File Change Monitor - .NET cache

FileChangeMonitor

private ObjectCache _cache = MemoryCache.Default;
private CacheItemPolicy _policy = new CacheItemPolicy();

public string RetrieveFileContents()
{
    // Build a key that we can use to cache the contents in memory against
    const string cacheKey = "fileContents";

    // Check if the data exists in cache
    string fileContents = _cache.Get(cacheKey) as string;

    // If it is null, then go and fetch it
    if (string.IsNullOrEmpty(fileContents))
    {
        using (StreamReader streamReader = new StreamReader(@"C:\ImportantFile.xml"))
        {
            fileContents = streamReader.ReadToEnd();
        }

        // Add back into cache with the dependency
        _policy.ChangeMonitors.Add(new HostFileChangeMonitor(new List<string> { @"C:\ImportantFile.xml"}));
        _cache.Add(cacheKey, fileContents, _policy);
    }

    return fileContents;
}