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<h1>Custom Typehandler Example</h1>
<p>For a custom typehandler example we will try to write a very
simple typehandler for the StringBuilder class. We want to handle a StringBuilder as a value type, therefore we implement the ValueTypeHandler interface. Not that there's a whole collection of interfaces for typehandlers. Take a look at the TypeHandler4 type hierarchy. </p>
<p>To keep it simple we will skip information required for
indexing - please look at IndexableTypeHandler in db4o sources to get more
information on how to handle indexes.</p>
<p>The first thing should be the write method, which determines
how the object is persisted:</p>
<div class="codesnippet" MadCap:conditions="Primary.c#">
<pre class="prettyprint" xml:space="preserve">public void Write(IWriteContext writeContext, object o)
{
StringBuilder builder = (StringBuilder) o;
string str = builder.ToString();
byte[] bytes = Encoding.UTF8.GetBytes(str);
writeContext.WriteInt(bytes.Length);
writeContext.WriteBytes(bytes);
}</pre>
<div class="codesnippet-meta">StringBuilderHandler.cs: Write the StringBuilder
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<div class="codesnippet" MadCap:conditions="Primary.VB.NET">
<pre class="prettyprint lang-vb" MadCap:conditions="Primary.Online" xml:space="preserve">Public Sub Write(ByVal writeContext As IWriteContext, ByVal o As Object) _
Implements IValueTypeHandler.Write
Dim builder As StringBuilder = DirectCast(o, StringBuilder)
Dim str As String = builder.ToString()
Dim bytes As Byte() = Encoding.UTF8.GetBytes(str)
writeContext.WriteInt(bytes.Length)
writeContext.WriteBytes(bytes)
End Sub</pre>
<div class="codesnippet-meta">StringBuilderHandler.vb: Write the StringBuilder
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<p>As you can see from the code above, there are 3 steps:</p>
<ol>
<li value="1">Get
the buffer from WriteContext/I WriteContext</li>
<li value="2">Convert the string-content to a byte-array using the UTF8 encoding.</li>
<li value="3">Write
the length of the resulted byte-array.</li>
<li value="4">Write the byte array of the string.</li>
</ol>
<p>Next step is to read the stored object. It is just
opposite to the write method:</p>
<div class="codesnippet" MadCap:conditions="Primary.c#">
<pre class="prettyprint" xml:space="preserve"> public Object Read(IReadContext readContext)
{
int length = readContext.ReadInt();
byte[] data = new byte[length];
readContext.ReadBytes(data);
return new StringBuilder(Encoding.UTF8.GetString(data));
}
}</pre>
<div class="codesnippet-meta">StringBuilderHandler.cs: Read the StringBuilder
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<div class="codesnippet" MadCap:conditions="Primary.VB.NET">
<pre class="prettyprint lang-vb" MadCap:conditions="Primary.Online" xml:space="preserve">Public Function Read(ByVal readContext As IReadContext) As Object _
Implements IValueTypeHandler.Read
Dim length As Integer = readContext.ReadInt()
Dim data As Byte() = New Byte(length - 1) {}
readContext.ReadBytes(data)
Return New StringBuilder(Encoding.UTF8.GetString(data))
End Function</pre>
<div class="codesnippet-meta">StringBuilderHandler.vb: Read the StringBuilder
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<p>Delete is simple - we just reposition the buffer offset to the end of the
slot:</p>
<div class="codesnippet" MadCap:conditions="Primary.c#">
<pre class="prettyprint" xml:space="preserve">public void Delete(IDeleteContext deleteContext)
{
SkipData(deleteContext);
}
private static void SkipData(IReadBuffer deleteContext)
{
int numBytes = deleteContext.ReadInt();
deleteContext.Seek(deleteContext.Offset() + numBytes);
}</pre>
<div class="codesnippet-meta">StringBuilderHandler.cs: Delete the content
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<div class="codesnippet" MadCap:conditions="Primary.VB.NET">
<pre class="prettyprint lang-vb" MadCap:conditions="Primary.Online" xml:space="preserve">Public Sub Delete(ByVal deleteContext As IDeleteContext) _
Implements IValueTypeHandler.Delete
SkipData(deleteContext)
End Sub
Private Shared Sub SkipData(ByVal deleteContext As IReadBuffer)
Dim numBytes As Integer = deleteContext.ReadInt()
deleteContext.Seek(deleteContext.Offset() + numBytes)
End Sub</pre>
<div class="codesnippet-meta">StringBuilderHandler.vb: Delete the content
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<p>The last method left: #defragment. This one only moves the offset to the beginning
of the object data, i.e. skips Id and size information (to be compatible to
older versions):</p>
<div class="codesnippet" MadCap:conditions="Primary.c#">
<pre class="prettyprint" xml:space="preserve">public void Defragment(IDefragmentContext defragmentContext)
{
SkipData(defragmentContext);
}</pre>
<div class="codesnippet-meta">StringBuilderHandler.cs: Defragment the content
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<div class="codesnippet" MadCap:conditions="Primary.VB.NET">
<pre class="prettyprint lang-vb" MadCap:conditions="Primary.Online" xml:space="preserve">Public Sub Defragment(ByVal defragmentContext As IDefragmentContext) _
Implements IValueTypeHandler.Defragment
SkipData(defragmentContext)
End Sub</pre>
<div class="codesnippet-meta">StringBuilderHandler.vb: Defragment the content
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<p>Now to use this type handler we need to configure db4o. To register a typehandler you have to provide a predicate which decides if a type is handled by the typehandler and the typehandler itself.</p>
<div class="codesnippet" MadCap:conditions="Primary.c#">
<pre class="prettyprint" xml:space="preserve">IEmbeddedConfiguration configuration = Db4oEmbedded.NewConfiguration();
configuration.Common.RegisterTypeHandler(
new SingleClassTypeHandlerPredicate(typeof(StringBuilder)), new StringBuilderHandler());</pre>
<div class="codesnippet-meta">TypeHandlerExample.cs: Register type handler
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<div class="codesnippet" MadCap:conditions="Primary.VB.NET">
<pre class="prettyprint lang-vb" MadCap:conditions="Primary.Online" xml:space="preserve">Dim configuration As IEmbeddedConfiguration = Db4oEmbedded.NewConfiguration()
configuration.Common.RegisterTypeHandler(
New SingleClassTypeHandlerPredicate(GetType(StringBuilder)), New StringBuilderHandler())</pre>
<div class="codesnippet-meta">TypeHandlerExample.vb: Register type handler
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<p>After that all string builders are handled by you're type handler.</p>
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