What is the best or most interesting use of Extension Methods you've seen? [closed]

Answer 1

Another set I use quite often is for coalescing IDictionary methods:

    public static TValue Get(this IDictionary d, TKey key, Func valueThunk)
    {
        TValue v = d.Get(key);
        if (v == null)
        {
            v = valueThunk();
            d.Add(key, v);
        }
        return v;
    }
    public static TValue Get(this IDictionary d, TKey key, TValue coalesce)
    {
        return Get(d, key, () => coalesce);
    }

And for working with collections in general:

    public static IEnumerable AsCollection(this T item)
    {
        yield return item;
    }

Then for tree-like structures:

    public static LinkedList Up(this T node, Func parent)
    {
        var list = new LinkedList();
        node.Up(parent, n => list.AddFirst(n));
        return list;
    }

So I can then easily traverse and operate upon a up a class like:

class Category
{
    public string Name { get; set; }
    public Category Parent { get; set; }
}

Next, to facilitate function composition and a more F# like way of programming in C#:

public static Func Func(this Func f)
{
    return f;
}
public static Func Compose(this Func f, Func g)
{
    return x => g(f(x));
}