Question

I have a table in my SQL Anywhere 12.0.1 database:

CREATE TABLE Entries (
    ListId        UNIQUEIDENTIFIER NOT NULL,
    . . .
);

I need to query the table to get the number of rows in the table that have the same ListID. In SQL:

SELECT ListID, COUNT(*)
FROM Entries
GROUP BY ListId;

I want to execute this query using Entity Framework 4.x, and I want to return the results as a Dictionary<Guid, long>. Something like this:

public Dictionary<Guid, long> GetRowCounts( MyEntities context ) {
    Dictionary<Guid, long> result = null;
    try {
        result = ( from entry in Entries
                   group entry by entry.ListId into listGroup
                   select listGroup ).ToDictionary( grp => grp.Key, ???? );

    } catch ( Exception ex ) {
        . . .
    }
    return result;
}

Remember, I want the count of rows that have each unique ListId returned. What do I put in place of the “????”?

Answer 1

Well, look at what you wrote:

Remember, I want the count of rows

(emphasis mine). So you need:

ToDictionary(grp => grp.Key, grp => grp.Count())

Personally I’d write the method like this though:

public Dictionary<Guid, int> GetRowCounts(MyEntities context) {
    return context.Entries
                  .GroupBy(entry => entry.ListId)
                  .ToDictionary(g => g.Key, g => g.Count());
}

… or if you still want it to be a Dictionary<Guid, long>, use g => g.LongCount(), or just cast:

.ToDictionary(g => g.Key, g => (long) g.Count())

You almost certainly don’t want the try/catch block there, and returning directly is a lot simpler. Likewise, unless you’re writing a moderately complex query, there’s really no benefit in using a query expression. Learn to use the extension methods directly too, so that for each query you can write the simplest possible code.