MyExtensions.cs

void Main()
{
  // Write code to test your extensions here. Press F5 to compile and run.
}

public static class MyExtensions
{
	// Write custom extension methods here. They will be available to all queries.
	public static double Std(this IEnumerable<double> src)
	{
		var n = src.Count();
		var avg = src.Average();
		
		return Math.Sqrt(src.Aggregate(0D, (acc, k) => acc + Math.Pow(selector(k) - avg, 2)) / n);
	}
	
	public static double Std<TSource>(this IEnumerable<TSource> src, Func<TSource,double> selector)
	{
		return src.Select(selector).Std();
	}
}

// You can also define non-static classes, enums, etc.