Question

I’ve created a very simple helper class that I can use in my ASP.Net pages. The idea is that it is supposed to be a very simple way to log an on-page error or success (not a form validation error) and then display it to the user.

In my public helper class I have a class which has certain properties, as shown below:

public class UserMessage
{
    public UserMessage()
    {
        Messages = new Dictionary<string, string>();
    }

    public string SummaryMessage;
    public Dictionary<string, string> Messages;
    public bool ShowMessages;
    public bool ShowAsError;
}

I then have a variable which is used to store an instance of the UserMessage class, like so:

private static UserMessage _userMessage { get; set; }

I then have two public static methods, one to log a message, the other to display all the messages, like so:

public static void LogSummary(string summaryMessage, bool showIndividualMessages, bool showAsError)
{
    _userMessage = new UserMessage();
    _userMessage.SummaryMessage = summaryMessage;
    _userMessage.ShowMessages = showIndividualMessages;
    _userMessage.ShowAsError = showAsError;
}

public static string DisplayUserMessages()
{
    if (_userMessage == null)
        return string.Empty;

    StringBuilder messageString = new StringBuilder();
    messageString.AppendFormat("\n");
    messageString.AppendLine(string.Format("<div class=\"messageSummary {0}\">", (_userMessage.ShowAsError) ? "invalid" : "valid"));
    messageString.AppendLine(string.Format("<h3>{0}</h3>", _userMessage.SummaryMessage));
    messageString.AppendLine("</div>");

    return messageString.ToString();
}

The problem I have is that the _userMessage variable has to be a static variable, otherwise I get the error message “An object reference is required for the non-static field…….”. The problem with the variable being static is that is stays in memory, so if the user receives an error message and then visits another page – the error message is still displayed!

I’m sure this is because I missed OOP 101, but how should I correct this?

Answer 1

Do not use static variable to keep messages per user! ASP.NET application is multi-threaded and using static variable is not thread safe. Store them in Session.

public static void LogSummary(string summaryMessage, ...)
{
   HttpContext.Current.Session["userMessages"] = new UserMessage(); 
   ...
}

public static string DisplayUserMessages()
{
   // get the value from session
   var userMessage = (UserMessage)HttpContext.Current.Session["userMessages"];
   // do the work
   // do the clean up
   HttpContext.Current.Session["userMessages"] = null;
   // the messages will not be displayed on next request
}

Each request is handled by different thread, so the users will overwrite the _userMessage field and you cannot guarantee that messages for the current user will be displayed.