I wonder why it is not possible to do the following:
struct TestStruct
{
public readonly object TestField;
}
TestStruct ts = new TestStruct {
/* TestField = "something" // Impossible */
};
Shouldn't the object initializer be able to set the value of the fields
?
Object Initializer internally uses a temporary object and then assign each value to the properties. Having a readonly field would break that.
Following
TestStruct ts = new TestStruct
{
TestField = "something";
};
Would translate into
TestStruct ts;
var tmp = new TestStruct();
tmp.TestField = "something"; //this is not possible
ts = tmp;
(Here is the answer from Jon Skeet explaining the usage of temporary object with object initalizer but with a different scenario)
readonly means that the field can only be set in the constructor (or in a field initializer). Properties specified in the object initializer are set after the constructor has returned. That is,
TestStruct ts = new TestStruct {
TestField = "something"
};
is basically equivalent to
TestStruct ts = new TestStruct();
ts.TestField = "something";
(In a Debug build, the compiler may use a temporary variable, but you get the idea.)
C# 9 Init-Only Properties, despite the name, will allow the initializer syntax to be able to set readonly fields as well.
Here are the relevant parts copied from the links.
Init-only properties
Here's a simple example of object initializer.
new Person
{
FirstName = "Scott",
LastName = "Hunter"
}
The one big limitation today is that the properties have to be mutable for object initializers to work: They function by first calling the object’s constructor (the default, parameterless one in this case) and then assigning to the property setters.
Init-only properties fix that! They introduce an init accessor that is a variant of the set accessor which can only be called during object initialization:
public class Person
{
public string FirstName { get; init; }
public string LastName { get; init; }
}
With this declaration, the client code above is still legal, but any subsequent assignment to the FirstName and LastName properties is an error.
Init accessors and readonly fields
Because init accessors can only be called during initialization, they are allowed to mutate readonly fields of the enclosing class, just like you can in a constructor.
public class Person
{
private readonly string firstName;
private readonly string lastName;
public string FirstName
{
get => firstName;
init => firstName = (value ?? throw new ArgumentNullException(nameof(FirstName)));
}
public string LastName
{
get => lastName;
init => lastName = (value ?? throw new ArgumentNullException(nameof(LastName)));
}
}
This is not possible. since readonly fields cannot be assigned from other than Constructor or Field Initializer.
What you show is actually object initializer. It is just a syntatic sugar, gets comiled into something like this
TestStruct ts;
TestStruct ts1 = new TestStruct();
ts1.TestField = value;
ts = ts1;
Is that clear why it doesn't compile?
I wonder why it is not possible to do the following:
Because the compiler cannot know for sure that the following code will be executed:
TestStruct ts = new TestStruct
{
TestField = "something"
};
You should initialize readonly members directly inline or inside the constructor.
From MSDN:
The readonly keyword is a modifier that you can use on fields. When
a field declaration includes a readonly modifier, assignments to the
fields introduced by the declaration can only occur as part of the
declaration or in a constructor in the same class.
So it's simply not (yet) possible since object initializers are just post-creation assignments.
Because object initializer is just a short way of initializing:
TestStruct ts = new TestStruct {
TestField = "something";
};
is the same to (compiler will translate the above to this):
TestStruct ts = new TestStruct();
ts.TestField = "something";//this is of course not allowed.
I ran across an interesting "exception" to this, in the case where the readonly field extends CollectionBase.
Here's the code:
using System.Collections;
namespace ReadOnly
{
class Program
{
static void Main(string[] args)
{
Foo foo1 = new Foo()
{
Bar = new Bar() // Compile error here - readonly property.
{
new Buzz() { Name = "First Buzz" }
}
};
Foo foo2 = new Foo()
{
Bar = // No Compile error here.
{
new Buzz { Name = "Second Buzz" }
}
};
}
}
class Foo
{
public Bar Bar { get; }
}
class Bar : CollectionBase
{
public int Add(Buzz value)
{
return List.Add(value);
}
public Buzz this[int index]
{
get { return (Buzz)List[index]; }
set { List[index] = value; }
}
}
class Buzz
{
public string Name { get; set; }
}
}
Foo1 is how I initially tried to do it (all these classes came from an external library so we didn't know at first that Bar was readonly). Got the compile error. Then accidentally I retyped it like foo2, and it worked.
After decompiling the dll and seeing that Bar extended CollectionBase, we realized that the second syntax (foo2) was invoking the Add method on the collection. So, in the case of collections, while you can't set a read only property, you can invoke the Add method, via object initializers.
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;
}
}
For example, I have an immutable type
class Contact
{
// Read-only properties.
public string Name { get; }
public string Address { get; }
}
And I hope I can use object initializer syntax to create a Contact
Contact a = new Contact { Name = "John", Address = "23 Tennis RD" };
But I cannot. Any possible way to make use of the powerful object initializer syntax in this case?
The closest thing would be a constructor with optional parameters:
class Contact
{
public string Name { get; }
public string Address { get; }
public Contact(string name = null, string address = null) {
Name = name;
Address = address;
}
}
Then you can call it with parameter names:
new Contact(
name: "John",
address: "23 Tennis RD"
)
The syntax is slightly different from an object initializer, but it's just as readable; and IMO, the difference is a good thing, because constructor parameters tend to suggest immutable properties. And you can specify the parameters in any order, or leave some out, so it's just as powerful as object initializer syntax.
This does require some extra code (defining the constructor, assigning all the properties), so it's more work than object initializer syntax. But not too terrible, and the value of immutable objects is worth it.
(For what it's worth, C# 7 may get immutable "record types" that have much simpler syntax. These may or may not make it into the final release, but they sound pretty cool.)
This is dated now, but with the release of C# 9 you can use init to achieve the desired functionality.
So your example would become:
class Contract
{
// Read-only properties.
public string Name { get; init; }
public string Address { get; init; }
}
And then you could initialize with:
// success!
Contract a = new Contract { Name = "John", Address = "23 Tennis RD" };
But you would still be unable to modify the parameters after setting them (so effectively they are still readonly).
// error!
a.Name = "Uncle Bob";
Under the hood, when you use object initializer syntax prior to C# 9 the compiler would call the default constructor first, and then set the property values you've specified. Obviously if those properties are readonly (i.e. only a get method), it can't set them. The init only setter allows setting the value only on initialization, either via a constructor method or object initializer syntax.
More info is available here: https://learn.microsoft.com/en-us/dotnet/csharp/whats-new/csharp-9#init-only-setters
Nope, you cannot use it with readonly properties.
Here are the different property and field types in comparism.
public class sometype {
public int readonlyProp{
get;
}
public int normalProp {
get;
set;
}
public const int constField = 3;
public readonly int readonlyField = 3;
public int normalField = 3;
public void test() {
sometype test = new sometype() { readonlyProp = 3}; // Doesn't work -> Property or indexer is readonly
sometype test1 = new sometype() { normalProp = 3 }; // ok
sometype test2 = new sometype() { constField = 3 }; // Doesn't work -> Static field or property
sometype test3 = new sometype() { readonlyField = 3 }; // Doesn't work -> readonly field
sometype test4 = new sometype() { normalField = 3 }; // ok
}
}
It is important to understand that const fields are considered static and thus are not instance members. And since the object initializer is used for instance members this doesn't work.
Object initializer will first construct the object, then set property values.
It needs setters.
It's short hand for:
Contact a = new Contact();
a.Name = "John";
a.Address = "23 Tennis RD";
A readonly field can't have it's values set once the object has been constructed. To have that class immutable, you'll need to create a constructor to take those values:
class Contact // Immutable class
{
// Read-only properties.
public string Name { get; }
public string Address { get; }
public Contact(string name, string address)
{
this.Name = name;
this.Address = address;
}
}
I've been trying to experiment with reflection and I have a question.
Lets say I have a class, and in this class I have a property initilized with the new feature of c# 6.0
Class MyClass()
{
public string SomeProperty{ get; set; } = "SomeValue";
}
Is there any way of getting this value, with reflection, without initilizating the class?
I know I could do this;
var foo= new MyClass();
var value = foo.GetType().GetProperty("SomeProperty").GetValue(foo);
But what I want to do is something similiar to this ;
typeof(MyClass).GetProperty("SomeProperty").GetValue();
I know I could use a field to get the value. But it needs to be a property.
Thank you.
It's just a syntax sugar.
This:
class MyClass()
{
public string SomeProperty{ get; set; } = "SomeValue";
}
will be unwrapped by compiler into this:
class MyClass()
{
public MyClass()
{
_someProperty = "SomeValue";
}
// actually, backing field name will be different,
// but it doesn't matter for this question
private string _someProperty;
public string SomeProperty
{
get { return _someProperty; }
set { _someProperty = value; }
}
}
Reflection is about metadata. There are no any "SomeValue" stored in metatada. All you can do, is to read property value in regular way.
I know I could use a field to get the value
Without instantiating an object, you can get values of static fields only.
To get values of instance fields, you, obviously, need an instance of object.
Alternatively, if you need default value of property in reflection metadata, you can use Attributes, one of it from System.ComponentModel, do the work: DefaultValue. For example:
using System.ComponentModel;
class MyClass()
{
[DefaultValue("SomeValue")]
public string SomeProperty{ get; set; } = "SomeValue";
}
//
var propertyInfo = typeof(MyClass).GetProperty("SomeProperty");
var defaultValue = (DefaultValue)Attribute.GetCustomeAttribute(propertyInfo, typeof(DefaultValue));
var value = defaultValue.Value;
Let's say there's a class that with one public constructor, which takes one parameter. In addition, there are also multiple public properties I'd like to set. What would be the syntax for that in F#? For instance in C#
public class SomeClass
{
public string SomeProperty { get; set; }
public SomeClass(string s)
{ }
}
//And associated usage.
var sc = new SomeClass("") { SomeProperty = "" };
In F# I can get this done using either the constructor or property setters, but not both at the same time as in C#. For example, the following aren't valid
let sc1 = new SomeClass("", SomeProperty = "")
let sc2 = new SomeClass(s = "", SomeProperty = "")
let sc3 = new SomeClass("")(SomeProperty = "")
It looks like I'm missing something, but what?
<edit: As pointed out by David, doing it all in F# works, but for some reason, at least for me :), it gets difficult when the class to be used in F# is defined in C#. As for an example on such is TopicDescription (to make up something public enough as for an added example). One can write, for instance
let t = new TopicDescription("", IsReadOnly = true)
and the corresponding compiler error will be Method 'set_IsReadOnly' is not accessible from this code location.
The problem you're having is that IsReadOnly has an internal setter.
member IsReadOnly : bool with get, internal set
If you want to set it directly you're going to need to subclass TopicDescription.
The constructor syntax you're looking at is perfectly acceptable.
let test = new Microsoft.ServiceBus.Messaging.TopicDescription("", EnableBatchedOperations=true)
compiles just fine for me.
I never program in F#, but this seems to work fine for me:
type SomeClass(s : string) =
let mutable _someProperty = ""
let mutable _otherProperty = s
member this.SomeProperty with get() = _someProperty and set(value) = _someProperty <- value
member this.OtherProperty with get() = _otherProperty and set(value) = _otherProperty <- value
let s = new SomeClass("asdf", SomeProperty = "test");
printf "%s and %s" s.OtherProperty s.SomeProperty;
That outputs "asdf and test".
Additionally, the following code works fine for me:
public class SomeClass
{
public string SomeProperty { get; set; }
public string OtherProperty { get; set; }
public SomeClass(string s)
{
this.OtherProperty = s;
}
}
Then in F#:
let s = SomeClass("asdf", SomeProperty = "test")
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;
}
}