返回语句后的C＃fire事件

Question

I have a Log method which creates a LogMessage object, and fires an event OnMessage

The method returns the LogMessage object after the event has been invoked.

Is it possible to delay the event, so it occurs after the return statement?

The reason i need this, is because i want to be able to alter the LogMessage object before the event will raise.

public class Logger
{
    public static event LogMessageCreatedEventHandler OnMessage;

    public LogMessage Log(string logLevel, string message)
    {
        LogMessage logMessage = new LogMessage
        {
            Message = message,
            LogLevel = logLevel,
            DateTime = DateTime.UtcNow
        };

        OnMessage?.Invoke(this, new LogMessageCreatedEventArgs { LogMessage = logMessage });

        return logMessage;
    }
}


// usage

var logMessage = _logger.Log("Debug", "Product has been created");  <-- the event is invoked now
logMessage.CustomProp.Add("isProductsService", true);               <-- this property is not available in the Event Handler

One idea would be to use Task.Delay() , but i don't think this is a good solution.

Task.Delay(1000).ContinueWith(_ => {
    OnMessage?.Invoke(this, new LogMessageCreatedEventArgs { LogMessage = logMessage });
});

return message;

Answer 1

Using Task.Delay(1000) creates a race condition. Delaying the event till after the return is insufficient, because the caller may or may not have enough time to complete its modifications to LogMessage in time for the event to pick up modified properties.

One way to make sure that modifications are over before the event is fired is to plug a piece of code in between of creating the event and firing it. This can be done with a delegate:

public static event LogMessageCreatedEventHandler OnMessage;
// By default message preprocessor does nothing
public static Func<LogMessage,LogMessage> Preprocess { get; set; } = m => m;

public LogMessage Log(string logLevel, string message) {
    LogMessage logMessage = new LogMessage {
        Message = message,
        LogLevel = logLevel,
        DateTime = DateTime.UtcNow
    };
    OnMessage?.Invoke(this, new LogMessageCreatedEventArgs {
        LogMessage = Preprocess(logMessage)
    });
    return logMessage;
}

Now you can add preprocessing logic to Logger to make modifications to LogMessage :

Logger.Preprocessor = lm => {
    lm.PropertyToBeChanged = newPropertyValue;
    lm.AnotherProperty = someOtherValue;
    return lm;
};

I need to update the property at "contextual" level, rather than on Global level, where I lose the information from where the log message was created.

You can pass the Func as a parameter to Log , as follows:

private static readonly Func<LogMessage,LogMessage> doNothing = m => m;
public LogMessage Log(string logLevel, string message, Func<LogMessage,LogMessage> preprocess = doNothing) {
    LogMessage logMessage = new LogMessage {
        Message = message,
        LogLevel = logLevel,
        DateTime = DateTime.UtcNow
    };
    OnMessage?.Invoke(this, new LogMessageCreatedEventArgs {
        LogMessage = preprocess(logMessage)
    });
    return logMessage;
}