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 have a custom attribute class defined as follows.
[AttributeUsage(AttributeTargets.Property, Inherited = false)]
internal class EncryptedAttribute : System.Attribute
{
private bool _encrypted;
public EncryptedAttribute(bool encrypted)
{
_encrypted = encrypted;
}
public virtual bool Encrypted
{
get
{
return _encrypted;
}
}
}
I applied the above attribute to another class as follows.
public class KeyVaultConfiguration
{
[Encrypted(true)]
public string AuthClientId { get; set; } = "";
public string AuthClientCertThumbprint { get; set; } = "";
}
How do I get the value of Encrypted=True on property AuthClientId?
var config = new KeyVaultConfiguration();
// var authClientIdIsEncrypted = ??
In .NET Framework, this was easy. In .NET CORE, I think this is possible but I don't see any documentation. I believe you need to use System.Reflection but exactly how?
Add using System.Reflection and then you may use extension methods from CustomAttributeExtensions.cs.
Something like this should work for you:
typeof(<class name>).GetTypeInfo()
.GetProperty(<property name>).GetCustomAttribute<YourAttribute>();
in your case
typeof(KeyVaultConfiguration).GetTypeInfo()
.GetProperty("AuthClientId").GetCustomAttribute<EncryptedAttribute>();
I am using the Description attribute in my enums to provide a user friendly name to an enum field. e.g.
public enum InstallationType
{
[Description("Forward of Bulk Head")]
FORWARD = 0,
[Description("Rear of Bulk Head")]
REAR = 1,
[Description("Roof Mounted")]
ROOF = 2,
}
And accessing this is easy with a nice helper method:
public static string GetDescriptionFromEnumValue(Enum value)
{
DescriptionAttribute attribute = value.GetType()
.GetField(value.ToString())
.GetCustomAttributes(typeof(DescriptionAttribute), false)
.SingleOrDefault() as DescriptionAttribute;
return attribute == null ? value.ToString() : attribute.Description;
}
I need to convert this into a portable class library but it doesn't seem to have access to the System.ComponentModel library. when I try add a reverence VS tells me that I have referenced everything already.
Thanks
Since DescriptionAttribute is not available for portable class libraries you need to use another attribute. The namespace System.ComponentModel.DataAnnotations which is available for portable class libraries provides the attribute DisplayAttribute that you can use instead.
public enum InstallationType
{
[Display(Description="Forward of Bulk Head")]
FORWARD = 0,
[Display(Description="Rear of Bulk Head")]
REAR = 1,
[Display(Description="Roof Mounted")]
ROOF = 2,
}
Your method needs to be changed to
public static string GetDescriptionFromEnumValue(Enum value)
{
DisplayAttribute attribute = value.GetType()
.GetField(value.ToString())
.GetCustomAttributes(typeof(DisplayAttribute ), false)
.SingleOrDefault() as DisplayAttribute ;
return attribute == null ? value.ToString() : attribute.Description;
}
Whether something is available to a portable class library depends a bit on exactly which frameworks you selected for the library - you get the strict intersection only. However, it could well be that this attribute simply doesn't exist in one of your targeted frameworks. In which case, one option is add your own - then you know it is available. For example:
[AttributeUsage(AttributeTargets.Field, AllowMultiple = false)]
public class EnumDescriptionAttribute :Attribute
{
private readonly string description;
public string Description { get { return description; } }
public EnumDescriptionAttribute(string description)
{
this.description = description;
}
}
enum Foo
{
[EnumDescription("abc")]
A,
[EnumDescription("def")]
B
}
Note that I intentionally haven't included the additional serialization construtors here, because those too depend on features that are not available on all frameworks. Changing your code from using [Description] / DescriptionAttribute to [EnumDescription] / EnumDescriptionAttribute should be fairly trivial.
Try this for retrieving attribute for enum in portable libraries:
public static class EnumsHelper
{
public static T GetAttributeOfType<T>(this Enum enumVal) where T : Attribute
{
var typeInfo = enumVal.GetType().GetTypeInfo();
var v = typeInfo.DeclaredMembers.First(x => x.Name == enumVal.ToString());
return v.GetCustomAttribute<T>();
}
}
Update: also you should declare new attribute (look like DescriptionAttribute not available in PCL), for example next:
public class MyDescriptionAttribute : Attribute
{
public virtual string Text { get; set; }
}
and add one more method in EnumsHelper class:
public static class EnumsHelper
{
...
public static string GetDescription(this Enum enumVal)
{
var attr = GetAttributeOfType<MyDescriptionAttribute>(enumVal);
return attr != null ? attr.Text : string.Empty;
}
}
and if you have next enum:
public enum InstallationType
{
[MyDescription(Text = "Forward of Bulk Head")]
FORWARD = 0
}
you can retrieve description with code like this:
static void Main(string[] args)
{
var it = InstallationType.FORWARD;
var description = it.GetDescription();
Console.WriteLine(description);
}
namespace MyOldService
{
public MyNewService.AddressList ToPrivateAddressList()
{
MyNewService.AddressList privAddrList = new MyNewService.AddressList();
privAddrList.creator = (MyNewService.AddressListCreator)this.creator;
privAddrList.listId = this.listId;
privAddrList.listIdSpecified = this.listIdSpecified;
privAddrList.listName = this.listName;
privAddrList.listType = (MyNewService.AddressingMode)this.listType;
privAddrList.lastModified = this.lastModified;
privAddrList.lastModifiedSpecified = this.lastModifiedFieldSpecified;
if (this.siteList != null && this.listType == MyOldService.AddressingMode.XDAddressingModeSiteIDList)
{
privAddrList.siteList = new long[this.siteList.Length];
Array.Copy(this.siteList, privAddrList.siteList, this.siteList.Length);
}
...
Originally written to copy a list defined in a SOAP namespace MyOldService to a class of the same layout in a new namespace MyNewService. The problem is, with soap classes, if I import the MyOldService namespace into a third namespace, say MyOtherAppService, then my AddressList class becomes a member of that third namespace and is referenced as such.
So, rather than duplicating the code, I'd like to decorate it (or adjust it somehow) with something akin to generics (which I understand won't work because I'm altering the namespace, not just one fixed type [there are multiple types I need from each namespace, as can be seen from the snippet]) to allow this to convert the address list to the corresponding class in whichever namespace is needed. Possible?
Edit: In response to some of the comments below, I'll try to define a better example of what I'm trying to do.
Three classes imported from WSDLs via web references (no, these won't compile, just examples for illustration). The classes AddressList all have the same layout.
namespace A
{
enum Mode {};
enum Creator {};
class ATypeClass {}
public partial class AddressList
{
int id;
enum Mode mode;
enum Creator creator
long[] siteList;
ATypeClass[] cspList;
}
}
namespace B
{
enum Mode {};
enum Creator {};
class BTypeClass {}
public partial class AddressList
{
int id;
enum Mode mode;
enum Creator creator
long[] siteList;
BTypeClass[] cspList;
}
}
namespace C
{
enum Mode {};
enum Creator {};
class CTypeClass {}
public partial class AddressList
{
int id;
string name;
enum Mode mode;
enum Creator creator
long[] siteList;
CTypeClass[] cspList;
}
}
I'll extend the partial class in namespace A with a new method:
namespace A
{
public partial class AddressList
{
public T.AddressList ToPrivateAddressList<T>()
{
T.AddressList privAddrList = new T.AddressList();
privAddrList.creator = (T.Creator)this.creator;
privAddrList.id = this.id;
privAddrList.name = this.name;
privAddrList.mode = (T.Mode)this.mode;
if (this.siteList != null && this.listType == Mode.XDAddressingModeSiteIDList)
{
privAddrList.siteList = new long[this.siteList.Length];
Array.Copy(this.siteList, privAddrList.siteList, this.siteList.Length);
}
...
}
}
}
Notice that part of the problem, in addition to the classes each part of a different namespace, are the enums that are also from the varying namespaces.
Finally, I envision calling it like so (though I know I can't actually do this, I'm looking for a solution that's roughly as elegant):
B.AddressList al1 = A.AddressList.ToPrivateAddressList<B>();
C.AddressList al1 = A.AddressList.ToPrivateAddressList<C>();
I think what you are looking for is an interface and the where keyword in the generic type definition.
From your code I see that you have this:
a method that converts type A to type B
it does this by assigning properties with the same name
and then returns the new type B
the types can reside in different namespaces (but currently have the same name)
classes are partial
Instead of relying on the same name (which you can, using reflection you can achieve the effect you want), you should let each class implement an interface that contains the common properties. This way, you retain compile time type-safety:
public interface ICommonAddress
{
int id { get; set; }
Mode mode { get; set; }
Creator creator { get; set; }
long[] siteList { get; set; }
ICommonAddress CreateAddress();
}
You can now refactor your classes like this (of course, you'll have to change your fields into properties, but I'm assuming you have them as properties already:
// if your original partial class is auto-generated, it is ok to place
// this definition in another file, it'll still work as long as
// namespace, classname and compile-unit (must be in same project) are the same
public partial class AddressList : ICommonAddress
{
int id { get; set; }
Mode mode { get; set; }
Creator creator { get; set; }
long[] siteList { get; set; }
ATypeClass[] cspList;
ICommonAddress CreateAddress()
{
return new AddressList(); // NOTE: you can even have your ctor private!
}
}
If you do that for each AddressList type you have, you can change your generic method as follows, and it will automatically work, including the IntelliSense showing you the common available properties. Also, implement it as an extension method, so that it applies to all your AddressList types (this is in your case better than using partial):
public T ToPrivateAddressList<T>(this ICommonAddress _this)
where T: ICommonAddress
{
T privAddrList = _this.CreateAddress();
// this now works normally, without casting
privAddrList.creator = _this.creator;
privAddrList.id = _this.id;
privAddrList.name = _this.name;
privAddrList.mode = _this.mode;
}
Now, if you import a reference to this extension method, you can call ToPrivateAddressList() on any type that you have:
A.AddressList a_address = A.AddressList.CreateAddress(); // or new A.AddressList()
B.AddressList al1 = a_address.ToPrivateAddressList<B.AddressList>();
C.AddressList al1 = a_address.AddressList.ToPrivateAddressList<C.AddressList>();
If I right undesrtood your problem you have
namespace A
{
public class MyClass {... }
}
and
namepsace B
{
public class MyClass {...}
}
Your question is: how can I define something "generic" that in the same function sometimes deals with A.MyClass and other cases B.MyClass.
If you think about namespaces like a part of the type definition, I think the story becomes clear.
Like if you have 2 different types and want to have one generic that works with both of them. That generic class knows when to choose A.MyClass method, and when to choose B.MyClass method.
If it's not what you're asking for, please clarify.
Are you asking how you can do this:
namespace Old
{
public New.Address Foo()
{
New.Address result;
//...
return result;
}
}
Without having to say 'New.' on each occurrence of Address? But if you don't, the code uses Old.Address?
Can you remove Old.Address from the build/linking of this project (with this code)?
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;
}
}