Updating an item property within IEnumerable but the property doesn't stay set?

Question

I have two tables: Transactions and TransactionAgents. TransactionAgents has a foreign key to Transactions called TransactionID. Pretty standard.

I also have this code:

BrokerManagerDataContext db = new BrokerManagerDataContext();

var transactions = from t in db.Transactions
                   where t.SellingPrice != 0 
                   select t;

var taAgents = from ta in db.TransactionAgents
               select ta;

foreach (var transaction in transactions)
{
    foreach(var agent in taAgents)
    {
        agent.AgentCommission = ((transaction.CommissionPercent / 100) * (agent.CommissionPercent / 100) * transaction.SellingPrice) - agent.BrokerageSplit;
    } 
}

dataGridView1.DataSource = taAgents;

Basically, a TransactionAgent has a property/column named AgentCommission, which is null for all TransactionAgents in my database.

My goal is to perform the math you see in the foreach(var agent in taAgents) to patch up the value for each agent so that it isn't null.

Oddly, when I run this code and break-point on agent.AgentCommission = (formula) it shows the value is being calculated for AgentCommissision and the object is being updated but after it displays in my datagrid (used only for testing), it does not show the value it calculated.

So, to me, it seems that the Property isn't being permanently set on the object. What's more, If I persist this newly updated object back to the database with an update, I doubt the calculated AgentCommission will be set there.

Without having my table set up the same way, is there anyone that can look at the code and see why I am not retaining the property's value?

Answer 1

IEnumerable<T>s do not guarantee that updated values will persist across enumerations. For instance, a List will return the same set of objects on every iteration, so if you update a property, it will be saved across iterations. However, many other implementations of IEnumerables return a new set of objects each time, so any changes made will not persist.

If you need to store and update the results, pull the IEnumerable<T> down to a List<T> using .ToList() or project it into a new IEnumerable<T> using .Select() with the changes applied.

To specifically apply that to your code, it would look like this:

var transactions = (from t in db.Transactions
                    where t.SellingPrice != 0 
                    select t).ToList();

var taAgents = (from ta in db.TransactionAgents
                select ta).ToList();

foreach (var transaction in transactions)
{
    foreach(var agent in taAgents)
    {
        agent.AgentCommission = ((transaction.CommissionPercent / 100) * (agent.CommissionPercent / 100) * transaction.SellingPrice) - agent.BrokerageSplit;
    } 
}

dataGridView1.DataSource = taAgents;

Answer 2

Specifically, the problem is that each time you access the IEnumerable, it enumerates over the collection. In this case, the collection is a call to the database. In the first part, you're getting the values from the database and updating them. In the second part, you're getting the values from the database again and setting that as the datasource (or, pedantically, you're setting the enumerator as the datasource, and then that is getting the values from the database).

Use .ToList() or similar to keep the results in memory, and access the same collection every time.

Answer 3

Assuming you are using LINQ to SQL, if EnableObjectTracking is false, then the objects will be constructed new every time the query is run. Otherwise, you would be getting the same object instances each time and your changes would survive. However, like others have shown, instead of having the query execute multiple times, cache the results in a list. Not only will you get what you want working, you'll have fewer database round trips.

Answer 4

I found that I had to locate the item in the list that I wanted to modify, extract the copy, modify the copy (by incrementing its count property), remove the original from the list and add the modified copy. var x = stats.Where(d => d.word == s).FirstOrDefault(); var statCount = stats.IndexOf(x); x.count++; stats.RemoveAt(statCount); stats.Add(x);

Answer 5

It is helpful to rewrite your LINQ expression using lambdas so that we can consider the code in more explicit terms.

//Original code from question
var taAgents = from ta in db.TransactionAgents
               select ta;

//Rewritten to explicitly call attention to what Select() is actually doing
var taAgents = db.TransactionAgents.Select(ta => new TransactionAgents(/*database row's data*/)});

In the rewritten code, we can clearly see that Select() is constructing a new object based on each row returned from the database. What's more, this object construction occurs every time the IEnumerable taAgents is iterated through.

So, explained more concretely, if there are 5 TransactionAgents rows in the database, in the following example, the TransactionAgents() constructor is called a total of 10 times.

// Assume there are 5 rows in the TransactionAgents table
var taAgents = from ta in db.TransactionAgents
               select ta;

//foreach will iterate through the IEnumerable, thus calling the TransactionAgents() constructor 5 times
foreach(var ta in taAgents)
{
  Console.WriteLine($"first iteration through taAgents - element {ta}");
}
// these first 5 TransactionAgents objects are now out of scope and are destroyed by the GC


//foreach will iterate through the IEnumerable, thus calling the TransactionAgents() constructor 5 MORE times
foreach(var ta in taAgents)
{
  Console.WriteLine($"second iteration through taAgents - element {ta}");
}

// these second 5 TransactionAgents objects are now out of scope and are destroyed by the GC

As we can see, all 10 of our TransactionAgents objects were created by the lambda in our Select() method, and do not exist outside of the scope of the foreach statement.