c# - Underscore Arrow (_ => ...) What Is This? -

reading through c# in nutshell noticed bit of code i've never came across:

_uisynccontent.post(_ => txtmessage.text += "test"); 

what underscore followed arrow? i've seen lambda expressions written in similar way nothing underscore.

it's lambda expression uses _ instead of x parameter. _ valid identifier can used parameter name.

as mentioned in comments, it's convention among developers call _ indicate it's not used lambda expression, it's no more that: convention.