Michael Bell’s Blog

With my in-memory log-hit table I talked about in my last entry, I started getting these errors a few times a day, and it was enough to bomb the whole app:
Column ‘blahblah’ is constrained to be unique.  Value ‘-4862′ is already present.
Column ‘blahblah’ is constrained to be unique.  Value ‘-4847′ is already present.
Column ‘blahblah’ is constrained to be unique.  Value ‘-4768′ is already present.

Let me show you how the ‘blahblah’ column is built in the table construction:
DataColumn dc;

dc = new DataColumn(“HitID”, typeof(Int32));
dc.Unique = true;
dc.AutoIncrement = true;
dc.AutoIncrementSeed = -1;
dc.AutoIncrementStep = -1;
_MyTable.Columns.Add(dc);

So how in the world is this happening? Let me show the piece of code that was throwing the exception:
DataRow row = MemoryTables.HitList.NewRow();
row["FileID"] = Convert.ToInt32(context.Request.QueryString["fi"]);
row["FileName"] = fi.Name;
row["Username"] = _Username;
row["MemberID"] = _MemberID;
row["Success"] = true;
row["Bytes"] = fi.Length;
MemoryTables.HitList.Rows.Add(row);

Fairly straight forward.. right? With each hit to an image, this thing fires and inserts a new row into the in-memory table. About 20 rows get inserted a second. After much trial and error, I completely eliminated this problem by re-arranging a few lines of code. The new code reads:
DataRow row = MemoryTables.HitList.NewRow();
MemoryTables.HitList.Rows.Add(row);
row["FileID"] = Convert.ToInt32(context.Request.QueryString["fi"]);
row["FileName"] = fi.Name;
row["Username"] = _Username;
row["MemberID"] = _MemberID;
row["Success"] = true;
row["Bytes"] = fi.Length;

By creating a new row and adding it to the table before I populate it with any values, I eliminate my problem. I’d love to hear an explanation if anyone has one…