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    <p>The previous values need to be in your object.</p> <p>Let's say you have a property <code>ConcurrencyToken</code>:</p> <pre><code>public class MyObject { public Guid Id { get; set; } // stuff public byte[] ConcurrencyToken { get; set; } } </code></pre> <p>Now you can set <code>ConcurrencyMode.Fixed</code> on that property. You also need to configure your DB to automatically update it.</p> <p>When you query the DB, it will have some value:</p> <pre><code>var mo = Context.MyObjects.First(); Assert.IsNotNull(mo.ConcurrencyToken); </code></pre> <p>Now you can detach or serialize the object, but you need to include <code>ConcurrencyToken</code>. So if you're putting the object data on a web form, you'll need to serialize <code>ConcurrencyToken</code> to a string and put it in a hidden input.</p> <p>When you <code>ApplyChanges</code>, you need to include the <code>ConcurrencyToken</code>:</p> <pre><code>Assert.IsNotNull(myObject.ConcurrencyToken); using (var context = new MyContext()) { context.MyObjects.ApplyChanges(myObject); context.SaveChanges(); } </code></pre> <p>Having <code>ConcurrencyMode.Fixed</code> changes the <code>UPDATE</code> SQL. Normally it looks like:</p> <pre><code>UPDATE [dbo].[MyObject] SET --stuff WHERE [Id] = @0 </code></pre> <p>With <code>ConcurrencyMode.Fixed</code> it looks like:</p> <pre><code>UPDATE [dbo].[MyObject] SET --stuff WHERE [Id] = @0 AND [ConcurrencyToken] = @1 </code></pre> <p>...so if someone has updated the row between the time you read the original concurrency token and the time you saved, the <code>UPDATE</code> will affect 0 rows instead of 1. The EF throws a concurrency error in this case.</p> <p><strong>Therefore</strong>, if any of this isn't working for you, the first step is to use SQL Profiler to look at the generated <code>UPDATE</code>.</p>
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    1. USCraig Stuntz
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    1. COthis is something like MVCC.
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