c# log4net store entries in memory and Email under certain conditions

MemoryAppender will only "append" to memory and is thus mostly useful only for development and testing purposes. And there is currently no appender that will only append on application shutdown.

The SMTPAppender is something in between, since it inherits the BufferingAppenderSkeleton. These appenders have a BufferSize property which controls how many messages are kept in memory before they are flushed.

Which messages to pass to the appenders are controlled with the level settings either on the root element or on individual logger elements. In your case use a level of WARN which will let through WARN, ERROR and FATAL. If you don't want the ERROR messages you will have to put a level filter on your appender.

Update: MemoryAppender is not using any layout to "render" message objects. What you get from MemoryAppender is just the raw message objects as they are produced by log4net. You will have to convert those to meaningfull text yourself.

Alternatively, if you require both layout functionality and in-memory appending you could look into subclassing AppenderSkeleton. That way you get the basic Layout support. When implementing the Append method you can do what MemoryAppender does, that is just appending to an internal list of messages.

Update 2: to implement the MemoryAppender alternative I suggest taking the MemoryAppender as a starting point. MemoryAppender is a subclass of AppenderSkeleton and have thus access to the RenderLoggingEvent method. So, we subclass MemoryAppender and add a method that renders the current batch of logging events:

public class RenderingMemoryAppender : MemoryAppender
{

    public IEnumerable<string> GetRenderedEvents()
    {
        foreach(var loggingEvent in GetEvents())
        {
            yield return RenderLoggingEvent(loggingEvent);
        }
    }
}