What causes .Attach() to be slow in EF4?

I can confirm this slow behaviour and I also found the main reason. I’ve made a little test with the following model …

public class MyClass
{
    public int Id { get; set; }
    public string P1 { get; set; }
    // ... properties P2 to P49, all of type string
    public string P50 { get; set; }
}

public class MyContext : DbContext
{
    public DbSet<MyClass> MyClassSet { get; set; }
}

… and this test program …

using (var context = new MyContext())
{
    var list = new List<MyClass>();
    for (int i = 0; i < 1000; i++)
    {
        var m = new MyClass()
        {
            Id = i+1,
            P1 = "Some text ....................................",
            // ... initialize P2 to P49, all with the same text
            P50 = "Some text ...................................."
        }
        list.Add(m);
    }

    Stopwatch watch = new Stopwatch();
    watch.Start();
    foreach (var entity in list)
    {
        context.Set<MyClass>().Attach(entity);
        context.Entry(entity).State = System.Data.EntityState.Modified;
    }
    watch.Stop();
    long time = watch.ElapsedMilliseconds;
}

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Test 1

Exactly the code above:

–> time = 29,2 sec

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Test 2

Comment out the line …

//context.Entry(entity).State = System.Data.EntityState.Modified;

–> time = 15,3 sec

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Test 3

Comment out the line …

//context.Set<MyClass>().Attach(entity);

–> time = 57,3 sec

This result is very strange because I expected that calling Attach is not necessary because changing the state attaches anyway.

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Test 4

Remove properties P6 to P50 (so we only have 5 strings in the entity), original code:

–> time = 3,4 sec

So, yes, obviously the number of properties strongly matters.

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Test 5

Add the following line before the loop (model again with all 50 properties):

context.Configuration.AutoDetectChangesEnabled = false;

–> time = 1,4 sec

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Test 6

Again with AutoDetectChangesEnabled = false but with only 5 properties:

–> time = 1,3 sec

So, without change tracking the number of properties doesn’t matter so much anymore.

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Conclusion

By far most of the time seems to be spent for taking the snapshot of the attached object’s properties by the change tracking mechanism. If you don’t need it disable change tracking for your code snippet. (I guess in your code your really don’t need change tracking because by setting the entitiy’s state to Modified you basically mark all properties as changed anyway. So all columns get sent to the database in an update statement.)

Edit

The test times above are in Debug mode. But Release mode doesn’t make a big difference (for instance: Test 1 = 28,7 sec, Test 5 = 0,9 sec).