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<head><title>How To Work With db4o Exceptions</title>
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<h1>How To Work With db4o Exceptions</h1>
<p>Appropriate
exception handling will help you to create easy to support systems, saving your
time and efforts in the future. The following hints identify important
places for exception handling. Take also a look at the list of <a href="exception_types.htm">common db4o exceptions</a>.</p>
<ol>
<li value="1">
<p>Opening a database file can throw a DatabaseFileLockedException.</p>
<div class="codesnippet" MadCap:conditions="Primary.c#">
<pre class="prettyprint" xml:space="preserve">try
{
IObjectContainer container = Db4oEmbedded.OpenFile("database.db4o");
}
catch (DatabaseFileLockedException e)
{
// Database is already open!
// Use another database-file or handle this case gracefully
}</pre>
<div class="codesnippet-meta">ImportantExceptionCases.cs: If the database is already open
<div class="codedownload"><a href="../../CodeExamples/strategies/exceptions/Example-strategies-exceptions-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">Try
Dim container As IObjectContainer = Db4oEmbedded.OpenFile("database.db4o")
Catch e As DatabaseFileLockedException
' Database is already open!
' Use another database-file or handle this case gracefully
End Try</pre>
<div class="codesnippet-meta">ImportantExceptionCases.vb: If the database is already open
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</div>
</li>
<li value="2">
<p>Opening a client connection can throw IOException.</p>
<div class="codesnippet" MadCap:conditions="Primary.c#">
<pre class="prettyprint" xml:space="preserve">try
{
IObjectContainer container = Db4oClientServer.OpenClient("localhost", 1337, "sa", "sa");
}
catch (Db4oIOException e)
{
// Couldn't connect to the server.
// Ask for new connection-settings or handle this case gracefully
}</pre>
<div class="codesnippet-meta">ImportantExceptionCases.cs: Cannot connect to the server
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<div class="codesnippet" MadCap:conditions="Primary.VB.NET">
<pre class="prettyprint lang-vb" MadCap:conditions="Primary.Online" xml:space="preserve">Try
Dim container As IObjectContainer = Db4oClientServer.OpenClient("localhost", 1337, "sa", "sa")
Catch e As Db4oIOException
' Couldn't connect to the server.
' Ask for new connection-settings or handle this case gracefully
End Try</pre>
<div class="codesnippet-meta">ImportantExceptionCases.vb: Cannot connect to the server
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</div>
</li>
<li value="3">
<p>Working with <a href="../unique_constraints.htm">db4o-unique constraints</a> the commit may throw exceptions when the constraints are violated.</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)
{
// Violated the unique-constraint!
// Retry with a new value or handle this gracefully
container.Rollback();
}</pre>
<div class="codesnippet-meta">ImportantExceptionCases.cs: Violation of the unique constraint
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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
' Violated the unique-constraint!
' Retry with a new value or handle this gracefully
container.Rollback()
End Try</pre>
<div class="codesnippet-meta">ImportantExceptionCases.vb: Violation of the unique constraint
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</li>
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