FrameCounter.cs

public class FrameCounter
{
	private readonly int sampleCount;
	private int index;
	private int sum;
	private readonly int[] samples;

	private int lastTick;

	public FrameCounter(int sampleCount = 100)
	{
		this.sampleCount = sampleCount;
		lastTick = Environment.TickCount;
		samples = new int[sampleCount];
	}

	public float GetCurrentRate()
	{
		var ticks = System.Environment.TickCount;
		var delta = ticks - lastTick;
		lastTick = ticks;

		return 1000f / CalculateAverage(delta);
	}

	private float CalculateAverage(int delta)
	{
		sum -= samples[index];
		sum += delta;
		samples[index] = delta;

		if (++index == sampleCount)
			index = 0;

		return (float)sum / sampleCount;
	}
}