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    <p>The problem you are having is that your DataContext is being set after you have created the validation rule and there is no notification that it has changed. The simplest way to solve the problem is to change the xaml to the following:</p> <pre><code>&lt;TextBox.Text&gt; &lt;Binding NotifyOnValidationError="True" Path="myViewModelProperty" UpdateSourceTrigger="PropertyChanged"&gt; &lt;Binding.ValidationRules&gt; &lt;local:TotalQuantityValidator x:Name="validator" /&gt; &lt;/Binding.ValidationRules&gt; &lt;/Binding&gt; &lt;/TextBox.Text&gt; </code></pre> <p>And then set up the Context directly after setting the DataContext:</p> <pre><code>public MainWindow() { InitializeComponent(); this.DataContext = new MyViewModel(); this.validator.Context = new TotalQuantityValidatorContext { ViewModel = (MyViewModel)this.DataContext }; } </code></pre> <p>You could actually remove the Context class now and just have a property directly on the ValidationRule containing the ViewModel.</p> <p><strong>EDIT</strong></p> <p>Based on your comment I now suggest a slight change to the above code (the XAML is fine) to the following:</p> <pre><code>public MainWindow() { this.DataContextChanged += new DependencyPropertyChangedEventHandler(MainWindow_DataContextChanged); InitializeComponent(); } private void MainWindow_DataContextChanged(object sender, DependencyPropertyChangedEventArgs e) { this.validator.Context = new TotalQuantityValidatorContext { ViewModel = (MyViewModel)this.DataContext }; } </code></pre> <p>This will update your context whenever your viewmodel changes.</p>
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    1. POBinding DataContext to ValidationRule
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    1. USLeom Burke
    1. USLeom Burke
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    1. COI do have an initial DataContext being set in the XAML which might then be replaced by the code-behind (this is a data entry form - when first used it uses the DataContext set by XAML, following a save it then assigns a new empty model to the DataContext). Why then is the setter not being called for the original DataContext value (especially since the change of DataContext is reflected by the rest of the bindings)?
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      1. USMartin Robins
    2. COWell, I have at least figured out why it wont work :). The object that you are binding on (TotalQuantityValidatorContext) is not a part of the visual or logical tree and as such it cant see "myUserControl" in its binding. You must specify a direct source but as your source (the VM itself) changes you cant directly bind to it. My suggestion (if you are still doing it in XAML) is to do as I have pointed out above (just editing it so that it will work better with a changed VM)
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      1. USLeom Burke
    3. CORight - answer updated - that took a bit of tracking down - thanks for the fun :)
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