[WebService]
public class MyWebService : WebService
{
  public MyWebService()
  {
    // Initialize to boost performance of first call.
    GetContent();
  }
  [WebMethod]
  public void DoSomething()
  {
    string content = GetContent();
    if (content != null)
      // do something with the content of the file that's cached.
  }
  private string GetContent()
  {
    string content = (string)Context.Cache[CacheName];
    if (content == null)
    {
      string path = Server.MapPath("/path/to/virtual/file.txt");
      using (StreamReader reader = new StreamReader(path))
      {
        content = reader.ReadToEnd();
        reader.Close();
      }
      Context.Cache.Add(
        CacheName,
        content,
        new CacheDependency(path),
        Cache.NoAbsoluteExpiration,
        Cache.NoSlidingExpiration,
        CacheItemPriority.Default,
        null);
    }
    return content;
  }
  private const string CacheName = "CONTENT";
}