What are you expecting it to do? You probably want to make an actual method call there, such as:
var mailgroup = emails.Where(p =>IsValidFormat(p.Value)) .Select(p => p.Value);
Or if you just want the key/value pairs, you can just use:
var mailgroup = emails.Where(p =>IsValidFormat(p.Value));
and remove the "Select" entirely.
If you do just want the values (as per the first code snippet) I'd suggest using:
var mailgroup = emails.Values.Where(p =>IsValidFormat(p));
Without any brackets, your reference to "Select" is a method group - the kind of thing you'd convert to a delegate, e.g.
Func<int> x = SomeMethod; // Where SomeMethod is declared as "int SomeMethod()"
It doesn't really make sense to use
Select as method group in this case, although it is just about feasible...
Select. As a result, what's being assigned to the variable is a reference to the Select method, which the compiler refers to as a 'method group'.
Incidentally, based on the code that you've posted I don't think that you need the trailing
.Select() at all.
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