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Setting Default Values on Automatic Properties

, 14 May 2008 CPOL
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An article on implementing default values on Automatic Properties

Introduction

C# 3.0 introduces a great new feature called Automatic Properties, and if you haven’t already read about them, I would encourage you to read Scott Guthrie's introductory post.

As great as they are and as much time as they save, Automatic Properties have a serious drawback – you can’t set the default value of the property. Instead, the compiler will initialize value properties to 0, reference properties to null, and enum’s to the first member, and while this might work for some applications, it wasn’t working for mine.

Background

When I thought about the implementation, two options became apparent. One, I could create a base object class and have all of my classes inherit from this base class. This however isn’t a great solution because a number of my classes inherit from other classes outside of my control, and since .NET does not support multiple inheritances, it was clear this wasn’t going to work. To my rescue was the also new C# 3.0 feature, Extension Methods. If you haven’t already heard about Extension Methods, I’d recommend reading another one of Scott Guthrie's blog posts about them.

Using the Code

Using the code requires that you decorate your properties with an attribute already available in the System.ComponentModel namespace – if you haven’t already guessed it, it’s the aptly named DefaultValueAttribute attribute. As well, it requires a quick call to the InitDefaults() extension method from the constructor which I will discuss a bit later.

The attached code supplies a demo implementation of the TestObject and TestObjectInherited classes:

public class TestObject
    {
        public TestObject()
        {
            this.InitDefaults();
        }

        [DefaultValue(-45)]
        public int DefaultInt
        {
            get;
            set;
        }

        [DefaultValue(10.23)]
        public double DefaultDouble
        {
            get;
            set;
        }

        [DefaultValue(true)]
        public bool DefaultBool
        {
            get;
            set;
        }

        [DefaultValue(TestEnum.Value2)]
        public TestEnum DefaultEnum
        {
            get;
            set;
        }

        [DefaultValue("DefaultString!")]
        public string DefaultString
        {
            get;
            set;
        }

        public string StringWithoutDefault
        {
            get;
            set;
        }

        public string ValueOfPrivateProperty
        {
            get
            {
                return PrivateProperty;
            }
        }

        [DefaultValue("This is a private property!")]
        protected string PrivateProperty
        {
            get;
            set;        
        }
    }

The magical InitDefaults() method is implemented as an extension method which uses reflection to set the value of the properties to the default value:

public static void InitDefaults(this object o)
        {
            PropertyInfo[] props = o.GetType().GetProperties(BindingFlags.Public | 
                BindingFlags.NonPublic | BindingFlags.Instance | BindingFlags.Static);

            for (int i = 0; i < props.Length; i++)
            {
                PropertyInfo prop = props[i];                

                if (prop.GetCustomAttributes(true).Length > 0)
                {
                    object[] defaultValueAttribute = 
                        prop.GetCustomAttributes(typeof(DefaultValueAttribute), true);

                    if (defaultValueAttribute != null)                    
                    {
                        DefaultValueAttribute dva = 
                            defaultValueAttribute[0] as DefaultValueAttribute;
                        
                        if(dva != null)
                            prop.SetValue(o, dva.Value, null);                                
                    }
                }
            }
        }

Points of Interest

I decided to support initializing the default value of properties in inherited classes, but if you don’t want this behavior, you can simply pass false to GetCustomAttributes().

if (prop.GetCustomAttributes(false).Length > 0)
                {
                    object[] defaultValueAttribute = prop.GetCustomAttributes
                        (typeof(DefaultValueAttribute), false);

                    if (defaultValueAttribute != null)                    
                    {
                        DefaultValueAttribute dva = 
                            defaultValueAttribute[0] as DefaultValueAttribute;
                        
                        if(dva != null)
                            prop.SetValue(o, dva.Value, null);                                
                    }
                }

History

  • 14th May, 2008: Initial post

License

This article, along with any associated source code and files, is licensed under The Code Project Open License (CPOL)

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About the Author

Steven Berkovitz
Web Developer
Canada Canada
Steven is VP Development at MBC Computer Solutions Ltd. (http://www.mbccs.com), a Richmond Hill based company specializing in e-Business Application Development, e-Store Solutions, Managed Co-Location and Proactive IT services.
 
Steven has over 10 years experience in software and hardware design and is experienced with a large array of platforms, technologies and languages.
 
In his spare time, Steven enjoys a wide array of music, is an avid skier and enjoys spending time with friends.
 
Steven is the primary contributor of MBC's blog which can be read at http://blogs.mbccs.com/mbccomputersolutions

Comments and Discussions

 
Questionover-design? Pinmemberffffffffffffffffffff29-Oct-08 10:03 
GeneralDiffent approach PinmemberGünther M. FOIDL8-Oct-08 1:01 
RantVote for initialization to be added as a language feature. PinmemberLaughing.John23-Aug-08 14:08 
GeneralI don't think its a good idea PinmemberJorge Bay Gondra22-Aug-08 4:18 
GeneralAnother option PinmemberAjek20-May-08 12:02 
GeneralRe: Another option Pinmembernsimeonov9-Jun-09 17:32 
GeneralA opinion PinmemberRicky Wang19-May-08 20:57 
GeneralI'd do it differently... PinmemberPIEBALDconsult14-May-08 14:49 
GeneralRe: I'd do it differently... PinmemberSeishin#15-May-08 2:56 
GeneralRe: I'd do it differently... PinmemberPIEBALDconsult15-May-08 6:56 
GeneralRe: I'd do it differently... PinmemberPIEBALDconsult15-May-08 8:39 
GeneralRe: I'd do it differently... PinmemberSeishin#15-May-08 9:30 
GeneralRe: I'd do it differently... PinmemberPIEBALDconsult15-May-08 10:45 
GeneralRe: I'd do it differently... PinmemberSeishin#15-May-08 10:47 
GeneralRe: I'd do it differently... PinmemberPIEBALDconsult15-May-08 11:15 
GeneralI Don't See The Value In Automatic Properties PinmvpJohn Simmons / outlaw programmer14-May-08 12:26 
GeneralRe: I Don't See The Value In Automatic Properties [modified] PinmemberPIEBALDconsult14-May-08 14:16 
GeneralRe: I Don't See The Value In Automatic Properties PinmemberWilliam E. Kempf15-May-08 7:47 
GeneralRe: I Don't See The Value In Automatic Properties PinmemberPIEBALDconsult15-May-08 8:53 
GeneralRe: I Don't See The Value In Automatic Properties PinmemberWilliam E. Kempf15-May-08 9:37 
GeneralRe: I Don't See The Value In Automatic Properties PinmemberPIEBALDconsult15-May-08 10:29 
GeneralRe: I Don't See The Value In Automatic Properties PinmemberWilliam E. Kempf15-May-08 10:42 
GeneralRe: I Don't See The Value In Automatic Properties PinmemberSteven Berkovitz15-May-08 10:44 
GeneralRe: I Don't See The Value In Automatic Properties PinmemberWilliam E. Kempf15-May-08 10:46 
GeneralRe: I Don't See The Value In Automatic Properties PinmemberPIEBALDconsult15-May-08 10:49 
GeneralRe: I Don't See The Value In Automatic Properties PinmemberSteven Berkovitz15-May-08 10:50 
GeneralRe: I Don't See The Value In Automatic Properties PinmemberPIEBALDconsult15-May-08 11:11 
GeneralRe: I Don't See The Value In Automatic Properties PinmemberWilliam E. Kempf15-May-08 11:44 
GeneralRe: I Don't See The Value In Automatic Properties PinmemberPIEBALDconsult15-May-08 13:03 
GeneralRe: I Don't See The Value In Automatic Properties PinmemberWilliam E. Kempf16-May-08 3:47 
GeneralRe: I Don't See The Value In Automatic Properties PinmemberPIEBALDconsult16-May-08 11:42 
GeneralRe: I Don't See The Value In Automatic Properties PinmemberWilliam E. Kempf16-May-08 15:59 
GeneralRe: I Don't See The Value In Automatic Properties PinmemberPIEBALDconsult16-May-08 20:30 
GeneralRe: I Don't See The Value In Automatic Properties PinmemberPIEBALDconsult15-May-08 15:22 
GeneralRe: I Don't See The Value In Automatic Properties PinmemberWilliam E. Kempf16-May-08 3:49 
GeneralRe: I Don't See The Value In Automatic Properties PinmemberPolymorpher21-May-08 13:39 

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