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    <p>For validation that is based on business rules, I usually expose a Validation Delegate that my ViewModel can set.</p> <p>For example, the ViewModel for the Recipe might contain code that looks like this:</p> <pre><code>public GetRecipe(id) { CurrentRecipe = DAL.GetRecipe(id); CurrentRecipe.AddValidationErrorDelegate(ValidateRecipe); } private string ValidateRecipe(string propertyName) { if (propertyName == "TemperatureSetpoint") { var maxTemp = Configuration.MaxTemperatureSetpoint; if (CurrentRecipe.TemperatureSetpoint &gt;= maxTemp ) { return string.Format("Temperature cannot be greater than {0}", maxTemp); } } return null; } </code></pre> <p>The idea is that your <code>Model</code> should only contain raw data, therefore it should only validate raw data. This can include validating things like maximum lengths, required fields, and allowed characters. Business Logic, which includes business rules, should be validated in the <code>ViewModel</code>, and this allows that to happen.</p> <p>The actual implementation of my <code>IDataErrorInfo</code> on the <code>Recipe</code> class would look like this:</p> <pre><code>#region IDataErrorInfo &amp; Validation Members /// &lt;summary&gt; /// List of Property Names that should be validated /// &lt;/summary&gt; protected List&lt;string&gt; ValidatedProperties = new List&lt;string&gt;(); #region Validation Delegate public delegate string ValidationErrorDelegate(string propertyName); private List&lt;ValidationErrorDelegate&gt; _validationDelegates = new List&lt;ValidationErrorDelegate&gt;(); public void AddValidationErrorDelegate(ValidationErrorDelegate func) { _validationDelegates.Add(func); } #endregion // Validation Delegate #region IDataErrorInfo for binding errors string IDataErrorInfo.Error { get { return null; } } string IDataErrorInfo.this[string propertyName] { get { return this.GetValidationError(propertyName); } } public string GetValidationError(string propertyName) { // If user specified properties to validate, check to see if this one exists in the list if (ValidatedProperties.IndexOf(propertyName) &lt; 0) { //Debug.Fail("Unexpected property being validated on " + this.GetType().ToString() + ": " + propertyName); return null; } string s = null; // If user specified a Validation method to use, Validate property if (_validationDelegates.Count &gt; 0) { foreach (ValidationErrorDelegate func in _validationDelegates) { s = func(propertyName); if (s != null) { return s; } } } return s; } #endregion // IDataErrorInfo for binding errors #region IsValid Property public bool IsValid { get { return (GetValidationError() == null); } } public string GetValidationError() { string error = null; if (ValidatedProperties != null) { foreach (string s in ValidatedProperties) { error = GetValidationError(s); if (error != null) { return error; } } } return error; } #endregion // IsValid Property #endregion // IDataErrorInfo &amp; Validation Members </code></pre>
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    1. COThanks, for your answer! I like the idea of injecting the business rules into the model because this remove the dependency from the model to the Repository. The only disadvantage I see is that the model validation is now scattered through the code(in the model for simple properties and in the ViewModel for validation that depends ond other services) instead of being in 1 place(the model)
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      1. USArjen den Hartog
    2. CO@ArjendenHartog You could also add the validation in whatever layer creates the object. For example, you could have a `CustomerRepository` which queries the database, creates the model, adds the validation, then returns the object to whoever called it.
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