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  1. PO
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    <p>To be honest, I found that the baked in WPF validation methods are not complete and/or elegant enough. I find that using the WPF methods would scatter validation code and logic throughout my application and would even put some in my UI. Like you, I used Custom Business Objects (CBOs) for everything, and I was really wanting to keep my validation in my objects, since I was using them across several projects (a web service, UI, mobile, etc).</p> <p>What I did was take my CBO (Recipe, in this case), and add some validation methods as properties. Eg:</p> <pre><code> public Func&lt;string&gt; NameValidation { get { return () =&gt; { string result = null; if (String.IsNullOrEmpty(Name)) result = "Name cannot be blank"; else if (Name.Length &gt; 100) result = "Name cannot be longer than 100 characters"; return result; }; } } </code></pre> <p>After that, I decorated it with a custom attribute:</p> <pre><code>[AttributeUsage(AttributeTargets.Property)] public class CustomValidationMethod : Attribute { } </code></pre> <p>then I created a Validate() method for object-level validation:</p> <pre><code> public override void Validate() { var a = GetType().GetProperties().Where(w =&gt; w.GetCustomAttributes(typeof(CustomValidationMethod), true).Length &gt; 0); foreach (var a2 in a) { var result = a2.GetValue(this, null) as Func&lt;string&gt;; if (result != null) { var message = result(); if (message != null) //There was an error, do something else if (message == null &amp;&amp; Errors.ContainsKey(a2.Name)) //There was no error } } } </code></pre> <p>then I created custom controls that support my validation. In this case, it was a ComboBox that I derived from the standard ComboBox and added this code:</p> <pre><code> public Func&lt;string&gt; ValidationMethod { get { return (Func&lt;string&gt;) GetValue(ValidationMethodProperty); } set { SetValue(ValidationMethodProperty, value); } } protected override void OnPropertyChanged(DependencyPropertyChangedEventArgs e) { base.OnPropertyChanged(e); if (ValidationMethod != null &amp;&amp; !String.IsNullOrEmpty(ValidationMethod())) SetControlAsInvalid(); else SetControlAsValid(); } </code></pre> <p>Once this is all set up, I can add field validation in the validation methods (which are stored in my CBOs instead of scattered throughout my code), I can add object-level validation in my Validate() method. As well, I can customize with ease how the control should behave with regards to validation.</p> <p>To use this, in my VM I would call .Validate() first, then deal with any problems before saving. In my case specifically, I would store error messages in a collection and then query them (this also allowed me to store several error messages instead of the first one)</p>
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    1. POSuggestions for Entity/Business object validation where the validation is dependent on other Entity/Service
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    1. USCamronBute
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    1. COHi CamronBute, thanks for your answer. I like the idea of overriding the OnPropertyChanged to enforce that validation is done automatically. I couldn't find an answer for my main question: "Do you have suggestions how to implement Entity/Business object validation where the validation is dependent on other Entity/Service, like in the example below."
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      1. USArjen den Hartog
    2. COYou can add the validation in several different places. You can check in the ViewModel by creating a method, or put it right in the .Validate() method. I personally didn't have a method or doing this, but I would add it in the .Validate() method, since you know .Validate() will be called every time.
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