Iterate over a datareader and send email once per unique combination

Question

I have a datareader which has columns like this among other columns populated via an SP:

ColX    ColY   ColZ 
NYC     100    200
NYC     101    200
EWR     100    200
EWR     100    200

I'm trying to iterate over this result, and perform an action like sending an email for every duplicate in the combination of ColX, ColY, ColZ; in the above example EWR-100-200

What is the efficient way to accomplish this given that I have no control over the SP that returns this data in C#?

Answer 1

Create a Hashset<string> to hold keys for the items you've sent emails to. You can create the keys by:

string key = ColX.ToString() + ColY.ToString() + ColZ.ToString();

So:

var keysUsed = new Hashset<string>();
while (reader.Read())
{
    // Assuming you got the values from the columns...
    string key = ColX.ToString() + ColY.ToString() + ColZ.ToString();
    if (keysUsed.Add(key))
    {
        // send email
    }
}

The point here is that keysUsed.Add will return true if the item is added to the Hashset. If the key already existed, the method would return false.