Related
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;
}
}
I'm trying to create a collection (list<> or IEnumerable<>) of a custom objet "InventorAttribue" that has 2 properties; Name and Value.
The "Value" property can be of various type so I thought of coding this object like this:
public class InventorAttribute<T> {
public InventorAttribute (string name, T value) {
Name = name;
Value = value;
}
public string Name { get; set; }
public T Value { get; set; }
}
Further I plan to use an "AttiributeSet" class to represent the final Autodesk Inventor AttributeSet to be stored in an Inventor's object. Here is the class and where my question stands, because of course, this code does not work as the type 'T' cannot be found (!)
public class AttributeSet
{
public AttributeSet(string category, string name {
Name = name;
Attributes = new List<InventorAttribute<T>>();
}
public string Category { get; set; }
public string Name { get; set; }
public List<InventorAttribute<T>> Attributes { get; set; }
public void AddAttribute(string name, T value){
Attributes.Add(new InventorAttribute<T>(name,value));
}
}
Question:
How can I manage to write this code, and being able to pass the "InventorAttribute.Value" type only at run time through the "AddAttribute" method.
Thanks in advance for greatly appreciated help.
Your AttributeSet class should be also parametrized:
public class AttributeSet<T>
NOTE: you cannot store InventorAttribute<T> parametrized with different T types in Attributes collection. Even if you could do that, how would you consume such collection? You will need to cast Value for each attribute to appropriate type. You will not have any benefits of having generic class here. So create non-generic InventorAttribute which will store values in property of object type.
You're probably imagining some form of inheritance. It doesn't exist here.
An InventorAttribute<string> is not a subclass of InventorAttribute<T>. Nor is it a subclass of InventorAttribute<object> (I mention this since it's usually people's next attempt to define the collection's item type). Each constructed generic type is effectively independent1.
If applicable, you may be able to introduce a new base class:
public abstract class InventorAttribute {
public string Name { get; set; }
public InventorAttribute (string name) {
Name = name;
}
}
public class InventorAttribute<T> : InventorAttribute {
public InventorAttribute (string name, T value) : base(name) {
Value = value;
}
public T Value { get; set; }
}
And you can now declare your collection to be of non-generic type InventorAttribute. But now you cannot access the Values until you cast to the more specific type.
1So far as the type system is concerned. As an implementation detail, the system is able to cleverly JIT only a single version of each method body that is applicable for all reference types. But that doesn't have any visible impact in the type system.
I'm using linq to populate an object through a parameterless constructor. I set a breakpoint on the constructor and noticed that none of the values for "this" had been populated. One of the properties I'd like to have a is a sum total of some of the other properties. Here is a stripped down example of what my class looks like:
// I'd like totalEstimatedUse to be the total of estimatedType1 + estimatedType2
public class EstimatedUse
{
public string ID { get; set; }
public double estimatedType1 { get; set; }
public double estimatedType2 { get; set; }
public double totalEstimatedUse { get; set; } // I'd like this to be the sum
}
// parameterless constructor - I cannot get anything to work inside this because
// 'this' has no values when it's populated (from linq in my code)
public EstimatedUse
{
}
// here is a constructor that has parameters
public EstimatedUse(string id, double estimatedType1, double estimatedType2)
{
this.ID = id;
this.estimatedType1 = estimatedType1;
this.estimatedType2 = estimatedType2;
this.totalEstimatedUse = estimatedType1 + estimatedType2;
}
The problem that happens is that I'm using linq to populate it and it goes to the paramterless constructor and that doesn't set my totalEstimatedUse. What I'm doing to get around it is to set it a second time using my constructor with parameters. Is there a more proper way to accomplish this rather than what I have below?
EstimatedUse estimatedUse = null;
// cutoff linq to keep example clean
.Select(e => new EstimatedUse
{
ID = e.ProjectID,
estimatedType1 = e.estimatedType1),
estimatedType1 = e.estimatedType2),
}).FirstOrDefault();
// below is to set the length but I wish I could set in the parameterless constuctor if
// it's possible
estimatedUse = new EstimatedUse(estimatedUse.ID, estimatedUse.estimatedType1,
stimatedUse.estimatedType2);
Perhaps you would be better served by changing your totalEstimatedUse property to a read-only property that automatically sums the values.
public class EstimatedUse
{
public string ID { get; set; }
public double estimatedType1 { get; set; }
public double estimatedType2 { get; set; }
public double totalEstimatedUse
{
get { return this.estimatedType1 + this.estimatedType2; }
}
// ...
}
This way you don't have to set the value of the property upon creation, and it will stay up-to-date with your other values.
You could also just change the totalEstimatedUse property's getter to automatically calculate the sum when you access it:
public class EstimatedUse
{
public string ID { get; set; }
public double estimatedType1 { get; set; }
public double estimatedType2 { get; set; }
public double totalEstimatedUse { get { return estimatedType1 + estimatedType2; }
}
You're not setting the values in the constructor, you're using the object initializer syntax. That syntax creates the object by calling its constructor and then sets the properties specified.
To perform initialization of the object using the values you'll need to actually have a constructor that accepts them.
Of course the alternative is to avoid the need for initialization of the object in the first place. In this particular case that would mean not holding the total sum in a field but rather computing the sum on request in the property getter.
If you dont want to pass any parameter in constructor, then you should set the field you should have passed to constructor as global. and Set the value before making call to the class. Then inside constructor you can initialize it.
Though this is not the ideal way to do it, this is still an option. But you should see if you can pass a parameter to the constructor to set the values.
Just eliminate the parameterless constructor, and use the one with parameters:
.Select(e => new EstimatedUse(e.ProjectID,e.estimatedType1e.estimatedType2))
.FirstOrDefault();
There is no reason to use an object initializer when you have a better constructor option.
I see some code will return the default value, so I am wondering for a user defined class, how will the compiler define its default value?
To chime in with the rest, it will be null, but I should also add that you can get the default value of any type, using default
default(MyClass) // null
default(int) // 0
It can be especially useful when working with generics; you might want to return default(T), if your return type is T and you don't want to assume that it's nullable.
The default value for class is a null
Note: A DefaultValueAttribute will not cause a member to be automatically initialized with the attribute's value. You must set the initial value in your code.
You can decorate your properties with the DefaultValueAttribute.
private bool myVal = false;
[DefaultValue(false)]
public bool MyProperty
{
get
{
return myVal;
}
set
{
myVal = value;
}
}
I know this doesn't answer your question, just wanted to add this as relevant information.
For more info see http://msdn.microsoft.com/en-us/library/system.componentmodel.defaultvalueattribute.aspx
The default value for classes is null. For structures, the default value is the same as you get when you instantiate the default parameterless constructor of the structure (which can't be overriden by the way). The same rule is applied recursively to all the fields contained inside the class or structure.
I would make this "default" class instance a field rather than property, like how System.String.Empty looks:
public class Person
{
public string Name { get; set; }
public string Address { get; set; }
public static readonly Person Default = new Person()
{
Name = "Some Name",
Address = "Some Address"
};
}
...
public static void Main(string[] args)
{
string address = String.Empty;
Person person = Person.Default;
//the rest of your code
}
If you are wondering why there isn't a language feature that allows you to somehow define a non-null default value for a class, remember that a class is a "reference type". A reference type is a reference to allocated memory, or in other words, anything except null is a constructed instance of that object which required memory to be allocated. So now thinking about how to integrate the "default" keyword into .NET so that it works for classes, imagine two implementations that could be considered:
You can define a static instance that is marked as the default. Maybe "default" becomes like an access modifier (e.g. public default readonly etc). This could work, maybe. But is it really a worthy language feature? At least this way every "default" would point to the same static instance of memory.
What if the default constructor were called every time you used the default keyword on a class? This would be terrible because two default values could possibly be unequal to each other if you didn't override Equals to not compare references, and also terrible because of the memory management and memory allocation performance hit, and side affects from the constructor.
So in summary I think it is a language feature that we really don't want. And default(MyClass) will always == null.
If it is a reference type, the default value will be null, if it is a value type, then it depends.
Assert.IsTrue(default(MyClass) == null);
For reference types or nullable value types the default will be null:
Person person = default; // = null
IEnumerable<Person> people = default; // = null
int? value = default; // = null
For value types, it depends on which value type it is:
int value = default; // = 0
DateTime dateTime = default; // = 1/1/0001 12:00:00 AM
Guid id = default; // = 00000000-0000-0000-0000-000000000000
Saving some other ppl's time, hopefully.
Obvious option nowadays (for which I still had to google a bit, as landed on this topic first) is to write an extention, that helps you to initialize a class, regardless of it's own complications (like constructor getters/setters, which prevent simple default value direct assignition).
Modifying previous answer a bit:
public class Person
{
public string Name { get; set; }
public string Address { get; set; }
public static readonly Person Default = new Person()
{
Name = "Some Name",
Address = "Some Address"
};
}
=>
public class Person
{
public string Name { get; set; }
public string Address { get; set; }
}
public static class PersonExtentions
{
public static Person withDefaults(this Person obj) {
obj.Name = "John Doe";
return obj;
}
}
And then
Person x = new Person.withDefaults();
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;
}
}