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<h1><a name="kanchor69"></a><a name="kanchor70"></a>Unique Constraints</h1>
<p>Unique constraints allow a user to define a field
to be unique across all the objects of a particular class stored to db4o. This
means that you cannot save an object where a previously committed object
has the same field value for fields marked as unique.
A Unique Constraint is checked at commit-time and a constraint violation will
cause a UniqueFieldValueConstraintViolationException to be thrown. This
functionality is based on <a href="callbacks/commit-time_callbacks.htm">Commit-Time Callbacks</a> feature. </p>
<h2>How To Use Unique Constraints</h2>
<p>First you need to add an index on the field which should be unique. After that you add the UniqueFieldValueConstraint to the configuration for the unique field.</p>
<div class="codesnippet" MadCap:conditions="Primary.c#">
<pre class="prettyprint" xml:space="preserve">configuration.Common.ObjectClass(typeof (UniqueId)).ObjectField("id").Indexed(true);
configuration.Common.Add(new UniqueFieldValueConstraint(typeof (UniqueId), "id"));</pre>
<div class="codesnippet-meta">UniqueConstrainExample.cs: Add the index the field and then the unique constrain
<div class="codedownload"><a href="../CodeExamples/features/uniqueconstrain/Example-features-uniqueconstrain-csharp.zip" class="codedownload" MadCap:conditions="Primary.Online">Download Code</a></div><div class="codedownload copylink-marker" MadCap:conditions="Primary.Online"><a href="#copy">Copy Code</a></div></div>
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<div class="codesnippet" MadCap:conditions="Primary.VB.NET">
<pre class="prettyprint lang-vb" MadCap:conditions="Primary.Online" xml:space="preserve">configuration.Common.ObjectClass(GetType(UniqueId)).ObjectField("id").Indexed(True)
configuration.Common.Add(New UniqueFieldValueConstraint(GetType(UniqueId), "id"))</pre>
<div class="codesnippet-meta">UniqueConstrainExample.vb: Add the index the field and then the unique constrain
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<p>After that, the unique constrain is applied. When you commit a transaction the uniqueness of the field is checked. If there's any violation, the UniqueFieldValueConstraintViolationException will be thrown.</p>
<div class="codesnippet" MadCap:conditions="Primary.c#">
<pre class="prettyprint" xml:space="preserve">container.Store(new UniqueId(42));
container.Store(new UniqueId(42));
try
{
container.Commit();
}
catch (UniqueFieldValueConstraintViolationException e)
{
Console.Out.WriteLine(e.StackTrace);
}</pre>
<div class="codesnippet-meta">UniqueConstrainExample.cs: Violating the constrain throws an exception
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<div class="codesnippet" MadCap:conditions="Primary.VB.NET">
<pre class="prettyprint lang-vb" MadCap:conditions="Primary.Online" xml:space="preserve">container.Store(New UniqueId(42))
container.Store(New UniqueId(42))
Try
container.Commit()
Catch e As UniqueFieldValueConstraintViolationException
Console.Out.WriteLine(e.StackTrace)
End Try</pre>
<div class="codesnippet-meta">UniqueConstrainExample.vb: Violating the constrain throws an exception
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