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, ...

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.

Hope that helps!

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 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

You can simple put like this

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

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.

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

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";
}