Simple and transparent sliding timer implementation using MemoryCache, available in .NET 4+.

SlidingTimer.cs

using System;
using System.Runtime.Caching;

public class SlidingTimer
{
    public EventHandler TimeoutElapsed;

    private readonly ObjectCache cache;
    private readonly CacheItemPolicy cacheItemPolicy;
    private readonly string cacheKey;

    public SlidingTimer(TimeSpan timeout)
    {
        this.cache = MemoryCache.Default;

        this.cacheItemPolicy = new CacheItemPolicy()
        {
            SlidingExpiration = timeout,
            RemovedCallback = CacheEntryRemoved,
        };

        this.cacheKey = Guid.NewGuid().ToString();
    }

    public void Ping()
    {
        var cacheItem = this.cache.GetCacheItem(this.cacheKey);
        if (null == cacheItem)
        {
            cacheItem = new CacheItem(this.cacheKey, this);
            this.cache.Set(cacheItem, this.cacheItemPolicy);
        }
    }

    protected virtual void OnTimeoutElapsed(EventArgs e)
    {
        var handler = this.TimeoutElapsed;
        if (null != handler)
        {
            handler(this, e);
        }
    }

    private void CacheEntryRemoved(CacheEntryRemovedArguments arguments)
    {
        this.OnTimeoutElapsed(EventArgs.Empty);
    }
}