Well, I just want to give my 2 cents on this thread, and I also believe that all fields should not have underscores as leading character.
C# has a "this" keyword that serves exactly the purpose to assign a method's parameter to a field when they have the same name, so it seems like the use of underscore partially defeat the purpose of "this".
A language's keyword should be preferred among a simply naming convention.
Moreover, It feels like for as C# was designed (and also the main IDE well-featured) there's really no need to use any Hungarian notation or other convention to better clarify a variable's scope. These leading underscores sounds like unneeded garbage, to me. :))