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POBinding DataContext to ValidationRule
 

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    <p>I have a custom ValidationRule that requires access to the ViewModel in order to validate a supplied value in conjunction with other properties of the ViewModel. I previously tried to acheive this by using a ValidationGroup, but abandoned this idea as the code I am modifying would need a lot of refactoring in order to enable this route.</p> <p>I found a <a href="http://social.msdn.microsoft.com/Forums/en-US/wpf/thread/982e2fcf-780f-4f1c-9730-cedcd4e24320/" rel="noreferrer">thread on a newsgroup</a> that showed a way of binding the DataContext of a control in which the ValidationRule is being run to that ValidationRule by way of an intermediate class inherited from DependencyObject, but I cannot get it to bind.</p> <p>Can anybody help?</p> <p>My ValidationRule is as follows...</p> <pre><code>class TotalQuantityValidator : CustomValidationRule { public TotalQuantityValidator() : base(@"The total number must be between 1 and 255.") { } public TotalQuantityValidatorContext Context { get; set; } public override ValidationResult Validate(object value, CultureInfo cultureInfo) { ValidationResult validationResult = ValidationResult.ValidResult; if (this.Context != null &amp;&amp; this.Context.ViewModel != null) { int total = ... if (total &lt;= 0 || total &gt; 255) { validationResult = new ValidationResult(false, this.ErrorMessage); } } return validationResult; } } </code></pre> <p>CustomValidationRule is defined as follows...</p> <pre><code>public abstract class CustomValidationRule : ValidationRule { protected CustomValidationRule(string defaultErrorMessage) { this.ErrorMessage = defaultErrorMessage; } public string ErrorMessage { get; set; } } </code></pre> <p>TotalQuantityValidatorContext is defined as follows...</p> <pre><code>public class TotalQuantityValidatorContext : DependencyObject { public static readonly DependencyProperty ViewModelProperty = DependencyProperty.Register(@"ViewModel", typeof(MyViewModel), typeof(TotalQuantityValidatorContext), new PropertyMetadata { DefaultValue = null, PropertyChangedCallback = new PropertyChangedCallback(TotalQuantityValidatorContext.ViewModelPropertyChanged) }); public MyViewModel ViewModel { get { return (MyViewModel)this.GetValue(TotalQuantityValidatorContext.ViewModelProperty); } set { this.SetValue(TotalQuantityValidatorContext.ViewModelProperty, value); } } private static void ViewModelPropertyChanged(DependencyObject element, DependencyPropertyChangedEventArgs args) { } } </code></pre> <p>And the whole thing is used thus...</p> <pre><code>&lt;UserControl x:Class="..." xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation" xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml" xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006" xmlns:d="http://schemas.microsoft.com/expression/blend/2008" xmlns:val="clr-namespace:Validators" x:Name="myUserControl"&gt; &lt;TextBox Name="myTextBox"&gt; &lt;TextBox.Text&gt; &lt;Binding NotifyOnValidationError="True" Path="myViewModelProperty" UpdateSourceTrigger="PropertyChanged"&gt; &lt;Binding.ValidationRules&gt; &lt;val:TotalQuantityValidator&gt; &lt;val:TotalQuantityValidator.Context&gt; &lt;val:TotalQuantityValidatorContext ViewModel="{Binding ElementName=myUserControl, Path=DataContext}" /&gt; &lt;/val:TotalQuantityValidator.Context&gt; &lt;/val:TotalQuantityValidator&gt; &lt;/Binding.ValidationRules&gt; &lt;/Binding&gt; &lt;/TextBox.Text&gt; &lt;/TextBox&gt; &lt;/UserControl&gt; </code></pre> <p>The DataContext of the UserControl is being set to an instance of MyViewModel in code-behind. I know that this binding works as the standard control bindings are operating as expected.</p> <p>The <code>TotalQuantityValidator.Validate</code> method is called correctly, but whenever I look at the <code>ViewModel</code> property of the <code>Context</code>, it is always null (the <code>Context</code> property of the <code>TotalQuantityValidator</code> is being set to an instance of <code>TotalQuantityValidatorContext</code> correctly). I can see from the debugger however that the setter on the <code>ViewModel</code> property of the <code>TotalQuantityValidatorContext</code> is never called.</p> <p>Can anybody advise as to how I can get this binding to work?</p> <p>Thanks in advance.</p>
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    1. COI know this question is similar to http://stackoverflow.com/questions/3577899/wpf-property-in-validationrule-never-set, but I am looking to access the DataContext and not just another property of it.
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