C# lambda unnamed parameters

No, you can't. Looking at the C# language specification grammar, there are two ways to declare lambdas: explicit and implicit. Neither one allows you to skip the identifier of the parameter or to reuse identifiers (names).

explicit-anonymous-function-parameter:
  anonymous-function-parameter-modifieropt   type   identifier

implicit-anonymous-function-parameter:
  identifier

It's the same as for unused function parameters in ordinary functions. They have to be given a name.

Of course you can use _ as the name for one of the parameters, as it is a valid C# name, but it doesn't mean anything special.

As of C# 7, _ does have a special meaning. Not for lambda expression parameter names but definitely for other things, such as pattern matching, deconstruction, out variables and even regular assignments. (For example, you can use _ = 5; without declaring _.)

The short answer is: no, you have to name every parameter, and the names have to be unique.

You can use _ as one parameter name because it is a valid identifier in C#.
However, you can only use it once.