linq List<A> select many <A,B>

Question

Let's say I have:

List<string> sqlServerNames;

and a method:

List<string> GetDatabases( string serverName );

And I want a single query to get a list of { serverName, databaseName }

Is there a way to do this with Linq?

sqlServerNames.????

So for example, if I have two servers, ".", and "SQL2016".

And "." has "db1", "db2"
And "SQL2016" has "db3"

I'd want:

".", "db1"
".", "db2"
"SQL2016", "db3"

Probably with an anonymous type.

Answer 1

只需尝试如下所示：

var tuples = sqlServerNames.SelectMany(server => GetDatabases(server).Select(db => Tuple.Create(server, db))).ToList()

Answer 2

Recommend using another overload of SelectMany . The other answers will need to allocate a new enumerable/enumerator/delegate for each server name, while this doesn't.

sqlServerNames.SelectMany(serverName => GetDatabases(serverName),
                          (serverName, databaseName) => new
                          {
                              ServerName = serverName,
                              DatabaseName = databaseName
                          });

equivalent to:

from serverName in sqlServerNames
from databaseName in GetDatabases(serverName)
select new
{
    ServerName = serverName,
    DatabaseName = databaseName
};

Answer 3

You can use SelectMany method :

sqlServerNames
    .SelectMany(sn => GetDatabases(sn)
                        .Select(dbn => new { 
                                            ServerName = sn,
                                            DatabaseName = dbn 
                                            }
                                )
                );