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Andy De George
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title: "How to: Implement PriorityBinding"
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ms.date: "03/30/2017"
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dev_langs:
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- "csharp"
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- "vb"
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helpviewer_keywords:
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- "data binding [WPF], PriorityBinding class"
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ms.assetid: d63b65ab-b3e9-4322-9aa8-1450f8d89532
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---
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# How to: Implement PriorityBinding
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<xref:System.Windows.Data.PriorityBinding> in [!INCLUDE[TLA#tla_winclient](../../../includes/tlasharptla-winclient-md.md)] works by specifying a list of bindings. The list of bindings is ordered from highest priority to lowest priority. If the highest priority binding returns a value successfully when it is processed then there is never a need to process the other bindings in the list. It could be the case that the highest priority binding takes a long time to be evaluated, the next highest priority that returns a value successfully will be used until a binding of a higher priority returns a value successfully.
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## Example
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To demonstrate how <xref:System.Windows.Data.PriorityBinding> works, the `AsyncDataSource` object has been created with the following three properties: `FastDP`, `SlowerDP`, and `SlowestDP`.
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The get accessor of `FastDP` returns the value of the `_fastDP` data member.
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The get accessor of `SlowerDP` waits for 3 seconds before returning the value of the `_slowerDP` data member.
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The get accessor of `SlowestDP` waits for 5 seconds before returning the value of the `_slowestDP` data member.
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> [!NOTE]
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> This example is for demonstration purposes only. The .NET guidelines recommend against defining properties that are orders of magnitude slower than a field set would be. For more information, see [Choosing Between Properties and Methods](https://docs.microsoft.com/previous-versions/dotnet/netframework-4.0/ms229054(v=vs.100)).
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[!code-csharp[PriorityBinding#1](~/samples/snippets/csharp/VS_Snippets_Wpf/PriorityBinding/CSharp/Window1.xaml.cs#1)]
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[!code-vb[PriorityBinding#1](~/samples/snippets/visualbasic/VS_Snippets_Wpf/PriorityBinding/VisualBasic/AsyncDataSource.vb#1)]
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The <xref:System.Windows.Controls.TextBlock.Text%2A> property binds to the above `AsyncDS` using <xref:System.Windows.Data.PriorityBinding>:
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[!code-xaml[PriorityBinding#2](~/samples/snippets/csharp/VS_Snippets_Wpf/PriorityBinding/CSharp/Window1.xaml#2)]
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When the binding engine processes the <xref:System.Windows.Data.Binding> objects, it starts with the first <xref:System.Windows.Data.Binding>, which is bound to the `SlowestDP` property. When this <xref:System.Windows.Data.Binding> is processed, it does not return a value successfully because it is sleeping for 5 seconds, so the next <xref:System.Windows.Data.Binding> element is processed. The next <xref:System.Windows.Data.Binding> does not return a value successfully because it is sleeping for 3 seconds. The binding engine then moves onto the next <xref:System.Windows.Data.Binding> element, which is bound to the `FastDP` property. This <xref:System.Windows.Data.Binding> returns the value "Fast Value". The <xref:System.Windows.Controls.TextBlock> now displays the value "Fast Value".
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After 3 seconds elapses, the `SlowerDP` property returns the value "Slower Value". The <xref:System.Windows.Controls.TextBlock> then displays the value "Slower Value".
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After 5 seconds elapses, the `SlowestDP` property returns the value "Slowest Value". That binding has the highest priority because it is listed first. The <xref:System.Windows.Controls.TextBlock> now displays the value "Slowest Value".
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See <xref:System.Windows.Data.PriorityBinding> for information about what is considered a successful return value from a binding.
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## See also
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- <xref:System.Windows.Data.Binding.IsAsync%2A?displayProperty=nameWithType>
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- [Data Binding Overview](/dotnet/desktop-wpf/data/data-binding-overview)
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- [How-to Topics](data-binding-how-to-topics.md)
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