Using Extension Methods in .Net 2.0

Using extension methods in .NET 2.0?

Like so:

// you need this once (only), and it must be in this namespace
namespace System.Runtime.CompilerServices
{
    [AttributeUsage(AttributeTargets.Assembly | AttributeTargets.Class
         | AttributeTargets.Method)]
    public sealed class ExtensionAttribute : Attribute {}
}
// you can have as many of these as you like, in any namespaces
public static class MyExtensionMethods {
    public static int MeasureDisplayStringWidth (
            this Graphics graphics, string text )
    {
           /* ... */
    }
}

Alternatively; just add a reference to LINQBridge.

Extension Method in C# 2.0

You can't. C# 2.0 doesn't have extension methods at all. You can use extension methods from C# 3.0 in Visual Studio 2008 targeting .NET 2.0 as described in my "C#/.NET versions" article but you can't persuade a C# 2.0 compiler to act as if it understands what extension methods are.

Using Extension Methods with .NET Framework 2.0

There is an ugly hack that gets Extension methods working in .Net 2.0; but it would better just to upgrade your framework to 3.5.

Alternate Sources: 1, 2.

In short (from link #2): Extension methods are just normal static methods tagged with the [Extension] attribute. This attribute is actually just added by the compiler behind the scenes. In .NET 3.5, it lives in System.Core, so just define your own attribute like this:

namespace System.Runtime.CompilerServices
{
  [AttributeUsage(AttributeTargets.Method, AllowMultiple = false, Inherited = false)]
  public class ExtensionAttribute : Attribute
  {
  }
}

Can I use extension methods and LINQ in .NET 2.0 or 3.0?

Extension methods were not added to .NET until 3.5. However, it was not a change to the CLR, but a change to the compiler that added them, so you can still use them in your 2.0 and 3.0 projects! The only requirement is you must have a compiler that can create 3.5 projects to be able to do this workaround (Visual Studio 2008 and above).

The error you get when you attempt to use an extension method is misleading as you do not truly need System.Core.dll to use extension methods. When you use a extension method, behind the scenes, the compiler is adding the [Extension] attribute to the function. If you have a compiler that understands what to do with the [Extension] attribute you can use it in your 2.0 and 3.0 projects if you create the attribute yourself.

Just add the following class to your project and you can then start using extension methods:

namespace System.Runtime.CompilerServices
{
    [AttributeUsage(AttributeTargets.Method, AllowMultiple = false, Inherited = false)]
    public class ExtensionAttribute : Attribute
    {
    }
}

The above code block is sitting inside System.Core.Dll, so that is why the error says you need to include the DLL file to use them.

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Now if you want LINQ functionality that will take a little extra work. You will need to re-implement the extension methods yourself. To mimic the full LINQ to SQL functionality the code can get quite complicated. However, if you are just using LINQ to Objects most LINQ methods are not complicated to implement. Here are a few LINQ to Objects replacement functions from a project I wrote out to get you started.

public static class LinqReplacement
{
    public delegate TResult Func<T, TResult>(T arg);
    public delegate TResult Func<T1, T2, TResult>(T1 arg1, T2 arg2);

    public static TSource First<TSource>(this IEnumerable<TSource> source, Func<TSource, bool> predicate)
    {
        if (source == null)
            throw new ArgumentNullException("source");
        if (predicate == null)
            throw new ArgumentNullException("predicate");

        foreach (TSource item in source)
        {
            if (predicate(item) == true)
                return item;
        }

        throw new InvalidOperationException("No item satisfied the predicate or the source collection was empty.");
    }

    public static TSource FirstOrDefault<TSource>(this IEnumerable<TSource> source)
    {
        if (source == null)
            throw new ArgumentNullException("source");

        foreach (TSource item in source)
        {
            return item;
        }

        return default(TSource);
    }

    public static IEnumerable<TResult> Cast<TResult>(this IEnumerable source)
    {
        foreach (object item in source)
        {
            yield return (TResult)item;
        }
    }

    public static IEnumerable<TResult> SelectMany<TSource, TResult>(this IEnumerable<TSource> source, Func<TSource, IEnumerable<TResult>> selector)
    {
        if (source == null)
            throw new ArgumentNullException("source");
        if (selector == null)
            throw new ArgumentNullException("selector");

        foreach (TSource item in source)
        {
            foreach (TResult subItem in selector(item))
            {
                yield return subItem;
            }
        }
    }

    public static int Count<TSource>(this IEnumerable<TSource> source)
    {
        var asCollection = source as ICollection;
        if(asCollection != null)
        {
            return asCollection.Count;
        }

        int count = 0;
        foreach (TSource item in source)
        {
            checked //If we are counting a larger than int.MaxValue enumerable this will cause a OverflowException to happen when the counter wraps around.
            {
                count++;
            }
        }
        return count;
    }
}

A library with the full re-implemenation of LINQ to Objects with the ExtensionAttribute already added in can be found in the LinqBridge project (Thanks Allon Guralnek).

C# Extension methods in .NET 2.0

Ultimately it isn't going to make much difference; you could argue that the one that matches the runtime is preferred, but the ideal answer is to switch to .NET 3.5 (otherwise at a later date it can get confusing with different versions of the same attribute in scope etc).

The [AttributeUsage] will prevent it being attached to things where it won't do anything - but it won't do aything by itself anyway...

Looking at metadata against the type, the exact attribute usage seems most like the stackoverflow variant - but ultimately this isn't hugely important - the name and namespace is all that matters (and that it inherits from Attribute).

.NET 2.0 equivalent of C# extension method

Just remove this keyword from extension methods.

public static class ServiceLocatorExtensions
{    
    public static T GetService<T>(IServiceLocator loc) {
        return (T)loc.GetService(typeof(T));
    }
}

And call it as any other static method by passing instance of object which you are 'extending':

IServiceLocator loc = GetServiceLocator();
Foo foo = ServiceLocatorExtensions.GetService<Foo>(loc);

Actually this is what .Net 3.5 compiler does behind the scene. Btw suffix Extensions you can remove too. E.g. use Helper to not confuse people.

Is it possible to create Extension Methods with 2.0 Framework?

You can create extension methods using .Net framework 2.0, if you use the C# 3.0 compiler and Visual Studio 2008 or greater.

The catch is that you have to add this code to your project:

 namespace System.Runtime.CompilerServices
{
  public class ExtensionAttribute : Attribute { }
}

Basically you need to re declare the ExtensionAttribute in Core.dll (.Net 3.5 +), in your project.

Creating an extension method in a .Net 2.0 project, compiling with C# 4.0

I can not seem to find any reference stating if this is still necessary using C# 4.0 on a .Net 2.0 project.

Yes, it is. The compiler needs that attribute. Whether you define it yourself or use the one in System.Core is up to you. Though in your particular case System.Core is not an option since that is only a part of .NET 3.5.

Once you upgrade to a later version, 3.5 or later, you can safely remove this attribute from your project all together and just use the one in System.Core.

If you have upgraded to Visual Studio 2010; then you are using the C# 4.0 compiler. That means all you need is an ExtensionAttribute:

namespace System.Runtime.CompilerServices
{
    [AttributeUsage(AttributeTargets.Method, AllowMultiple = false, Inherited = false)]
    public class ExtensionAttribute : Attribute
    {
    }
}

It must be in this exact namespace.

Once you've got that somewhere, you can declare an extension method the normal way (since you are using the C# 4.0 compiler):

public static class Extensions
{
    public static string ToTitleCase(this string str)
    {
        //omitted
    }
}

And then use it like an extension method:

var str = "hello world";
str.ToTitleCase();

You yourself don't actually ever need to put the ExtensionAttribute on anything; the C# 4.0 compiler does it for you. Essentially, all the compiler needs is to be able to find an attribute named ExtensionAttribute.

Extension Method in .net 2.0 in VS2008

Weird, the following compiles and runs fine when targeting .NET 2.0:

using System;

namespace System.Runtime.CompilerServices
{
    [AttributeUsage(AttributeTargets.Method)]
    public class ExtensionAttribute : Attribute
    {
        public ExtensionAttribute()
        {

        }
    }
}

public static class Helper
{
    public static float ToFloat(this string input)
    {
        float result;
        return float.TryParse(input, out result) ? result : 0;
    }
}

class Program
{
    static void Main()
    {
        string foo = "123";
        Console.WriteLine(foo.ToFloat());
    }
}

Extension Method usage in .NET Core 2.0?

You are calling the Usage.ListAsync() method incorrectly - it looks like a string parameter is needed:

ListAsync(IUsageOperations, String, CancellationToken)

Gets, for the
specified location, the current compute resource usage information as
well as the limits for compute resources under the subscription.

And indeed, in the code, it mentions that you need a location:

So update your code to be:

var usageReport = await azureClient.Usage.ListAsync("[resourceId('Microsoft.Storage/storageAccounts','examplestorage')]");

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