Take a look at this code.
public struct Customer
{
private int _id;
private string _name;
public int Id
{
get { return this._id; }
set { this._id = value; }
}
public Customer(int Id, string Name)
{
this.Id = Id; // Error - CS0188: The 'this' object cannot be used before all of its fields are assigned to.
this._name = Name;
}
}
`
Why do I get this error on the above line.
Now look at this code.
public struct Customer
{
private string _name;
public int Id { get; set; }
public Customer(int Id, string Name)
{
this.Id = Id; // No error recieved.
this._name = Name;
}
}
In the above code I implemented Id as an Auto-Implemented property.
This time I don't get any errors. Why?
Just to make it clear following link does not have my answer.
Compilation error. Using properties with struct
You need to initialize all the fields of struct first. When you assign a value to an Auto-Implemented property it always sets the backing field, however, with custom setters, it's not always guaranteed. That's why you don't get any error
For custom properties:
public struct Customer
{
private int _id;
private string _name;
public int Id
{
get { return this._id; }
set { this._id = value; }
}
public Customer(int Id, string Name)
{
_id=0;
this._name = Name;
this.Id = Id; // This will work now
}
}
Under C# 5, both methods should generate an error. From Automatically Implemented Properties:
This restriction also means that definite assignment of struct types with auto-implemented properties can only be achieved using the standard constructor of the struct, since assigning to the property itself requires the struct to be definitely assigned. This means that user-defined constructors must call the default constructor.
However, C# 6 introduced read-only auto-implemented properties and introduced this language (whilst removing the above language):
a getter-only auto-property can also be assigned to in the body of a constructor of the enclosing class. Such an assignment assigns directly to the readonly backing field of the property.
I suspect that they changed all assignments for Automatically Implemented Properties (not just for readonly) in constructors to assign to the backing field rather than use the accessor. This is known to be "safe" since the accessor doesn't contain any custom code - unlike the situation in your first example where the set accessor could do anything, including leaking the not-definitely-assigned this outside of the current struct.
In C# 6, we can assign in-line values to automatic property. Since the value of these automatic property is set directly to the backing field (without setter), can I assume the backing field is a 'static type'? If I am wrong, can someone explain me how value is been assigned to backing field? At which stage compiler assign the in-line to the Property ?
class ProDemo
{
public string Name { get; set; } = "Jon";
}
Can someone explain me how automatic property implemented differently in C# 6 while comparing previous version of C#?
This:
class ProDemo
{
public string Name { get; set; } = "Jon";
}
Is compiled to this:
class ProDemo
{
private string _name = "Jon";
public string get_Name() { return _name; }
public void set_Name(string value) { _name = value; }
}
There isn't anything to do with static types, at all.
How do you give a C# auto-property an initial value?
I either use the constructor, or revert to the old syntax.
Using the Constructor:
class Person
{
public Person()
{
Name = "Initial Name";
}
public string Name { get; set; }
}
Using normal property syntax (with an initial value)
private string name = "Initial Name";
public string Name
{
get
{
return name;
}
set
{
name = value;
}
}
Is there a better way?
In C# 5 and earlier, to give auto implemented properties an initial value, you have to do it in a constructor.
Since C# 6.0, you can specify initial value in-line. The syntax is:
public int X { get; set; } = x; // C# 6 or higher
DefaultValueAttribute is intended to be used by the VS designer (or any other consumer) to specify a default value, not an initial value. (Even if in designed object, initial value is the default value).
At compile time DefaultValueAttribute will not impact the generated IL and it will not be read to initialize the property to that value (see DefaultValue attribute is not working with my Auto Property).
Example of attributes that impact the IL are ThreadStaticAttribute, CallerMemberNameAttribute, ...
Edited on 1/2/15
C# 6 :
With C# 6 you can initialize auto-properties directly (finally!), there are now other answers that describe that.
C# 5 and below:
Though the intended use of the attribute is not to actually set the values of the properties, you can use reflection to always set them anyway...
public class DefaultValuesTest
{
public DefaultValuesTest()
{
foreach (PropertyDescriptor property in TypeDescriptor.GetProperties(this))
{
DefaultValueAttribute myAttribute = (DefaultValueAttribute)property.Attributes[typeof(DefaultValueAttribute)];
if (myAttribute != null)
{
property.SetValue(this, myAttribute.Value);
}
}
}
public void DoTest()
{
var db = DefaultValueBool;
var ds = DefaultValueString;
var di = DefaultValueInt;
}
[System.ComponentModel.DefaultValue(true)]
public bool DefaultValueBool { get; set; }
[System.ComponentModel.DefaultValue("Good")]
public string DefaultValueString { get; set; }
[System.ComponentModel.DefaultValue(27)]
public int DefaultValueInt { get; set; }
}
When you inline an initial value for a variable it will be done implicitly in the constructor anyway.
I would argue that this syntax was best practice in C# up to 5:
class Person
{
public Person()
{
//do anything before variable assignment
//assign initial values
Name = "Default Name";
//do anything after variable assignment
}
public string Name { get; set; }
}
As this gives you clear control of the order values are assigned.
As of C#6 there is a new way:
public string Name { get; set; } = "Default Name";
Sometimes I use this, if I don't want it to be actually set and persisted in my db:
class Person
{
private string _name;
public string Name
{
get
{
return string.IsNullOrEmpty(_name) ? "Default Name" : _name;
}
set { _name = value; }
}
}
Obviously if it's not a string then I might make the object nullable ( double?, int? ) and check if it's null, return a default, or return the value it's set to.
Then I can make a check in my repository to see if it's my default and not persist, or make a backdoor check in to see the true status of the backing value, before saving.
In C# 6.0 this is a breeze!
You can do it in the Class declaration itself, in the property declaration statements.
public class Coordinate
{
public int X { get; set; } = 34; // get or set auto-property with initializer
public int Y { get; } = 89; // read-only auto-property with initializer
public int Z { get; } // read-only auto-property with no initializer
// so it has to be initialized from constructor
public Coordinate() // .ctor()
{
Z = 42;
}
}
Starting with C# 6.0, We can assign default value to auto-implemented properties.
public string Name { get; set; } = "Some Name";
We can also create read-only auto implemented property like:
public string Name { get; } = "Some Name";
See: C# 6: First reactions , Initializers for automatically implemented properties - By Jon Skeet
In Version of C# (6.0) & greater, you can do :
For Readonly properties
public int ReadOnlyProp => 2;
For both Writable & Readable properties
public string PropTest { get; set; } = "test";
In current Version of C# (7.0), you can do : (The snippet rather displays how you can use expression bodied get/set accessors to make is more compact when using with backing fields)
private string label = "Default Value";
// Expression-bodied get / set accessors.
public string Label
{
get => label;
set => this.label = value;
}
In C# 9.0 was added support of init keyword - very useful and extremly sophisticated way for declaration read-only auto-properties:
Declare:
class Person
{
public string Name { get; init; } = "Anonymous user";
}
~Enjoy~ Use:
// 1. Person with default name
var anonymous = new Person();
Console.WriteLine($"Hello, {anonymous.Name}!");
// > Hello, Anonymous user!
// 2. Person with assigned value
var me = new Person { Name = "#codez0mb1e"};
Console.WriteLine($"Hello, {me.Name}!");
// > Hello, #codez0mb1e!
// 3. Attempt to re-assignment Name
me.Name = "My fake";
// > Compilation error: Init-only property can only be assigned in an object initializer
In addition to the answer already accepted, for the scenario when you want to define a default property as a function of other properties you can use expression body notation on C#6.0 (and higher) for even more elegant and concise constructs like:
public class Person{
public string FullName => $"{First} {Last}"; // expression body notation
public string First { get; set; } = "First";
public string Last { get; set; } = "Last";
}
You can use the above in the following fashion
var p = new Person();
p.FullName; // First Last
p.First = "Jon";
p.Last = "Snow";
p.FullName; // Jon Snow
In order to be able to use the above "=>" notation, the property must be read only, and you do not use the get accessor keyword.
Details on MSDN
In C# 6 and above you can simply use the syntax:
public object Foo { get; set; } = bar;
Note that to have a readonly property simply omit the set, as so:
public object Foo { get; } = bar;
You can also assign readonly auto-properties from the constructor.
Prior to this I responded as below.
I'd avoid adding a default to the constructor; leave that for dynamic assignments and avoid having two points at which the variable is assigned (i.e. the type default and in the constructor). Typically I'd simply write a normal property in such cases.
One other option is to do what ASP.Net does and define defaults via an attribute:
http://msdn.microsoft.com/en-us/library/system.componentmodel.defaultvalueattribute.aspx
My solution is to use a custom attribute that provides default value property initialization by constant or using property type initializer.
[AttributeUsage(AttributeTargets.Property, AllowMultiple = false, Inherited = true)]
public class InstanceAttribute : Attribute
{
public bool IsConstructorCall { get; private set; }
public object[] Values { get; private set; }
public InstanceAttribute() : this(true) { }
public InstanceAttribute(object value) : this(false, value) { }
public InstanceAttribute(bool isConstructorCall, params object[] values)
{
IsConstructorCall = isConstructorCall;
Values = values ?? new object[0];
}
}
To use this attribute it's necessary to inherit a class from special base class-initializer or use a static helper method:
public abstract class DefaultValueInitializer
{
protected DefaultValueInitializer()
{
InitializeDefaultValues(this);
}
public static void InitializeDefaultValues(object obj)
{
var props = from prop in obj.GetType().GetProperties()
let attrs = prop.GetCustomAttributes(typeof(InstanceAttribute), false)
where attrs.Any()
select new { Property = prop, Attr = ((InstanceAttribute)attrs.First()) };
foreach (var pair in props)
{
object value = !pair.Attr.IsConstructorCall && pair.Attr.Values.Length > 0
? pair.Attr.Values[0]
: Activator.CreateInstance(pair.Property.PropertyType, pair.Attr.Values);
pair.Property.SetValue(obj, value, null);
}
}
}
Usage example:
public class Simple : DefaultValueInitializer
{
[Instance("StringValue")]
public string StringValue { get; set; }
[Instance]
public List<string> Items { get; set; }
[Instance(true, 3,4)]
public Point Point { get; set; }
}
public static void Main(string[] args)
{
var obj = new Simple
{
Items = {"Item1"}
};
Console.WriteLine(obj.Items[0]);
Console.WriteLine(obj.Point);
Console.WriteLine(obj.StringValue);
}
Output:
Item1
(X=3,Y=4)
StringValue
little complete sample:
using System.ComponentModel;
private bool bShowGroup ;
[Description("Show the group table"), Category("Sea"),DefaultValue(true)]
public bool ShowGroup
{
get { return bShowGroup; }
set { bShowGroup = value; }
}
You can simple put like this
public sealed class Employee
{
public int Id { get; set; } = 101;
}
In the constructor. The constructor's purpose is to initialized it's data members.
private string name;
public string Name
{
get
{
if(name == null)
{
name = "Default Name";
}
return name;
}
set
{
name = value;
}
}
Have you tried using the DefaultValueAttribute or ShouldSerialize and Reset methods in conjunction with the constructor? I feel like one of these two methods is necessary if you're making a class that might show up on the designer surface or in a property grid.
Use the constructor because "When the constructor is finished, Construction should be finished". properties are like states your classes hold, if you had to initialize a default state, you would do that in your constructor.
To clarify, yes, you need to set default values in the constructor for class derived objects. You will need to ensure the constructor exists with the proper access modifier for construction where used. If the object is not instantiated, e.g. it has no constructor (e.g. static methods) then the default value can be set by the field. The reasoning here is that the object itself will be created only once and you do not instantiate it.
#Darren Kopp - good answer, clean, and correct. And to reiterate, you CAN write constructors for Abstract methods. You just need to access them from the base class when writing the constructor:
Constructor at Base Class:
public BaseClassAbstract()
{
this.PropertyName = "Default Name";
}
Constructor at Derived / Concrete / Sub-Class:
public SubClass() : base() { }
The point here is that the instance variable drawn from the base class may bury your base field name. Setting the current instantiated object value using "this." will allow you to correctly form your object with respect to the current instance and required permission levels (access modifiers) where you are instantiating it.
public Class ClassName{
public int PropName{get;set;}
public ClassName{
PropName=0; //Default Value
}
}
This is old now, and my position has changed. I'm leaving the original answer for posterity only.
Personally, I don't see the point of making it a property at all if you're not going to do anything at all beyond the auto-property. Just leave it as a field. The encapsulation benefit for these item are just red herrings, because there's nothing behind them to encapsulate. If you ever need to change the underlying implementation you're still free to refactor them as properties without breaking any dependent code.
Hmm... maybe this will be the subject of it's own question later
class Person
{
/// Gets/sets a value indicating whether auto
/// save of review layer is enabled or not
[System.ComponentModel.DefaultValue(true)]
public bool AutoSaveReviewLayer { get; set; }
}
I know this is an old question, but it came up when I was looking for how to have a default value that gets inherited with the option to override, I came up with
//base class
public class Car
{
public virtual string FuelUnits
{
get { return "gasoline in gallons"; }
protected set { }
}
}
//derived
public class Tesla : Car
{
public override string FuelUnits => "ampere hour";
}
I think this would do it for ya givng SomeFlag a default of false.
private bool _SomeFlagSet = false;
public bool SomeFlag
{
get
{
if (!_SomeFlagSet)
SomeFlag = false;
return SomeFlag;
}
set
{
if (!_SomeFlagSet)
_SomeFlagSet = true;
SomeFlag = value;
}
}
Can somebody help me understand the get & set?
Why are they needed? I can just make a public variable.
Warning: I am assuming you already know about object-oriented programming.
What are properties?
Properties are language elements that allow you to avoid the repetitive getXYZ() accessors and setXYZ() mutators techniques found in other languages, like Java.
Why do they exist?
They aim to solve the following problems:
Saying get and set in the beginning of every access or mutation of a value is annoying and distracting.
In Java, you often say:
class person
{
private int _age;
public void setAge(int value) { /*check value first, then set _age*/ }
public int getAge() { return this._age; }
}
and then consistently say:
if (person.getAge() > blah || person.getAge() < 10)
{
person.setAge(5);
}
After a while, the get and set become rather annoying.
Providing direct access to the actual variable breaks encapsulation, so that's not an option.
How are they used?
They are used just like variables. You read/write to them just like variables.
How are they created?
They are created as methods. You define a pair of methods that:
Return the current value of the property. Oftentimes, this is nothing more than something like the following:
class Person
{
private int _age; //Declare the backing field
public int Age
{
get { return this._age; }
set { ... }
}
}
Set the value of the property:
class Person
{
public int Age
{
get { ... }
set
{
if (value < 0) //'value' is what the user provided
{ throw new ArgumentOutOfRangeException(); } //Check validity
this._age = value;
}
}
}
Other notes:
Auto-implemented Properties
C# 3.0 introduced auto-implemented properties:
public int Age { get; set; }
This is equivalent to:
private int _age; //The name is auto-generated
public int Age { get { return this._age; } set { this._age = value; } }
Why does it exist?
It helps you avoiding breaking changes in client executables.
Let's say you're lazy and don't want to type the whole thing, and decide to expose a variable publicly. You then create an executable that reads from or writes to that field. Then you change your mind and decide that you in fact needed a property, so you change it to one.
What happens?
The depending executable breaks, because the code is no longer valid.
Auto-implemented properties help you avoid that, without extra redundancy in your initial code.
Indexers
Indexers extend the property syntax to let you index objects (surprise!), just like arrays.
For C++ users: This is similar to overloading operator [].
Example:
private int[] _elements;
public int this[int index] //Indexed property
{
get { return this._elements[index]; }
set
{
//Do any checks on the index and value
this._elements[index] = value;
}
}
You then use them like obj[5] = 10;, which is equivalent to calling the set method of obj's indexer.
In fact, System.Collections.Generic.List<T> is indexed:
var list = new List<int>();
list.Add(10);
list[0] = 5; //You're indexing list, as though it were an array!
Isn't that neat? :)
Anything else?
There are many more features to properties, not all of which are available in C#:
Parametrized properties, of which indexers are a special kind
Getter/setter access modifiers (in C#)
Multiple getters or setters (not in C#)
Et cetera
They are called Accessors
The accessor of a property contains the executable statements associated with getting (reading or computing) or setting (writing) the property. The accessor declarations can contain a get accessor, a set accessor, or both.
The body of the get accessor resembles that of a method. It must return a value of the property type.
http://msdn.microsoft.com/en-us/library/w86s7x04.aspx
private string m_Name; // the name field
public string Name // the Name property
{
get
{
return m_Name;
}
}
The set accessor resembles a method whose return type is void. It uses an implicit parameter called value, whose type is the type of the property.
private m_Name;
public string Name {
get {
return m_Name;
}
set {
m_Name = value;
}
}
Then in the incarnation of C# 3, you can do this much easier through auto-properties
public string Name {get; set; } // read and write
public string Name {get; } // read only
public string Name { get; private set; } //read and parent write
http://msdn.microsoft.com/en-us/library/bb384054.aspx
Properties act as accessors to the internal state of an object, hiding the implementation of that state.
So, for example, you may have a first name property in a class
public class Example
{
private string firstName;
public string FirstName
{
get {return this.firstName;}
}
}
So anyone using the class doesn't need to know how first name is stored, they just know they can get a string representation of it. By adding a set you also add a mutator, something which changes an objects internal state
public class Example
{
private string firstName;
public string FirstName
{
get {return this.firstName;}
set {set this.firstName = value;}
}
}
Again you're still isolating how the first name is stored internally (encapsulation), but users can change it by passing in a string.
Simply put, get and set accessors are the functions called on a Property; that is, when you retrieve the value or when you set it. It forces a type of behavior on the way values are retrieved or set.
For example, you may want to have a mechanism to get/set passwords. Generally speaking, you'll only want to compare the hash of a password instead of storing things plaintext, so you'd have the getter variable retrieve the stored hash, and the setter would take the provided input and hash it for storage.
Here's what I mean:
public class User {
//Usery properties here, and...
private string _password;
public string Password {
get {
return _password;
}
set {
_password = SomeHashingFunction(value);
}
}
}
value is the variable provided to the setter from what has been given in the variable assignment. e.g.: someuser.Password = "blah";
Get and set are used in properties. They can each be public, protected, or private. Similar to accessor and mutator methods, they allow some computation when code tries to access/mutate the property. Of course, as long as you define one of get/set, the other is optional.
Example without properties:
private int test;
public int getTest() {
// some computation on test here, maybe?
return test;
}
private void setTest(int test) {
// some error/range checking, maybe?
this.test = test;
}
With properties:
private int test;
public int Test {
get {
// some computation on test here, maybe?
return test;
}
private set {
// some error/range checking, maybe?
test = value; // value is a keyword here
}
}
get{} and set{} are accessors that offer up the ability to easily read and write to private fields. Working with a simple example:
public class Foo()
{
//Field
private int _bar;
//Property
public int Bar
{
get { return _bar; }
set { _bar = value; }
//value is an implicit parameter to the set acccessor.
//When you perform an assignment to the property, the value you
//assign is the value in "value"
}
}
In this case, Bar is a public property that has a getter and a setter that allows access to the private field _bar that would otherwise be inaccessible beyond class Foo.
Now in a class that has an instace of Foo, you can do this:
public class IHasAFoo()
{
private Foo _myFoo = new Foo();
public void SomeMethod()
{
_myFoo.Bar = 42;
}
}
So the public accessor allows you to set the value of the private field back in Foo.
Hope that helps!
I want to do this:
public Name
{
get;
set
{
dosomething();
??? = value
}
}
Is it possible to use the auto-generated private field?
Or is it required that I implement it this way:
private string name;
public string Name
{
get
{
return name;
}
set
{
dosomething();
name = value
}
}
Once you want to do anything custom in either the getter or the setter you cannot use auto properties anymore.
You can try something like this:
public string Name { get; private set; }
public void SetName(string value)
{
DoSomething();
this.Name = value;
}
As of C# 7, you could use expression body definitions for the property's get and set accessors.
See more here
private string _name;
public string Name
{
get => _name;
set
{
DoSomething();
_name = value;
}
}
This is not possible. Either auto implemented properties or custom code.
It is required that you implement it fully given your scenario. Both get and set must be either auto-implemented or fully implemented together, not a combination of the two.