Instance { get; } = new <class>() - Is this a C#7/8/9 feature? [duplicate]

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?

Solution 1:

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, ...

Solution 2:

Edited on 1/2/15

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; }
}