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

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, ...

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.

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.

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

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.

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

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

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;        
    }
}