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    <p>Get rid of the OnClick event handler, instead use a binding to a command.<BR/> Register the command in the VM with a CanExecute method that returns Model.IsValid.</p> <p>If the validation failure is in the binding of the view to the viewmodel, then the viewmodel can still be valid - since it didn't store the update (due to type mismatch, range check, etc).</p> <p>In this case consider storing error state in VM, before throwing exception, then clearing error state if same property was successfully set.</p> <p>Modified code:</p> <p><HR/> XAML:<BR/> <I>(Note that the scope of ValidationRules:NameRule instance is in TextBox, other usages will receive another instance with different field values)</I></p> <pre><code>&lt;TextBox Width="250" Height="25" Margin="5" Validation.ErrorTemplate="{StaticResource customErrorTemplate}"&gt; &lt;TextBox.Text&gt; &lt;Binding Path="WitnessName" Mode="TwoWay" UpdateSourceTrigger="PropertyChanged"&gt; &lt;Binding.ValidationRules&gt; &lt;ValidationRules:NameRule x:Name="nameValidator" /&gt; &lt;/Binding.ValidationRules&gt; &lt;/Binding&gt; &lt;/TextBox.Text&gt; &lt;/TextBox&gt; </code></pre> <p><HR/> Validation rule:</p> <pre><code>public class NameRule : ValidationRule, INotifyPropertyChanged { public bool HasFailed // set default of field behind to false { get; // change to support INotifyPropertyChanged set; // change to support INotifyPropertyChanged } public override ValidationResult Validate(object value, System.Globalization.CultureInfo cultureInfo) { var isValid = (value as string == null) || Regex.IsMatch(value.ToString(), @"^[\p{L} \.'\-]+$"); HasFailed = !isValid; return new ValidationResult(isValid, "Name can contain only letters, apostrophes and hyphens."); } } </code></pre> <p><HR/> Command:</p> <pre><code>public bool CanExecute(object parameter) { return this.witnessDialogue.IsValid &amp;&amp; !this.witnessDialogue.nameValidator.HasFailed; } public void Execute(object parameter) { this.witnessDialogue.OnOkClick(); } </code></pre>
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    1. POHow can I disable a WPF dialogue OK button in response to invalid input determined by a custom ValidationRule class?
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    1. USDanny Varod
    1. USDanny Varod
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    1. CODanny, thanks for the response. I think the problem is that the property in the VM is never being set (or attempted to set) because the validation in the view is intervening so I can't handle the error there.
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      1. USVal M
    2. COI have updated my question with my attempt to implement what you have suggested. Could you tell me where I am going wrong with that?
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    3. COUpdated my answer.
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      1. USDanny Varod
 

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