Pair off!

Linq extension method which pairs off elements of an IEnumerable<T>. If there's an odd element left over, it's ignored.

public static IEnumerable<KeyValuePair<T, T>> PairOff<T>(this IEnumerable<T> ie)
{
	IEnumerator<T> e = ie.GetEnumerator();
	while (e.MoveNext())
	{
		T first = e.Current;
		if (e.MoveNext())
			yield return new KeyValuePair<T, T>(first, e.Current);
	}
}

GroupBy for Mono

The Mono implementation of the GroupBy Linq extension is very slow, especially when compared to the .NET version. Here is a replacement extension method you can use as a workaround.

public class MyGrouping<TSrc,TKey> : List<TSrc>
{
	public TKey Key;
	public MyGrouping(TKey k) { Key = k; }
}

public static IEnumerable<MyGrouping<TSrc, TKey>> MyGroupBy<TSrc, TKey>(this IEnumerable<TSrc> ie, Converter<TSrc,TKey> fn)
{
	Dictionary<TKey, MyGrouping<TSrc, TKey>> d = new Dictionary<TKey, MyGrouping<TSrc, TKey>>();
	foreach (TSrc src in ie)
	{
		TKey k = fn(src);
		MyGrouping<TSrc,TKey> l;
		if (d.TryGetValue(k, out l))
			l.Add(src);
		else
			d.Add(k, new MyGrouping<TSrc,TKey>(k) { src });
	}
	return d.Values;
}

Value compare for arrays

public static bool ValueCompare(this T[] a, T[] b) where T : struct
{
	if (a.Length != b.Length)
		return false;

	EqualityComparer q = EqualityComparer.Default;
	for (int i = 0; i < a.Length; i++)
		if (!q.Equals(a[i], b[i]))
			return false;

	return true;
}

public static bool ValueCompare(this T[] a, int a_offset, T[] b) where T : struct
{
	if (a_offset >= a.Length || a_offset + b.Length > a.Length)
		return false;

	EqualityComparer q = EqualityComparer.Default;
	for (int i = 0; i < b.Length; i++, a_offset++)
		if (!q.Equals(a[a_offset], b[i]))
			return false;

	return true;
}

Filter for non-null references (and non-default values)

public static IEnumerable NotDefault(this IEnumerable source)
{
	return source.Where(e => !Object.Equals(e,default(T)));
}

public static IEnumerable NotDefault(this IEnumerable source, Converter selector)
{
	return source.Where(e => !Object.Equals(selector(e),default(TAny)));
}

DateTime to UNIX epoch

public static int ToEpoch(this DateTime dt)
{
	return (int)(dt - new DateTime(1970, 1, 1, 0, 0, 0)).TotalSeconds;
}