What is the best way to give a C auto-property an initial value

Question

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

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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

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Edited on 1 2 15  C  6    With C  6 you can initialize auto-properties directly  finally    there are now other answers in the thread 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

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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

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In the constructor   The constructor s purpose is to initialized it s data members

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In Version of C   6 0   amp  greater  you can do     For Readonly properties  public int ReadOnlyProp   gt  2    For both Writable  amp  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   gt  label     set   gt  this label   value

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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

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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

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public Class ClassName      public int PropName get set       public ClassName          PropName 0     Default Value

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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   gt   ampere hour

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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   Hope that helps

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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

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private string name  public string Name        get                if name    null                        name    Default Name                     return name            set               name   value

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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

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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

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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

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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

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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

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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  amp  amp  pair Attr Values Length  gt  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 lt string gt  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

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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

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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    gt     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

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You can simple put like this  public sealed  class Employee       public int Id   get  set      101

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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

[c#] What is the best way to give a C# auto-property an initial value?

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