[c#] Does C# have extension properties?

Does C# have extension properties?

For example, can I add an extension property to DateTimeFormatInfo called ShortDateLongTimeFormat which would return ShortDatePattern + " " + LongTimePattern?

Update (thanks to @chaost for pointing this update out):

Mads Torgersen: "Extension everything didn’t make it into C# 8.0. It got “caught up”, if you will, in a very exciting debate about the further future of the language, and now we want to make sure we don’t add it in a way that inhibits those future possibilities. Sometimes language design is a very long game!"

Source: comments section in https://blogs.msdn.microsoft.com/dotnet/2018/11/12/building-c-8-0/

I stopped counting how many times over the years I opened this question with hopes to have seen this implemented.

Well, finally we can all rejoice! Microsoft is going to introduce this in their upcoming C# 8 release.

So instead of doing this...

public static class IntExtensions
{
   public static bool Even(this int value)
   {
        return value % 2 == 0;
   }
}

We'll be finally able to do it like so...

public extension IntExtension extends int
{
    public bool Even => this % 2 == 0;
}

Source: https://blog.ndepend.com/c-8-0-features-glimpse-future/

No, they don't exist.

I know that the C# team was considering them at one point (or at least Eric Lippert was) - along with extension constructors and operators (those may take a while to get your head around, but are cool...) However, I haven't seen any evidence that they'll be part of C# 4.

EDIT: They didn't appear in C# 5, and as of July 2014 it doesn't look like it's going to be in C# 6 either.

Eric Lippert, the Principal Developer on the C# compiler team at Microsoft thru November 2012, blogged about this in October of 2009:

• Why No Extension Properties? – Fabulous Adventures In Coding

No they do not exist in C# 3.0 and will not be added in 4.0. It's on the list of feature wants for C# so it may be added at a future date.

At this point the best you can do is GetXXX style extension methods.

Because I recently needed this, I looked at the source of the answer in:

c# extend class by adding properties

and created a more dynamic version:

public static class ObjectExtenders
{
    static readonly ConditionalWeakTable<object, List<stringObject>> Flags = new ConditionalWeakTable<object, List<stringObject>>();

    public static string GetFlags(this object objectItem, string key)
    {
        return Flags.GetOrCreateValue(objectItem).Single(x => x.Key == key).Value;
    }

    public static void SetFlags(this object objectItem, string key, string value)
    {
        if (Flags.GetOrCreateValue(objectItem).Any(x => x.Key == key))
        {
            Flags.GetOrCreateValue(objectItem).Single(x => x.Key == key).Value = value;
        }
        else
        {
            Flags.GetOrCreateValue(objectItem).Add(new stringObject()
            {
                Key = key,
                Value = value
            });
        }
    }

    class stringObject
    {
        public string Key;
        public string Value;
    }
}

It can probably be improved a lot (naming, dynamic instead of string), I currently use this in CF 3.5 together with a hacky ConditionalWeakTable (https://gist.github.com/Jan-WillemdeBruyn/db79dd6fdef7b9845e217958db98c4d4)

As @Psyonity mentioned, you can use the conditionalWeakTable to add properties to existing objects. Combined with the dynamic ExpandoObject, you could implement dynamic extension properties in a few lines:

using System.Dynamic;
using System.Runtime.CompilerServices;

namespace ExtensionProperties
{
    /// <summary>
    /// Dynamically associates properies to a random object instance
    /// </summary>
    /// <example>
    /// var jan = new Person("Jan");
    ///
    /// jan.Age = 24; // regular property of the person object;
    /// jan.DynamicProperties().NumberOfDrinkingBuddies = 27; // not originally scoped to the person object;
    ///
    /// if (jan.Age &lt; jan.DynamicProperties().NumberOfDrinkingBuddies)
    /// Console.WriteLine("Jan drinks too much");
    /// </example>
    /// <remarks>
    /// If you get 'Microsoft.CSharp.RuntimeBinder.CSharpArgumentInfo.Create' you should reference Microsoft.CSharp
    /// </remarks>
    public static class ObjectExtensions
    {
        ///<summary>Stores extended data for objects</summary>
        private static ConditionalWeakTable<object, object> extendedData = new ConditionalWeakTable<object, object>();

        /// <summary>
        /// Gets a dynamic collection of properties associated with an object instance,
        /// with a lifetime scoped to the lifetime of the object
        /// </summary>
        /// <param name="obj">The object the properties are associated with</param>
        /// <returns>A dynamic collection of properties associated with an object instance.</returns>
        public static dynamic DynamicProperties(this object obj) => extendedData.GetValue(obj, _ => new ExpandoObject());
    }
}

A usage example is in the xml comments:

var jan = new Person("Jan");

jan.Age = 24; // regular property of the person object;
jan.DynamicProperties().NumberOfDrinkingBuddies = 27; // not originally scoped to the person object;

if (jan.Age < jan.DynamicProperties().NumberOfDrinkingBuddies)
{
    Console.WriteLine("Jan drinks too much");
}

jan = null; // NumberOfDrinkingBuddies will also be erased during garbage collection