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Check for Changes Between Collections with SequenceMatch

You'd prefer not to execute any update code more often than you absolutely have to. If you only have one object to deal with, checking to see if an update is required is simple: retrieve the corresponding object from the database, check to see if the two objects have different values in their properties, and update the database only if there is a difference.

However, the problem is more complicated if you have a collection of objects to update in your application and in the database. With two collections, you can check for differences between the objects that appear in both collections. But you'll also need to check if the collections have missing or added objects. Fortunately, SequenceEqual can do all those checks for you, returning false if the two sequences don't have the same elements or if matching elements are different.

There are two wrinkles here, though. The first is that the two collections must be sorted the same way. This example sorts two collections by ID and then calls the SequenceEqual method on one collection, passing the other collection to determine if an update is required:

Dim usrs = (From cu In context.CompanyUsers
            Order By cu.ID
            Select cu).ToList
Dim updatedUsrs = (From us In ChangedUsers
                   Order By us.ID
                   Select us).ToList
If Not usrs.SequenceEqual(updatedUsrs) Then
    '…do update
End If

The second wrinkle requires some code. SequenceEqual doesn't actually compare the properties in the two objects: it just checks that the lists reference the same objects. If you've retrieved the objects from two sources (for example, if your application generated one set and the other set comes from your database), SequenceEqual will always return False. Fortunately, to get SequenceEqual to check the values of your object's properties you just need to have your classes implement the IEquatable interface, which I described in an earlier tip.

Posted by Peter Vogel on 08/08/2013 at 1:36 PM


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