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:

Answer 1

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

Answer 2

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.

Answer 3

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; }
}

Answer 4

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";