Quick reference of new features and improvements to C# 7.
---
title: C# 7 Cheatsheet
subtitle: Quick reference of new features and improvements to C# 7
author: devhints.io
date: April 7, 2019
source: https://devhints.io/csharp7
notoc: false
---
## Out Variables
```cs
public void PrintCoordinates(Point p)
{
p.GetCoordinates(out int x, out int y);
WriteLine($"({x}, {y})");
}
```
> `out` is used to declare a variable at the point where it is passed as an
> argument.
## Pattern Matching
### Is-expressions with patterns
```cs
public void PrintStars(object o)
{
if (o is null) return; // constant pattern "null"
if (!(o is int i)) return; // type pattern "int i"
WriteLine(new string('*', i));
}
```
### Switch statements with patterns
```cs
switch(shape)
{
case Circle c:
WriteLine($"circle with radius {c.Radius}");
break;
case Rectangle s when (s.Length == s.Height):
WriteLine($"{s.Length} x {s.Height} square");
break;
case Rectangle r:
WriteLine($"{r.Length} x {r.Height} rectangle");
break;
default:
WriteLine("<unknown shape>");
break;
case null:
throw new ArgumentNullException(nameof(shape));
}
```
## Tuples
### Tuple type
```cs
(string, string, string) LookupName(long id) // tuple return type
{
... // retrieve first, middle and last from data storage
return (first, middle, last); // tuple literal
}
var names = LookupName(id);
WriteLine($"found {names.Item1} {names.Item3}.");
```
### Tuple elements with name
```cs
(string first, string middle, string last) LookupName(long id) // tuple elements have names
var names = LookupName(id);
WriteLine($"found {names.first} {names.last}.");
```
### Tuple Literals
```cs
return (first: first, middle: middle, last: last); // named tuple elements in a literal
```
### Tuple Deconstruction
```cs
(var first, var middle, var last) = LookupName(id1);
WriteLine($"found {first} {last}.");
```
or...
```cs
var (first, middle, last) = LookupName(id1); // var outside
```
or...
```cs
(first, middle, last) = LookupName(id2); // assign onto existing variables
```
## Local Functions
```cs
public int Fibonacci(int x)
{
if (x < 0) throw new ArgumentException("Less negativity please!", nameof(x));
return Fib(x).current;
(int current, int previous) Fib(int i)
{
if (i == 0) return (1, 0);
var (p, pp) = Fib(i - 1);
return (p + pp, p);
}
}
```
## Literal Improvements
### Digit Separator inside numbers literals
```cs
var d = 123_456;
var x = 0xAB_CD_EF;
```
### Binary Literals
```cs
var b = 0b1010_1011_1100_1101_1110_1111;
```
## Ref Returns and Locals
```cs
public ref int Find(int number, int[] numbers)
{
for (int i = 0; i < numbers.Length; i++)
{
if (numbers[i] == number)
{
return ref numbers[i]; // return the storage location, not the value
}
}
throw new IndexOutOfRangeException($"{nameof(number)} not found");
}
int[] array = { 1, 15, -39, 0, 7, 14, -12 };
ref int place = ref Find(7, array); // aliases 7's place in the array
place = 9; // replaces 7 with 9 in the array
WriteLine(array[4]); // prints 9
```
## More Expression Bodied Members
C# 7.0 adds accessors, constructors and finalizers to the list of things that
can have expression bodies:
```cs
class Person
{
private static ConcurrentDictionary<int, string> names = new ConcurrentDictionary<int, string>();
private int id = GetId();
public Person(string name) => names.TryAdd(id, name); // constructor
~Person() => names.TryRemove(id, out *); // destructor
public string Name
{
get => names[id]; // getter
set => names[id] = value; // setter
}
}
```
## Throw Expressions
```cs
class Person
{
public string Name { get; }
public Person(string name) => Name = name ?? throw new ArgumentNullException(name);
public string GetFirstName()
{
var parts = Name.Split(" ");
return (parts.Length > 0) ? parts[0] : throw new InvalidOperationException("No name!");
}
public string GetLastName() => throw new NotImplementedException();
}
```