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.

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).