Querying Nested Dictionaries using LINQ

Question

I'm developing a POS system and I need to check whether the database tables on each terminal are out of sync.

I maintain a Dictionary of terminal information, each of which has Dictionary containing table ids with a CRC for each table. Below is simplified description of what I've got so far (i'm using VB.NET but I've stripped out a lot of stuff to hopefully clarify things):

e.g.   TerminalList = Dictionary(Of Integer, TerminalInfo)

       class TerminalInfo
         TerminalID: Integer
         TableCRCs: Dictionary(Of String, TableInfo)

       class TableInfo
         TableID: String
         CRC: UInt32


       TerminalID: 1
          TableID: A   CRC: aa10
          TableID: B   CRC: 1234

       TerminalID: 2
          TableID: A   CRC: aa10
          TableID: B   CRC: 1234

       TerminalID: 3
          TableID: A   CRC: 12be
          TableID: B   CRC: 1234

Is it possible for me to create a LINQ query that will build a list of distinct TableIDs and CRC's?

i.e. A  aa10
     A  12be
     B  1234

If the count of this query is greater than the number of tables that i'm interested in then I know that a terminal is out of sync. I'm not interested in which terminal or which table is out of sync, only that there is a difference.

TIA,

Simon

jason · Accepted Answer · 2010-01-14 18:38:37Z

Sure. You didn't specify a language so I hope that C# is okay (I'm not fluent in VB.NET but I can help you if you need help translating):

var query = TerminalList.SelectMany(kvp => kvp.Value.TableCRCs.Values)
                        .GroupBy(info => new { info.TableID, info.CRC });

foreach (var result in query) {
    Console.WriteLine(
        String.Format(
            "{0}|{1:x}",
            result.Key.TableID,
            result.Key.CRC
        )
    );
}

The key to this is using SelectMany to flatten the nested enumerations of TerminalInfos into one enumeration. From there it's just a usual GroupBy operation to get the desired result.

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