But how can I access to MessageLoggers or _logger.MessageLoggers respectively. The _logger is of type ILogger.
To which type Have I to cast ?
You can use var and let the compiler to the cast to a type
https://learn.microsoft.com/en-us/dotnet/csharp/language-reference/keywords/var
var data = _logger.MessageLoggers[4].Logger._className;
The data is going to be of type string.
You can use var in the local scope and the complier will to the cast.
You can stop on a line in the debugger and use the mouse pointer and hover over a var and Visual Stuido will tell you the type. You can use the type shown and make a cast to the type if you need to do it manually for some reason.