Question

first, it’s not a duplication of
What does the @ symbol before a variable name mean in C#?

as group is not a preserved keyword.

I wrote some code and the resharper suggested me to add @ before the variable group.

Any idea why?

var group = GetDefaultGroup(ClientServiceCommon.Poco.Group);

filteredPairs = @group.Pairs.ToList();

Answer 1

As you correctly point out, group is not a reserved keyword (the language designers try hard not to introduce new reserved keywords when they add new features to the language in order to not break existing programs).
It is, however, a contextual keyword: it becomes a keyword within LINQ query expressions. Resharper is suggesting you rename the variable in question to avoid ambiguities that would result if this variable were used within query expressions.

From the language specification:

7.16.1 Ambiguities in query expressions

Query expressions contain a number of “contextual keywords”, i.e., identifiers that have special
meaning in a given context. Specifically these are from, where, join,
on, equals, into, let, orderby, ascending, descending, select, group
and by. In order to avoid ambiguities in query expressions caused by
mixed use of these identifiers as keywords or simple names, these
identifiers are considered keywords when occurring anywhere within a
query expression. For this purpose, a query expression is any
expression that starts with “from identifier” followed by any token
except “;”, “=” or “,”.

If you tried to do the following, you would get a compiler error:

var filteredParis = from pair in group.Pairs // group is now treated as a keyword
                    select pair;