Wednesday, May 14, 2008

Setting 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 behaviour 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);
}
}

1 comment:

Laughing John said...

I've opened an MS connect to request initialization as a language feature. If anyone is interested please add your vote/comments.

http://connect.microsoft.com/VisualStudio/feedback/ViewFeedback.aspx?FeedbackID=361647