Question

I understand how editing rows can cause concurrency issues, but concurrency issues being caused by selecting rows is something I do not understand.

If a query selects data from a database, how can a concurrency issue arise? Is it if there is a change made to the data I’m selecting, things will blow up?

In any case, if there is a concurrency issue caused by a select query, what is the best way to handle it? This is what I have in mind, but I wouldn’t be surprised at all if it were wrong.

try
{
    var SelectQuery = 
    from a DB.Table
    where a.Value == 1
    select new {Result = a};
}
catch 
{
    //retry query??
}

Answer 1

In this case your select operation essentially amounts to a read / query. Even read only operations can cause concurrency issues in an application.

The simplest example is when the object being read from has thread affinity and the read occurs from a different thread. This can cause a race since the data is being accessed in an improper way.

The best way to handle a concurrency issue is to simply avoid it. If you have 2 threads playing with the same peice of data using a lock to serialize access to the data is probably the best approach. Although a definitive solution requires a bit more detail.

Can you explain what is happening here and why the race is occurring? Do other threads modify this object while you are reading it?