displayGreeting(message: String, name: String = "Guest") {
println("Hello $name, $message")
}
displayGreeting("welcome!")
// Hello Guest, welcome!</code></pre></div><div class="card"><div class="lang">C#</div><pre class="code"><code>void DisplayGreeting(string message, string name = "Guest")
{
Console.WriteLine($"Hello {name}, {message}");
}
DisplayGreeting("welcome!");
// Hello Guest, welcome!</code></pre></div></div></div><div class="case"><div class="name">Variable Number of Arguments</div><div class="pair"><div class="card"><div class="lang">Kotlin</div><pre class="code"><code>fun sumOf(vararg numbers: Int): Int {
var sum = 0
for (number in numbers) {
sum += number
}
return sum
}
val sum = sumOf(42, 597, 12)
// sumOf() can also be written in a shorter way:
fun sumOf(vararg numbers: Int) = numbers.sum()
</code></pre></div><div class="card"><div class="lang">C#</div><pre class="code"><code>int SumOf(params int[] numbers)
{
var sum = 0;
foreach (var number in numbers)
sum += number;
return sum;
}
var sum = SumOf(42, 597, 12);
// SumOf() can also be written in a shorter way:
int SumOf(params int[] numbers) => numbers.Sum();</code></pre></div></div></div><div class="case"><div class="name">Lambdas</div><div class="pair"><div class="card"><div class="lang">Kotlin</div><pre class="code"><code>fun containsEven(numbers: List<Int>) = numbers.any { it % 2 == 0 }</code></pre></div><div class="card"><div class="lang">C#</div><pre class="code"><code>bool ContainsEven(List<int> numbers) => numbers.Any(e => e % 2 == 0);</code></pre></div></div></div><div class="case"><div class="name">Higher-Order Functions - Return a Function</div><div class="pair"><div class="card"><div class="lang">Kotlin</div><pre class="code"><code>fun makeIncrementer(): (Int) -> Int {
val addOne = fun(number: Int): Int {
return 1 + number
}
return addOne
}
val increment = makeIncrementer()
val result = increment(7)
// makeIncrementer can also be written in a shorter way:
fun makeIncrementer() = fun(number: Int) = 1 + number
</code></pre></div><div class="card"><div class="lang">C#</div><pre class="code"><code>Func<int, int> MakeIncrementer()
{
int addOne(int number) => 1 + number;
return addOne;
}
var increment = MakeIncrementer();
var result = increment(7);
// MakeIncrementer can also be written in a shorter way:
Func<int, int> MakeIncrementer() => i => 1 + i;
</code></pre></div></div></div><div class="case"><div class="name">HOF - Function as Parameter</div><div class="pair"><div class="card"><div class="lang">Kotlin</div><pre class="code"><code>fun transform(initial: String, f: (String) -> String) = f(initial)
val result = transform("hello", { x -> x.toUpperCase() })
// HELLO
// Trailing lambda can be placed outside the parentheses
val result2 = transform("hello") { x -> x.toUpperCase() }</code></pre></div><div class="card"><div class="lang">C#</div><pre class="code"><code>string Transform(string initial, Func<string, string> f) => f(initial);
var result = Transform("hello", x => x.ToUpper());
// HELLO</code></pre></div></div></div><div class="case"><div class="name">Tuple Return</div><div class="pair"><div class="card"><div class="lang">Kotlin</div><pre class="code"><code>// Kotlin doesn't have tuples, use data classes
data class GasPrices(val a: Double, val b: Double, val c: Double)
fun getGasPrices() = GasPrices(3.59, 3.69, 3.79)
val prices = getGasPrices();
val (a, b, c) = getGasPrices();</code></pre></div><div class="card"><div class="lang">C#</div><pre class="code"><code>(double a, double b, double c) GetGasPrices() => (3.59, 3.69, 3.79);
var result = GetGasPrices();
var (a, b, c) = GetGasPrices();</code></pre></div></div></div></div><div class="section"><div class="title">COLLECTIONS</div><div class="case"><div class="name">Arrays</div><div class="pair"><div class="card"><div class="lang">Kotlin</div><pre class="code"><code>val shoppingList = arrayOf("catfish", "water",
"tulips", "blue paint")
shoppingList[1] = "bottle of water"
</code></pre></div><div class="card"><div class="lang">C#</div><pre class="code"><code>var shoppingList = new[] { "catfish", "water",
"tulips", "blue paint" };
shoppingList[1] = "bottle of water";</code></pre></div></div></div><div class="case"><div class="name">Lists</div><div class="pair"><div class="card"><div class="lang">Kotlin</div><pre class="code"><code>val shoppingList = listOf("catfish", "water",
"tulips", "blue paint")
val shoppingListMutable = mutableListOf("catfish", "water",
"tulips", "blue paint")
shoppingListMutable[1] = "bottle of water"
shoppingListMutable.add("bucket")
</code></pre></div><div class="card"><div class="lang">C#</div><pre class="code"><code>IReadOnlyList<string> shoppingList = new List<string> { "catfish",
"water", "tulips", "blue paint" };
var shoppingListMutable = new List<string> { "catfish", "water",
"tulips", "blue paint" };
shoppingListMutable[1] = "bottle of water";
shoppingListMutable.Add("bucket");</code></pre></div></div></div><div class="case"><div class="name">Maps / Dictionaries</div><div class="pair"><div class="card"><div class="lang">Kotlin</div><pre class="code"><code>val occupations = mapOf(
"Malcolm" to "Captain",
"Kaylee" to "Mechanic"
)
val occupationsMutable = mutableMapOf(
"Malcolm" to "Captain",
"Kaylee" to "Mechanic"
)
occupationsMutable["Jayne"] = "Public Relations"
occupationsMutable.put("Rick", "Navigation")
</code></pre></div><div class="card"><div class="lang">C#</div><pre class="code"><code>IReadOnlyDictionary<string,string> occupations =
new Dictionary<string, string>
{
["Malcolm"] = "Captain",
["Kaylee"] = "Mechanic"
};
var occupationsMutable = new Dictionary<string, string>
{
["Malcolm"] = "Captain",
["Kaylee"] = "Mechanic"
};
occupationsMutable["Jayne"] = "Public Relations";
occupationsMutable.Add("Rick", "Navigation");
</code></pre></div></div></div><div class="case"><div class="name">Empty Collections</div><div class="pair"><div class="card"><div class="lang">Kotlin</div><pre class="code"><code>val emptyList = mutableListOf<String>()
val emptyMap = mutableMapOf<String, Float>()
// read-only empty list
val empty = emptyList<String>()</code></pre></div><div class="card"><div class="lang">C#</div><pre class="code"><code>var emptyList = new List<string>();
var emptyDictionary = new Dictionary<string, float>();
// read-only empty list
var empty = Enumerable.Empty<string>();</code></pre></div></div></div><div class="case"><div class="name">ForEach</div><div class="pair"><div class="card"><div class="lang">Kotlin</div><pre class="code"><code>val names = arrayOf("Anna", "Alex", "Brian", "Jack")
for (name in names) {
println("Person is called $name")
}
names.forEach { println("Person is called $it") }
// Person is called Anna
// Person is called Alex
// Person is called Brian
// Person is called Jack</code></pre></div><div class="card"><div class="lang">C#</div><pre class="code"><code>var names = new List<string> { "Anna", "Alex", "Brian", "Jack" };
foreach (var name in names)
{
Console.WriteLine($"Person is called {name}");
}
names.ForEach(name => Console.WriteLine($"Person is called {name}"));
// Person is called Anna
// Person is called Alex
// Person is called Brian
// Person is called Jack</code></pre></div></div></div><div class="case"><div class="name">Range Operator</div><div class="pair"><div class="card"><div class="lang">Kotlin</div><pre class="code"><code>val names = arrayOf("Anna", "Alex", "Brian", "Jack")
val count = names.count()
for (i in 0..count - 1) {
println("Person ${i + 1} is called ${names[i]}")
}
// Person 1 is called Anna
// Person 2 is called Alex
// Person 3 is called Brian
// Person 4 is called Jack
</code></pre></div><div class="card"><div class="lang">C#</div><pre class="code"><code>var names = new[] { "Anna", "Alex", "Brian", "Jack" };
var count = names.Count();
foreach (var i in Enumerable.Range(0, count))
{
Console.WriteLine($"Person {i + 1} is called {names[i]}");
}
// Person 1 is called Anna
// Person 2 is called Alex
// Person 3 is called Brian
// Person 4 is called Jack
</code></pre></div></div></div><div class="case"><div class="name">Inclusive Range Operator</div><div class="pair"><div class="card"><div class="lang">Kotlin</div><pre class="code"><code>for (index in 1..5) {
println("$index times 5 is ${index * 5}")
}
// 1 times 5 is 5
// 2 times 5 is 10
// 3 times 5 is 15
// 4 times 5 is 20
// 5 times 5 is 25
</code></pre></div><div class="card"><div class="lang">C#</div><pre class="code"><code>foreach (var index in Enumerable.Range(1, 5))
{
Console.WriteLine($"{index} times 5 is {index * 5}");
}
// 1 times 5 is 5
// 2 times 5 is 10
// 3 times 5 is 15
// 4 times 5 is 20
// 5 times 5 is 25
</code></pre></div></div></div><div class="case"><div class="name">Collection Range and Index</div><div class="pair"><div class="card"><div class="lang">Kotlin</div><pre class="code"><code>val names = arrayOf("Anna", "Alex", "Brian", "Jill", "Jack")
val count = names.count()
for (name in names.slice(1 until count - 1)) {
println("Person is called $name")
}
// Person is called Alex
// Person is called Brian
// Person is called Jill</code></pre></div><div class="card"><div class="lang">C#</div><pre class="code"><code>var names = new[] { "Anna", "Alex", "Brian", "Jill", "Jack" };
foreach (var name in names[1..^1])
{
Console.WriteLine($"Person is called {name}");
}
// Person is called Alex
// Person is called Brian
// Person is called Jill
</code></pre></div></div></div></div><div class="section"><div class="title">COLLECTION OPERATIONS / LINQ</div><div class="case"><div class="name">Map</div><div class="pair"><div class="card"><div class="lang">Kotlin</div><pre class="code"><code>val numbers = listOf(20, 19, 7, 12)
val multiplied = numbers.map { 3 * it }
// [ 60, 57, 21, 36 ]
</code></pre></div><div class="card"><div class="lang">C#</div><pre class="code"><code>var numbers = new[] { 20, 19, 7, 12 };
var multiplied = numbers.Select(e => 3 * e);
// [ 60, 57, 21, 36 ]</code></pre></div></div></div><div class="case"><div class="name">Sort</div><div class="pair"><div class="card"><div class="lang">Kotlin</div><pre class="code"><code>val ordered = listOf(1, 5, 3, 12, 2).sorted()
// [ 1, 2, 3, 5, 12 ]</code></pre></div><div class="card"><div class="lang">C#</div><pre class="code"><code>var ordered = new[] { 1, 5, 3, 12, 2 }.OrderBy(i => i);
// [ 1, 2, 3, 5, 12 ]</code></pre></div></div></div><div class="case"><div class="name">Filter / GroupBy / Average</div><div class="pair"><div class="card"><div class="lang">Kotlin</div><pre class="code"><code>var datas = listOf(
SensorData(1, "A", 2.89),
SensorData(2, "B", 12.01),
SensorData(3, "B", 11.89),
SensorData(4, "A", 3.11),
SensorData(5, "A", -456.0)
)
var avgs = datas
.filter { it.value > -50.0 }
.groupBy { it.location }
.map { g ->
Location(g.key,
g.value.map { it.value }.average()) }
// (location=A, value=3.0)
// (location=B, value=11.95)</code></pre></div><div class="card"><div class="lang">C#</div><pre class="code"><code>var datas = new List<SensorData>
{
new SensorData { Id = 1, Location = "A", Value = 2.89 },
new SensorData { Id = 2, Location = "B", Value = 12.01 },
new SensorData { Id = 3, Location = "B", Value = 11.89 },
new SensorData { Id = 4, Location = "A", Value = 3.11 },
new SensorData { Id = 5, Location = "A", Value = -456.0 }
};
var avgs = datas
.Where(e => e.Value > -50.0)
.GroupBy(e => e.Location)
.Select(g => new {
Location = g.Key,
Value = g.Average(e => e.Value) });
// { Location = A, Value = 3.0 }
// { Location = B, Value = 11.95 }</code></pre></div></div></div><div class="case"><div class="name">Sequences</div><div class="pair"><div class="card"><div class="lang">Kotlin</div><pre class="code"><code>// Sequence is lazy
val seq = sequenceOf(1, 2, 3, 4)
.filter { println("Filter $it, "); it % 2 == 1 }
.map { println("Map $it, "); it * 2 }
// Computations are evaluated during terminal operation
val items = seq.toList()
// Filter 1,
// Map 1,
// Filter 2,
// Filter 3,
// Map 3,
// Filter 4,
// List is not lazy, so functions are evaluated immediately
val items2 = listOf(1, 2, 3, 4)
.filter { println("Filter $it, "); it % 2 == 1 }
.map { println("Map $it, "); it * 2 }
// Filter 1,
// Filter 2,
// Filter 3,
// Filter 4,
// Map 1,
// Map 3,</code></pre></div><div class="card"><div class="lang">C#</div><pre class="code"><code>// LINQ is lazy, so no need to use other collection types
var query = new List<int> { 1, 2, 3, 4 }
.Where(i =>
{
Console.WriteLine($"Filter {i}, ");
return i % 2 == 1;
}).Select(i =>
{
Console.WriteLine($"Map {i}, ");
return i * 2;
});
var items = query.ToList();
//Filter 1,
//Map 1,
//Filter 2,
//Filter 3,
//Map 3,
//Filter 4,</code></pre></div></div></div></div><div class="section"><div class="title">CLASSES</div><div class="case"><div class="name">Declaration</div><div class="pair"><div class="card"><div class="lang">Kotlin</div><pre class="code"><code>class Shape {
var numberOfSides = 0
fun simpleDescription() =
"A shape with $numberOfSides sides."
}
</code></pre></div><div class="card"><div class="lang">C#</div><pre class="code"><code>class Shape
{
public int NumberOfSides { get; set; }
public string SimpleDescription() =>
$"A shape with {NumberOfSides} sides.";
}
</code></pre></div></div></div><div class="case"><div class="name">Usage</div><div class="pair"><div class="card"><div class="lang">Kotlin</div><pre class="code"><code>var shape = Shape()
shape.numberOfSides = 7
var shapeDescription = shape.simpleDescription()
// A shape with 7 sides.</code></pre></div><div class="card"><div class="lang">C#</div><pre class="code"><code>var shape = new Shape();
shape.NumberOfSides = 7;
var shapeDescription = shape.SimpleDescription();
// A shape with 7 sides.</code></pre></div></div></div><div class="case"><div class="name">Subclass</div><div class="pair"><div class="card"><div class="lang">Kotlin</div><pre class="code"><code>open class NamedShape(val name: String) {
var numberOfSides = 0
open fun simpleDescription() =
"A shape with $numberOfSides sides."
}
class Square(var sideLength: Double, name: String) :
NamedShape(name) {
init {
numberOfSides = 4
}
fun area() = sideLength.pow(2)
override fun simpleDescription() =
"A square with sides of length $sideLength."
}
val square = Square(5.2, "My square")
val area = square.area()
val desc = square.simpleDescription()
</code></pre></div><div class="card"><div class="lang">C#</div><pre class="code"><code>class NamedShape
{
private readonly string _name;
public NamedShape(string name) => _name = name;
protected int NumberOfSides { get; set; }
public virtual string SimpleDescription() =>
$"A shape with {NumberOfSides} sides.";
}
class Square: NamedShape
{
private readonly double _sideLength;
public Square(double sideLength, string name) : base(name)
{
_sideLength = sideLength;
NumberOfSides = 4;
}
public double Area() => Math.Pow(_sideLength, 2);
override public string SimpleDescription() =>
$"A square with sides of length {_sideLength}.";
}
var square = new Square(5.2, "My square");
var area = square.Area();
var desc = square.SimpleDescription();
</code></pre></div></div></div><div class="case"><div class="name">Data Class / Record</div><div class="pair"><div class="card"><div class="lang">Kotlin</div><pre class="code"><code>data class Customer(var id: Long, var name: String)
val customer = Customer(1, "Sachin")
val name = customer.name
customer.id = 2
</code></pre></div><div class="card"><div class="lang">C#</div><pre class="code"><code>// Pre C# 9 doesn't have data classes
public class Customer
{
public int Id { get; set; }
public string Name { get; set; }
}
var customer = new Customer { Id = 1, Name = "Sachin" };
var name = customer.Name;
customer.Id = 2
// C# 9 has records
// Records can be mutable, but they are primarily
// intended for supporting immutable data models
public record Customer
{
public int Id { get; set; }
public string Name { get; set; }
}</code></pre></div></div></div><div class="case"><div class="name">Immutable Data Class / Record</div><div class="pair"><div class="card"><div class="lang">Kotlin</div><pre class="code"><code>data class Customer(val id: Long, val name: String)
val customer = Customer(1, "Sachin")
val name = customer.name
customer.id = 2 // Error
</code></pre></div><div class="card"><div class="lang">C#</div><pre class="code"><code>// C# 9 immutable records can be created with positional parameters
public record Customer(int Id, string Name);
Customer customer = new(1, "Sachin");
customer.Id = 2 // Error
// or with standard property syntax and init only setter
public record Customer
{
public int Id { get; init; }
public string Name { get; init; }
};
var customer = new Customer { Id = 1, Name = "Sachin" };
customer.Id = 2 // Error
// Pre C# 9
public class Customer
{
public Customer(int id, string name) => (Id, Name) = (id, name);
public int Id { get; }
public string Name { get; }
}
var customer = new Customer(1, "Sachin");
var name = customer.Name;
customer.Id = 2 // Error</code></pre></div></div></div><div class="case"><div class="name">Extensions / Extension Methods</div><div class="pair"><div class="card"><div class="lang">Kotlin</div><pre class="code"><code>fun MutableList<Int>.swap(idx1: Int, idx2: Int) {
val tmp = this[idx1]
this[idx1] = this[idx2]
this[idx2] = tmp
}
val list = mutableListOf(1, 5, 3)
list.swap(0, 2)
// [ 3, 5, 1 ]</code></pre></div><div class="card"><div class="lang">C#</div><pre class="code"><code>public static class Extensions
{
public static void Swap(this List<int> list, int idx1, int idx2)
{
var temp = list[idx1];
list[idx1] = list[idx2];
list[idx2] = temp;
}
}
var list = new List<int> { 1, 5, 3 };
list.Swap(0, 2);
// [ 3, 5, 1 ]
</code></pre></div></div></div><div class="case"><div class="name">Interfaces</div><div class="pair"><div class="card"><div class="lang">Kotlin</div><pre class="code"><code>interface Nameable {
fun name(): String
}
fun <T: Nameable> genericFunction(x: T) {
println("Name is " + x.name())
}
class Person : Nameable {
override fun name() = "Person A"
}
</code></pre></div><div class="card"><div class="lang">C#</div><pre class="code"><code>interface INameable
{
string Name();
}
void GenericMethod<T>(T x) where T : INameable
{
Console.WriteLine("Name is " + x.Name());
}
class Person : INameable
{
public string Name() => "Person A";
}</code></pre></div></div></div></div><div class="section"><div class="title">TYPES</div><div class="case"><div class="name">Checking Type</div><div class="pair"><div class="card"><div class="lang">Kotlin</div><pre class="code"><code>var movieCount = 0
var songCount = 0
for (item in library) {
if (item is Movie) {
++movieCount
} else if (item is Song) {
++songCount
}
}
</code></pre></div><div class="card"><div class="lang">C#</div><pre class="code"><code>var movieCount = 0;
var songCount = 0;
foreach (var item in library)
{
if (item is Movie)
++movieCount;
else if (item is Song)
++songCount;
}
</code></pre></div></div></div><div class="case"><div class="name">Casting</div><div class="pair"><div class="card"><div class="lang">Kotlin</div><pre class="code"><code>// Unsafe (throw exception)
val circle: Circle = shape as Circle
// Safe (return null)
val circle: Circle? = shape as Circle?
val circle: Circle? = shape as? Circle</code></pre></div><div class="card"><div class="lang">C#</div><pre class="code"><code>// Unsafe (throw exception)
var circle = (Circle)shape;
// Safe (return null)
var circle = shape as Circle;
// If Nullable reference types enabled (optional feature)
Circle? circle = shape as Circle;</code></pre></div></div></div><div class="case"><div class="name">Smart Cast</div><div class="pair"><div class="card"><div class="lang">Kotlin</div><pre class="code"><code>for (current in library) {
if (current is Movie) {
println("Movie: '${current.name}', " +
"dir: ${current.director}")
}
}
</code></pre></div><div class="card"><div class="lang">C#</div><pre class="code"><code>foreach (var current in library)
{
if (current is Movie movie)
{
Console.WriteLine($"Movie: '{movie.Name}', " +
$"dir: {movie.Director}");
}
}
</code></pre></div></div></div></div><div class="section"><div class="title">EXCEPTIONS</div><div class="case"><div class="name">Exception Handling</div><div class="pair"><div class="card"><div class="lang">Kotlin</div><pre class="code"><code>try {
// some code
}
catch (e: SomeException) {
if (e.SomeCode == 404) {
// handle SomeException when SomeCode is 404
} else {
// handle SomeException
}
}
catch (e: Exception) {
// handle rest of the Exceptions
}
finally {
// optional finally block
}</code></pre></div><div class="card"><div class="lang">C#</div><pre class="code"><code>try
{
// Some code
}
catch (SomeException e) when (e.SomeCode == 404)
{
// Handle SomeException only when SomeCode is 404
}
catch (SomeException e)
{
// Handle SomeException
}
catch (Exception e)
{
// Handle rest of the Exceptions
}
finally
{
// Optional finally block
}</code></pre></div></div></div><div class="case"><div class="name">Exception Expression</div><div class="pair"><div class="card"><div class="lang">Kotlin</div><pre class="code"><code>// try is an expression, i.e., it may have a return value
val a: Int? = try { input.toInt() }
catch (e: NumberFormatException) { null }</code></pre></div><div class="card"><div class="lang">C#</div><pre class="code"><code>// try is not an expression
int? a;
try { a = int.Parse(input); }
catch { a = null; }</code></pre></div></div></div></div><div class="section"><div class="title">PATTERN MATCHING</div><div class="case"><div class="name">When / Switch Expression</div><div class="pair"><div class="card"><div class="lang">Kotlin</div><pre class="code"><code>val nb = 42
val text = when (nb) {
in 0..7, 8, 9 -> "single digit"
10 -> "double digits"
in 11..99 -> "double digits"
in 100..999 -> "triple digits"
else -> "four or more digits"
}
// double digits</code></pre></div><div class="card"><div class="lang">C#</div><pre class="code"><code>// Pre C# 9
var nb = 42;
var text = nb switch
{
int i when i < 10 => "single digit",
10 => "double digits",
int i when i < 100 => "double digits",
int i when i < 1000 => "triple digits",
_ => "four or more digits"
};
// With C# 9 relational and conjunctive patterns
var nb = 42;
var text = nb switch
{
< 10 => "single digit",
10 or (>= 11 and < 100) => "double digits",
< 1000 => "triple digits",
_ => "for or more digits",
};
// double digits</code></pre></div></div></div><div class="case"><div class="name">Is Expression / When Clause / Property Pattern</div><div class="pair"><div class="card"><div class="lang">Kotlin</div><pre class="code"><code>// Not supported yet
// https://youtrack.jetbrains.com/issue/KT-20004
// http://openjdk.java.net/jeps/305</code></pre></div><div class="card"><div class="lang">C#</div><pre class="code"><code>var result = item switch
{
Square s => Handle(s),
Circle { Radius: < 10 } c => HandleUnder10(c),
Circle { Radius: 20 } c => Handle20(c),
Circle c => Handle(c),
_ => throw new Exception("Unknown shape")
};
// Same with if statements
if (item is Square s)
{ }
else if (item is Circle { Radius: < 10 })
{ }
else if (item is Circle { Radius: 20 })
{ }
else if (item is Circle ci)
{ }</code></pre></div></div></div></div><div class="section"><div class="title">NULL</div><div class="case"><div class="name">Nullable Types</div><div class="pair"><div class="card"><div class="lang">Kotlin</div><pre class="code"><code>data class Measurement(val celsius: Double)
val data: Measurement = null // Error: can't be null
val data: Measurement? = null // Ok: can be null
fun printMayBeNull(data: Measurement?) {
// data can be null, must have null check
if (data == null)
return
println(data.celsius)
}
fun printNoNull(data: Measurement) {
// data can't be null. No need for check
println(data.celsius)
}
val current: Measurement? = getDataFromApi()
printMayBeNull(current) // Ok: can be null
if (current == null)
return
printNoNull(current)
</code></pre></div><div class="card"><div class="lang">C#</div><pre class="code"><code>// Nullable reference types are optional feature in C#
public class Measurement
{
public double Celsius { get; set; }
}
Measurement? data = null; // Ok: can be null
Measurement data = null; // Error: can't be null
void PrintMayBeNull(Measurement? data)
{
// data can be null, must have null check
if (data == null)
return;
Console.WriteLine(data.Celsius);
}
void PrintNoNull(Measurement data)
{
// data can't be null. No need for check
Console.WriteLine(data.Celsius);
}
Measurement? current = GetDataFromApi();
PrintMayBeNull(current); // No need for check as method accepts nulls
if (current == null)
return;
PrintNoNull(current); // OK: Null is checked before method call</code></pre></div></div></div><div class="case"><div class="name">Null Conditional</div><div class="pair"><div class="card"><div class="lang">Kotlin</div><pre class="code"><code>data class DataPoint(val id: Int, val celsius: Double,
val child: DataPoint? = null)
val data = DataPoint(1, 22.1, DataPoint(2, 22.8))
val result = data.child?.child?.
let { toFahrenheit(it.celsius) } ?: Double.MIN_VALUE</code></pre></div><div class="card"><div class="lang">C#</div><pre class="code"><code>public class DataPoint
{
public int Id { get; set; }
public double Celsius { get; set; }
public DataPoint? Child { get; set; }
}
var data = new DataPoint
{
Id = 1,
Celsius = 22.1,
Child = new DataPoint { Id = 2, Celsius = 22.8 }
};
var result = data.Child?.Child != null
? ToFahrenheit(data.Child.Child.Celsius)
: double.MinValue;</code></pre></div></div></div><div class="case"><div class="name">Elvis Operator / Null Coalescing</div><div class="pair"><div class="card"><div class="lang">Kotlin</div><pre class="code"><code>val data = DataPoint(1, 22.1, DataPoint(2, 22.8))
val result = data.child?.child?.celsius ?: Double.MIN_VALUE
// Double.MIN_VALUE</code></pre></div><div class="card"><div class="lang">C#</div><pre class="code"><code>var data = new DataPoint
{
Id = 1,
Celsius = 22.1,
Child = new DataPoint { Id = 2, Celsius = 22.8 }
};
var result = data.Child?.Child?.Celsius ?? double.MinValue;
// double.MinValue</code></pre></div></div></div><div class="case"><div class="name">Generics, Out and Conditional</div><div class="pair"><div class="card"><div class="lang">Kotlin</div><pre class="code"><code>// Use .let and forget weird helper methods
val data = DataPoint(1, 22.1, DataPoint(2, 22.8))
val result = data.child?.child?.
let { toFahrenheit(it.celsius) } ?: Double.MIN_VALUE</code></pre></div><div class="card"><div class="lang">C#</div><pre class="code"><code>// Generic helper method that will return boolean and set output
bool GetValue<T>(T input, out T output)
{
output = input;
return output != null;
}
var data = new DataPoint
{
Id = 1,
Celsius = 22.1,
Child = new DataPoint { Id = 2, Celsius = 22.8 }
};
var result = GetValue(data.Child?.Child, out var output)
? ToFahrenheit(output.Celsius)
: double.MinValue;
string set = "My text";
var text = GetValue(set, out var output) ? output : "Not set";
// "My text"
string notSet = null;
var text = GetValue(notSet, out var output) ? output : "Not set";
// "Not set"
</code></pre></div></div></div></div><div class="section"><div class="title">JSON / DYNAMIC</div><div class="case"><div class="name">Dynamic</div><div class="pair"><div class="card"><div class="lang">Kotlin</div><pre class="code"><code>// The dynamic type is not supported in code targeting the JVM
// https://kotlinlang.org/docs/reference/dynamic-type.html
// JSON example with data classes
data class Work(val name: String, val location: String)
data class User(val id: String, val work: Work)
val json = """[
{ "id": "A", "work": { "name": "ACME 2", "location": "NY" } },
{ "id": "B", "work": { "name": "Box Co", "location": "SF" } },
{ "id": "C", "work": { "name": "DotCom", "location": "NY" } }
]"""
val users = jacksonObjectMapper().readValue<List<User>>(json)
val name = users.first().work.name
// ACME 2
val fromNy = users
.filter { it.work.location == "NY" }
.map { it.id }
// [A, C]</code></pre></div><div class="card"><div class="lang">C#</div><pre class="code"><code>/* with dynamic, type is not known until runtime */
var json = @"[
{ 'id': 'A', 'work': { 'name': 'ACME 2', 'location': 'NY' } },
{ 'id': 'B', 'work': { 'name': 'Box Co', 'location': 'SF' } },
{ 'id': 'C', 'work': { 'name': 'DotCom', 'location': 'NY' } }
]";
var users = JsonConvert.DeserializeObject<List<dynamic>>(json);
var name = users.First().work.name;
// ACME 2
var fromNY = users
.Where(e => e.work.location == "NY")
.Select(e => e.id);
// [A, C]</code></pre></div></div></div></div><div class="section"><div class="title">COROUTINES / TASKS</div><div class="case"><div class="name">Async / Await</div><div class="pair"><div class="card"><div class="lang">Kotlin</div><pre class="code"><code>data class Stats(val full_name: String, val stargazers_count: Int = -1, val forks: Int = -1)
val mapper = jacksonObjectMapper().configure(DeserializationFeature.FAIL_ON_UNKNOWN_PROPERTIES, false)
val repos = listOf("jetbrains/kotlin", "dotnet/csharplang")
val asyncRequests = repos.map { repo ->
GlobalScope.async {
val body = Fuel.get("https://api.github.com/repos/$repo")
.responseString()
.third.component1() // Fuel Result & Body
body?.let { mapper.readValue<Stats>(it) } ?: Stats(repo)
}
}
runBlocking {
val results = asyncRequests.map { it.await() }
results.forEach{ println("${it.full_name} : ${it.stargazers_count} - ${it.forks}") }
}</code></pre></div><div class="card"><div class="lang">C#</div><pre class="code"><code>var client = new HttpClient();
var repos = new [] { "jetbrains/kotlin", "dotnet/csharplang" };
var asyncRequests = repos.Select(async repo =>
{
var response = await client.GetAsync($"https://api.github.com/repos/{repo}");
var json = await response.Content.ReadAsStringAsync();
dynamic content = JsonConvert.DeserializeObject(json);
return new { repo, stars = content.stargazers_count, forks = content.forks };
});
var results = await Task.WhenAll(asyncRequests);
foreach(var data in results)
Console.WriteLine($"{data.repo} : {data.stars} - {data.forks}");</code></pre></div></div></div></div></body></html>