Property. Consider the physical computer you are using. It has many properties—a size, a weight, a date it was built. Properties describe this computer.
We use properties on a class (like a Computer class) to describe the class. They can be set and read from like other fields, and special code can be run.
using System;
class Example
{
int _number;
public int Number
{
get
{
return this._number;
}
set
{
this._number = value;
}
}
}
class Program
{
static void Main()
{
Example example = new Example();
example.Number = 5; // set { }
Console.WriteLine(example.Number); // get { }
}
}5
Automatic property. We see automatically implemented property syntax in C#. A hidden field is generated—then the get and set statements are expanded to use that hidden field.
Detail The *= operator is used to multiply the property by itself. Because properties are meant to look like fields, this is allowed.
using System;
class Example
{
public int Number
{
get;
set;
}
}
class Program
{
static void Main()
{
Example example = new Example();
example.Number = 8;
example.Number *= 4;
Console.WriteLine(example.Number);
}
}32
Enum. This example shows the DayOfWeek enum type in a property. We also insert code in the getter (or setter) that checks the backing store or the parameter value.
Detail Like a method, a property can act on any type, even enum types like DayOfWeek. Many properties will use string or int.
using System;
class Example
{
DayOfWeek _day;
public DayOfWeek Day
{
get
{
// We don't allow this to be used on Friday.
if (this._day == DayOfWeek.Friday)
{
throw new Exception("Invalid access");
}
return this._day;
}
set
{
this._day = value;
}
}
}
class Program
{
static void Main()
{
Example example = new Example();
example.Day = DayOfWeek.Monday;
Console.WriteLine(example.Day == DayOfWeek.Monday);
}
}True
Private. We make a private property. Here the IsFound property can only be set in the Example class. We set it in the Example constructor.
Then We can only get the property in the Program.Main method by using an Example instance.
using System;
class Example
{
public Example()
{
// Set the private property.
this.IsFound = true;
}
bool _found;
public bool IsFound
{
get
{
return this._found;
}
private set
{
// Can only be called in this class.
this._found = value;
}
}
}
class Program
{
static void Main()
{
Example example = new Example();
Console.WriteLine(example.IsFound);
}
}True
Entire property. We can also make an entire property private. If we do this, we can only use the property in the same enclosing class.
Detail The Display method in the example shows how to use the private property.
Note This syntax is less useful in most programs. But it exists, and may be helpful in a complex class.
using System;
class Example
{
int _id;
private int Id
{
get
{
return this._id;
}
set
{
this._id = value;
}
}
public void Display()
{
// Access the private property in this method.
this.Id = 7;
Console.WriteLine(this.Id);
}
}
class Program
{
static void Main()
{
Example example = new Example();
example.Display();
}
}7
Static. Properties can also be static—this means they are associated with the type and not an instance. Static classes can only have static properties.
Info The get property has a side effect. It causes the field to be incremented upon each access.
Warning Side effects are not usually a good design feature in programs. They can make the logic hard to follow.
using System;
class Example
{
static int _count;
public static int Count
{
get
{
// Side effect of this property.
_count++;
return _count;
}
}
}
class Program
{
static void Main()
{
Console.WriteLine(Example.Count);
Console.WriteLine(Example.Count);
Console.WriteLine(Example.Count);
}
}1
2
3
Automatic, private. Let us consider how to make getters or setters on an automatic property. We cannot omit either the getter or setter in this kind of property.
Note The error reported by the C# compiler reads: "Automatically implemented properties must define both get and set accessors."
using System;
class Example
{
public Example()
{
// Use private setter in the constructor.
this.Id = new Random().Next();
}
public int Id
{
get;
private set;
}
}
class Program
{
static void Main()
{
Example example = new Example();
Console.WriteLine(example.Id);
}
}2077325073
Automatic, default values. Automatic properties have support for default values much like fields. Here we assign the Quantity property of Medication to 30 by default.
using System;
class Medication
{
public int Quantity { get; set; } = 30; // Has default value.
}
class Program
{
static void Main()
{
Medication med = new Medication();
// The quantity is by default 30.
Console.WriteLine(med.Quantity);
// We can change the quantity.
med.Quantity *= 2;
Console.WriteLine(med.Quantity);
}
}30
60
Expression-bodied properties. We can use lambda-style syntax to specify properties. These are expression-bodied properties—we use "get" and "set" and then the result on the right side.
class Program
{
private static int test;
public static int Test { get => test; set => test = value; }
static void Main()
{
// Use the property.
Program.Test = 200;
System.Console.WriteLine(Program.Test);
}
}200
Init. Suppose we wish to have a property that is not changeable (mutable) after its creation. It can only be initialized, not set.
Info We use the "init" keyword to specify that the class Test here has an Info property that can be initialized, but not set afterwards.
using System;
class Test
{
public string Info { get; init; }
}
class Program
{
static void Main()
{
// Use the init setter with an initializer.
var test = new Test() { Info = "Ok" };
// The Info is ok.
Console.WriteLine(test.Info);
}
}Ok
Benchmark, properties. Compiler optimizations ensure that properties are efficient. These same optimizations are used on methods, which share the underlying implementation with properties.
Version 1 This code uses a property. It sets a property, and then gets the value of the property.
Version 2 This version of the code uses a field directly. It performs the same logical steps that version 1 does.
Result There was no difference in performance with the property and the field. It is apparent the property access is inlined.
Tip The JIT compiler can inline properties that don't have logic inside of them. So they are as efficient as fields.
using System;
using System.Diagnostics;
class Program
{
static string _backing; // Backing store for property.
static string Property // Getter and setter.
{
get
{
return _backing;
}
set
{
_backing = value;
}
}
static string Field; // Static field.
static void Main()
{
const int m = 100000000;
for (int x = 0; x < 10; x++) // Ten tests.
{
Stopwatch s1 = new Stopwatch();
s1.Start();
// Version 1: test property.
for (int i = 0; i < m; i++)
{
Property = "string";
if (Property == "cat")
{
}
}
s1.Stop();
Stopwatch s2 = new Stopwatch();
s2.Start();
// Version 2: test field.
for (int i = 0; i < m; i++)
{
Field = "string";
if (Field == "cat")
{
}
}
s2.Stop();
Console.WriteLine("{0},{1}", s1.ElapsedMilliseconds, s2.ElapsedMilliseconds);
}
}
}Property get/set: 604.6 ms
Field read/assign: 603.6 ms
Summary. Properties are used throughout most C# programs. They are a powerful way to replace methods. They present a more intuitive way to use objects.
Dot Net Perls is a collection of tested code examples. Pages are continually updated to stay current, with code correctness a top priority.
Sam Allen is passionate about computer languages. In the past, his work has been recommended by Apple and Microsoft and he has studied computers at a selective university in the United States.
This page was last updated on Jun 4, 2024 (simplify).
