Token bucket rate limiter using Stopwatch

using System;
using System.Diagnostics;

sealed class TokenBucket
{
    private readonly double _rate, _burst;
    private double _tokens;
    private long _last = Stopwatch.GetTimestamp();
    public TokenBucket(double rate, double burst) { _rate = rate; _burst = burst; _tokens = burst; }
    public bool TryTake(double cost = 1)
    {
        long now = Stopwatch.GetTimestamp();
        _tokens = Math.Min(_burst, _tokens + (now - _last) / (double)Stopwatch.Frequency * _rate);
        _last = now;
        if (_tokens < cost) return false;
        _tokens -= cost;
        return true;
    }
}