By bentford


2008-09-02 21:29:41 8 Comments

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?

22 comments

@Julian 2019-11-28 13:31:13

In c# 8.0 you also can make them nullable:

public string? MyProperty { get; set; } = null;

or not nullable:

 public string MyProperty { get; set; } = null!; 

@Keith 2008-09-04 12:16:06

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";

@dbobrowski 2012-09-13 19:12:59

It's only best practice if constructors need to initialize in different ways (across multiple constructors). If they all initialize the same way, isn't what you are suggesting bad practice if I also want Person(string name), and then later I add property int ID and I have yet another constructor Person(string name, int id)? Violation of open to extension, closed to modification. I argue that it's best practice to initialize in line so long as all constructors initialize the member the same way.

@Keith 2013-03-08 16:35:31

@dbobrowski if I has multiple constructors I'd use public Person(string name, int id) : this() syntax, but you have a point. This gives you more fine control of exactly when properties are initialised at the cost of the inline option's simpler maintainability.

@ANeves wants peace for Monica 2014-01-31 17:43:31

@Keith I'd prefer public Person():this("Default Name"), rather than setting Name twice.

@sara 2015-12-14 07:40:45

if you need to run the same code in several places, you don't resort to reflection and weird attributes that no one has ever heard of. you extract a helper method, or in the real complicated places, create a factory (or just redesign/break down your class alltogether since it's probably too complex)

@Darren Kopp 2008-09-02 21:46:23

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, ...

@CallMeLaNN 2011-02-07 04:14:28

I can't do this with abstract class. My abstract class with auto-property seems impossible to assign default property since abstract cannot have a constructor.

@Darren Kopp 2011-03-01 18:31:38

you are incorrect. abstract classes can have constructors.

@Ludovic Chabant 2011-07-29 19:06:31

abstract classes usually have protected constructors, actually.

@Darren Kopp 2011-07-29 23:04:40

abstract classes usually have whatever accessibility you give to them. if you don't specify a constructor you always have a default public constructor

@Guillaume Massé 2011-09-08 21:00:56

@CallMeLaNN call :base() in your concrete class to construct it's base (abstract)

@Dave Black 2012-02-08 14:16:47

Constructors to abstract classes should never be given public access since they can never be instantiated. To do so would be a FxCop violation (hopefully you're using it) and a violation of .NET Framework Design Guidelines - amazon.com/Framework-Design-Guidelines-Conventions-Libraries‌​/dp/…

@Jeppe Stig Nielsen 2012-05-26 23:07:34

@DarrenKopp If, in a non-static class, you don't write any instance constructors yourself, then the auto-generated parameterless instance constructor will have (1) protected accessibility if your class is abstract, and (2) public accessibility otherwise.

@Hank Schultz 2013-06-19 20:50:37

If default values are set in the constructor, wouldn't this override on object initializer? That is: Person a = new Person() { Name = "My Value" }; ...would wind up with a.Name == "Default Value"?

@Darren Kopp 2013-06-19 23:07:18

@HankSchultz no, object initializer is just sugar the compiler does. what actually is emitted is this: var person = new Person(); persion.Name = "My Value"; Constructor always runs first

@TamusJRoyce 2014-06-02 05:23:10

ComponentModel only defaults your values for designers or things which read those attributes and initialize those properties for you. You do have to initialize properties in the constructor. It is best to do that anyways.

@Nyra 2014-07-18 14:16:50

@DarrenKopp thank you for that break down- am I correct in understanding you are saying that is what the VB is actually doing at run-time (but in C# syntax)?

@Darren Kopp 2014-07-18 15:10:52

@alykins I believe the VB.NET compiler is just generating code where the automatic backing field is getting assigned, like private int _A = 1; in csharp.

@Jespertheend 2016-08-12 19:40:54

According to Console.WriteLine(typeof(string).Assembly.ImageRuntimeVersio‌​n); I'm using C# 4.0 (v4.0.30319) but I'm still able to use default values for propperties, is this correct? I'm using mono.

@Darren Kopp 2016-08-13 04:43:47

@Jespertheend the compiler may be able to compile to 4.0 runtime, because ultimately the intent is still there (assign value in constructor). again, the new syntax is just a shortcut for what you had to do before.

@Jeppe Stig Nielsen 2018-11-01 11:53:01

@ComeIn All abstract classes (not static classes) have an instance constructor, possibly overloaded. If one did not, that class would be useless because nobody would be able to inherit from it. You can write one or more instance constructor overloads "by hand". If you do not write any explicitly, the C# compiler will write one for you. The automatic, "invisible" constructor will look like this: protected YourClassName() { } If the automatic constructor is illegal because the base class does not have the right constructor overload (zero arguments), the entire class will not compile.

@ComeIn 2018-11-20 02:43:23

Constructors are only applicable to the class in which they are defined, that is, they are not inherited. Base class constructors are used (you have to call one of them, even if only calling the default one automatically) but not overridden by deriving classes. You can define a constructor on an abstract base class -- it can't be used directly, but can be invoked by deriving classes. What you can't do is force a derived class to implement a specific constructor signature.

@Whelkaholism 2019-10-08 09:50:51

Am I the only person that thinks this new syntax is hideously ugly and it should just go in the constructor?

@Sunil Dhappadhule 2019-02-18 15:10:30

You can simple put like this

public sealed  class Employee
{
    public int Id { get; set; } = 101;
}

@Chuck Rostance 2011-06-22 18:14:58

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; }
}

@Tamilmaran 2012-02-10 05:44:53

Yes,It's the answer:[System.ComponentModel.DefaultValue(GiveAnyTypeOfDefa‌​ultValueHere)]

@Hamish Grubijan 2012-08-30 22:22:12

This is cool! I would change the public constructor to private and have the other constructors chain with the private one.

@Patrick M 2012-11-01 20:04:46

+1 for ingenuity, but this can be really slow. Imagine newing a bunch of items in an inner loop without realizing the constructors use reflection...

@Gayot Fow 2013-07-04 19:16:45

If the base class uses a static constructor to load the attributes and values into a dictionary, there's no overhead on each class!

@Grigory 2013-08-21 13:03:00

U're recommending really bad thing. It is quite slow and introduces reflection to quite simple class. If it wasn't so slow it would be okay to use some kind of il-weaving with such attribute (PostSharp, Fody, etc), but the performance...

@G.Y 2013-08-25 08:08:11

Only good for the VS designer and even than not in all cases (ie asp.net), anyway.. this will not do anything at runtime.

@Chuck Rostance 2013-08-26 17:24:52

I'm pretty sure it works at run time, did you try it?

@B.K. 2014-03-11 21:12:14

@ChuckRostance I tried... doesn't work at runtime... much sadness.

@ComeIn 2018-11-01 14:51:57

private string name;
public string Name 
{
    get 
    {
        if(name == null)
        {
            name = "Default Name";
        }
        return name;
    }
    set
    {
        name = value;
    }
}

@Jeppe Stig Nielsen 2018-11-02 08:59:16

I think the asker wanted an auto-property, i.e. a non-abstract property in a class or struct where you use just get; with a semicolon (often combined with set;) to indicate that the compiler should generate the body of the get accessor automatically.

@MarkO 2018-12-05 12:27:18

Also, this code has the side effect of reverting to the default value when explicitly set to null.

@Shiva 2016-07-25 19:35:56

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;
    }
}

@freefaller 2018-04-20 11:20:56

I don't have C#6.0 yet, and was checking to see what version I needed for default values on auto-properties. Does C# 6.0 also remove the need to have { get; set; } or { get; private set; } as otherwise setting the value would be blocked by the compiler?

@ANewGuyInTown 2017-03-15 01:07:28

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; 
 }

@Aluan Haddad 2018-02-22 08:50:47

The is fine but only the second of the three examples is an auto-property.

@Jeppe Stig Nielsen 2019-07-13 15:19:50

Also, consider the example class C { public DateTime P { get; } = DateTime.Now; public DateTime Q => DateTime.Now; } where both properties P and Q have a getter only, but the behaviors of P and Q are very different!

@Lex 2008-09-04 17:08:08

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

@Mark Brackett 2016-02-25 19:19:00

This is wrong; there is no "double assignment" problem w/using the constructor....

@Lex 2016-02-27 21:22:26

Wow, this is a blast from the past. I seem to recall this was based on the reading of the spec (partial excerpt here: msdn.microsoft.com/en-us/library/aa645756(v=vs.71).aspx ). Given the time and number of versions (and Roslyn) this could well not the case anymore. Although a counter reference would be appreciated.

@Mark Brackett 2016-02-27 23:47:12

Default assignment happens automatically whether or not you use an initial value or assign in constructor. There is a slight semantic difference - field assignments happen prior to constructor calls - but the null assignment will still happen. See 10.4.5 "all instance fields...are first initialized to their default values, and then the instance field initializers are executed" msdn.microsoft.com/en-us/library/aa645757(VS.71).aspx

@ghiboz 2010-06-17 07:44:02

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; }
}

@Boris B. 2011-07-29 13:05:14

That won't work. DefaultValueAttribute is just a serialization hint, it will not set ShowGroup to true because the default value for any boolean is false.

@brakeroo 2016-10-30 02:16:41

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

@RayLoveless 2016-07-18 16:39:49

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

@Preet Singh 2016-01-11 11:33:09

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.

@Habib 2014-04-29 14:40:19

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

@FloodMoo 2014-09-04 20:44:06

public Class ClassName{
    public int PropName{get;set;}
    public ClassName{
        PropName=0;  //Default Value
    }
}

@Nag 2011-05-26 19:29:05

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; }
}

@fire.eagle 2011-05-26 19:34:55

Welcome to Stack Overflow! Just so you know, bumping up an old question like this is generally frowned upon unless you have some good new information. However, in this case, several others have already posted about the DefaultValue attribute. If someone else has already posted what you were going to say, it's more appropriate to upvote them by clicking on the up arrow above the number next to their answer.

@Ben Voigt 2011-06-03 02:12:48

@fire: Commenting requires 50 reputation. Voting also requires reputation, IIRC.

@Gyum Fox 2014-08-07 08:03:59

-1 as this does not initializes the property to the default value.

@KinSlayerUY 2017-11-01 15:47:29

only works on the designer as stated in all the previous answers

@user3076134 2013-12-06 21:50:59

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;        
    }
}

@Aluan Haddad 2018-02-22 08:55:19

This is not an auto-property.

@Zack Jannsen 2012-07-31 11:02:41

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.

@introspected 2013-01-17 22:04:13

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

@KinSlayerUY 2017-11-01 15:46:08

As stated above, using reflection to initialize default values is both slow and overkill. Initialize on the constructor, use a non auto property or on c# 6 and above, use the simplified notation shown in the accepted answer

@Joel Coehoorn 2008-09-03 03:43:20

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

@Chris Farmer 2008-09-03 03:59:35

@Joel: data binding and other reflection-based tools often expect properties rather than fields.

@David Reis 2009-07-28 23:37:34

You cannot refactor a field into an auto property without breaking the calling code. It might look the same same but the generated code is different. With auto properties the calling code calls get_propname and set_propname behind the covers, whereas it just access the field directly if it's a field.

@Joel Coehoorn 2009-07-28 23:53:54

Yes, this is very old- I've since revised my position: stackoverflow.com/questions/205568/…

@Jacob Krall 2009-11-06 18:00:12

You cannot access a field across AppDomain boundaries, either -- only a property or method.

@yoel halb 2013-08-07 02:22:46

And you cannot declare a field in an interface only a property

@crucible 2008-09-02 23:07:27

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!

@abatishchev 2010-08-09 08:11:15

return _name ?? "Default Name"; probably even is more clear that your

@Sebastian Mach 2010-12-16 13:28:27

@abatishchev: though that is not the same. crucibles code would return "Default Name" if the string is "" or null, but using your approach would return "Default Name" only in case it is null. Also, it is discussible whether "??" or "IsNullOrEmpty" is more clear.

@abatishchev 2010-12-17 06:51:30

@phresnel: If property value can be "" and it's the same as null then you're right. Otherwise "" is meaningful value and my code is better

@Gilbert 2016-05-24 13:40:40

private string _name = "Default Name"; public string Name { get { return _name; } set { _name = value; } }

@ToolmakerSteve 2018-01-28 21:31:25

@Gilbert - you have repeated one of the code options shown in the question. Crucible's answer is specifically for the case "I don't want the default value persisted in my db". That is, he wants to be able to determine elsewhere whether _name has been written to, or is still null. [His write or persist code checks _name for null, skips writing it.]

@OwenP 2008-09-02 21:32:41

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.

@Hans Kesting 2010-07-26 14:21:12

No, those only work in the designer, not in "real life".

@Ben Voigt 2011-06-03 02:09:37

@Hans: They do work "in real life", but only for setting a default value (this affects xml serialization, for example). What's wanted here is not a "default value" but an "initial value".

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