Implement Custom Rules

• Mar 09, 2024
• 15 minutes to read

The XAF supplies a number of rules. You can use a corresponding rule attribute to apply them to business classes and their properties. To get the list of available rules and their attributes, refer to the Validation Rules topic. However, the supplied rules are universal may not work in every scenario. In these instances, you can implement a custom validation rule. This topic explains two approaches: implementing a typical XAF rule that is applied via a specific rule attribute, and implementing a rule that is automatically applied to a specified business class.

Implement a Regular Validation Rule

To implement a new validation rule, inherit it from any XAF validation rule type listed in the table below. Some of these types are abstract and represent a base for rules. Use them as base classes to implement a custom rule. To modify the behavior of an existing XAF rule, inherit your rule directly from this particular rule type. Every rule should have the following auxiliary entities:

• Rule Attribute

  To create an instance of a rule of a particular type for a business class or a property, a rule attribute should be applied to this class or property. Each rule type has a specific attribute. Basically, an attribute establishes a set of parameters that are required to check the rule. When implementing a custom rule, you should implement a custom attribute for it. For this purpose, you can inherit either from the RuleBaseAttribute, or from the attribute that corresponds to the rule you inherit from. In your attribute, you should, at a minimum, override the protected RuleType property, returning your rule type.

• Rule Properties

  Information on the rules to create in the application is saved to the Application Model. Each validation rule exposes a RuleBaseProperties class descendant via the RuleBase.Properties property. This descendant specifies the rule’s properties exported to the corresponding Validation | Rules | Rule child node. When implementing a custom rule, you have the following options:

  1. If you need to extend the Properties class used by the rule’s base rule, perform the following actions. First, declare a new interface derived from the interface implemented by the base Properties class. Declare the required additional properties in this interface. Second, declare the new Properties class by inheriting from the base Properties class. The new class must implement the newly declared interface. To make your rule, use the custom Properties class, override the PropertiesType property of the rule and the rule’s attribute classes. In the overridden properties, return an instance of the descendant Properties class. Hide the base class’ Properties property via the new modifier. The property should have the type of the newly declared interface.

  2. If you are satisfied with the properties presented in the Properties type that is set for your rule’s base rule, you do not have to override the PropertiesType properties.

The following table lists the available rule types, corresponding attributes and Properties types:

Rule Type	Rule Description	Properties Type	Attribute
RuleBase	Represents an abstract class implementing the IRule interface. Used as a base class for all rules.	RuleBaseProperties Implements the IRuleBaseProperties interface.	-
RuleCombinationOfPropertiesIsUnique	Represents the RuleSearchObject class descendant. This rule demands that a particular properties’ values combination is unique.	RuleCombinationOfPropertiesIsUniqueProperties Derived from the RuleSearchObjectProperties class. Implements the IRuleCombinationOfPropertiesIsUniqueProperties interface.	RuleCombinationOfPropertiesIsUniqueAttribute Derived from the RuleBaseAttribute class. Implements the IRuleCombinationOfPropertiesIsUniqueProperties interface.
RuleCriteria	Represents the RuleBase class descendant. This rule demands that an object of a particular type satisfy a specified criteria.	RuleCriteriaProperties Derived from the RuleBaseProperties class. Implements the IRuleCriteriaProperties interface.	RuleCriteriaAttribute Derived from the RuleBaseAttribute class. Implements the IRuleCriteriaProperties interface.
RuleFromBoolProperty	Represents the RulePropertyValue<TPropertyValue> class descendant. This rule demands that a Boolean type property should have a True value.	RuleFromBoolPropertyProperties Derived from the RulePropertyValueProperties class. Implements the IRuleFromBoolPropertyProperties interface.	RuleFromBoolPropertyAttribute Derived from the RuleBaseAttribute class. Implements the IRuleFromBoolPropertyProperties interface.
RuleIsReferenced	Represents the RuleSearchObject class descendant. This rule demands that an object should be referenced in objects of a specified type.	RuleIsReferencedProperties Derived from the RuleSearchObjectProperties class. Implements the IRuleIsReferencedProperties interface.	RuleIsReferencedAttribute Derived from the RuleBaseAttribute class. Implements the IRuleIsReferencedProperties interface.
RuleObjectExists	Represents the RuleSearchObject class descendant. This rule demands that an object of a particular type that satisfies a specified criteria, exists in the database.	RuleObjectExistsProperties Derived from the RuleSearchObjectProperties class. Implements the IRuleObjectExistsProperties interface.	RuleObjectExistsAttribute Derived from the RuleBaseAttribute class. Implements the IRuleObjectExistsProperties interface.
RulePropertyValue	Represents an abstract class derived form the RuleBase class. Used as a base class for the rules that check a specified property’s value.	RulePropertyValueProperties Derived from the RuleBaseProperties class. Implements the IRulePropertyValueProperties interface.	-
RulePropertyValue<TPropertyValue>	Represents an abstract class derived from the RulePropertyValue class. Used as a base class for the rules that checks a particular type of specified property’s value.	RulePropertyValueProperties Derived from the RuleBaseProperties class. Implements the IRulePropertyValueProperties interface.	-
RuleRange	Represents the RulePropertyValue<TPropertyValue> class descendant. This rule demands that a particular property’s value should be within the specified value range.	RuleRangeProperties Derived from the RulePropertyValueProperties class. Implements the IRuleRangeProperties interface.	RuleRangeAttribute Derived from the RuleBaseAttribute class. Implements the IRuleRangeProperties interface.
RuleRegularExpression	Represents the RulePropertyValue<TPropertyValue> class descendant. This rule demands that a particular property should match a specified pattern.	RuleRegularExpressionProperties Derived from the RulePropertyValueProperties class. Implements the IRuleRegularExpressionProperties interface.	RuleRegularExpressionAttribute Derived from the RuleBaseAttribute class. Implements the IRuleRegularExpressionProperties interface.
RuleRequiredField	Represents the RulePropertyValue<TPropertyValue> class descendant. This rule demands that a particular property should have a value.	RuleRequiredFieldProperties Derived from the RulePropertyValueProperties class. Implements the IRuleRequiredFieldProperties interface.	RuleRequiredFieldAttribute Derived from the RuleBaseAttribute class. Implements the IRuleRequiredFieldProperties interface.
RuleSearchObject	Represents an abstract class derived form the RuleBase class. Used as a base class for the RuleObjectExists, RuleUniqueValue and RuleIsReferenced rules. This rule’s checking algorithm is based on searching an object by a particular criteria.	RuleSearchObjectProperties Derived from the RuleBaseProperties class. Implements the IRuleSearchObjectProperties interface.	-
RuleStringComparison	Represents the RulePropertyValue<TPropertyValue> class descendant. This rule demands that a value of a particular String type property should satisfy a specified condition.	RuleStringComparisonProperties Derived from the RulePropertyValueProperties class. Implements the IRuleStringComparisonProperties interface.	RuleStringComparisonAttribute Derived from the RuleBaseAttribute class. Implements the IRuleStringComparisonProperties interface.
RuleUniqueValue	Represents the RuleSearchObject class descendant. This rule demands that a particular property’s value should be unique.	RuleUniqueValueProperties Derived from the RuleSearchObjectProperties class. Implements the IRuleUniqueValueProperties interface.	RuleUniqueValueAttribute Derived from the RuleBaseAttribute class. Implements the IRuleUniqueValueProperties interface.
RuleValueComparison	Represents the RulePropertyValue<TPropertyValue> class descendant. This rule demands that a particular property’s value satisfy a specified condition.	RuleValueComparisonProperties Derived from the RulePropertyValueProperties class. Implements the IRuleValueComparisonProperties interface.	RuleValueComparisonAttribute Derived from the RuleBaseAttribute class. Implements the IRuleValueComparisonProperties interface.

Note

All the rules, Property types and attributes that are presented in the table above are declared in the DevExpress.Persistent.Base.v24.1.dll assembly, in the DevExpress.Persistent.Validation.RuleBase namespace.

The following code demonstrates the key members that you should have in your rule, Properties interface, Properties type and attribute:

C#

//Declare the new Properties interface
[GenerateMessageTemplatesModel("MyRule")]
public interface IRuleMyProperties : IRuleSearchObjectProperties {
    [Category("Data")]
    string AdditionalProperty { get; set; }
}
[DomainComponent] //Here the RuleSearchObjectProperties is used as a base class
public class MyRuleProperties : RuleSearchObjectProperties, IRuleMyProperties {
    private string additionalProperty;
    public string AdditionalProperty {
        get { return additionalProperty; }
        set { additionalProperty = value; }
    }
}
//Here the RuleSearchObject is used as a base class
public class MyRule : RuleSearchObject {
    //A custom rule must have at least two constructors
    //The default constructor
    public MyRule() : base() { }
    //A constructor which takes an IRuleBaseProperties-descendant interface type parameter
    public MyRule(IRuleSearchObjectProperties properties) : base(properties) { }
    protected override bool IsValidInternal(object target, out string errorMessageTemplate) {
        //Check whether the rule is satisfied
    }
    public override ReadOnlyCollection<string> UsedProperties {
        get {
            //Specify the properties that will be highlighted when the rule is broken
            //In ASP.NET Core Blazor, specify all affected properties before the first evaluation.
        }
    }
    //If you have a custom Properties type, return the Properties object
    //cast to the corresponding interface type
    protected new IRuleMyProperties Properties {
        get {
            return (IRuleMyProperties)base.Properties;
        }
    }
    //If you have a custom Properties type, return it here
    public override Type PropertiesType {
        get {
            return typeof(MyRuleProperties);
        }
    }
}
//Here an abstract RuleBaseAttribute class is used as a base class
public class MyRuleAttribute : RuleBaseAttribute {
    //Return the type of the rule for which you implement this attribute
    protected override Type RuleType {
        get { return typeof(MyRule); }
    }
    //If you have a custom Properties type, return it
    protected override Type PropertiesType {
        get {
            return typeof(MyRuleProperties);
        }
    }
    public MyRuleAttribute(string id, string targetContextIDs, string additionalProperty)
        : base(id, targetContextIDs) {
        Properties.AdditionalProperty = additionalProperty;
    }
    public MyRuleAttribute(string id, DefaultContexts targetContexts, string additionalProperty)
        : base(id, targetContexts) {
        Properties.AdditionalProperty = additionalProperty;
    }
    //If you have a custom Properties type, return the Properties object
    //cast to the corresponding interface type
    protected new IRuleMyProperties Properties {
        get { return (IRuleMyProperties)base.Properties; }
    }
    public string AdditionalProperty {
        get { return Properties.AdditionalProperty; }
    }
}

VB.NET

'Declare the new Properties interface
<GenerateMessageTemplatesModel("MyRule")> _
Public Interface IRuleMyProperties
    Inherits IRuleSearchObjectProperties
    <Category("Data")> _
    Property AdditionalProperty() As String
End Interface
<DomainComponent> _
Public Class MyRuleProperties
    Inherits RuleSearchObjectProperties
    Implements IRuleMyProperties
    Private additionalProperty_Renamed As String
    Public Property AdditionalProperty() As String Implements IRuleMyProperties.AdditionalProperty
        Get
            Return additionalProperty_Renamed
        End Get
        Set(ByVal value As String)
            additionalProperty_Renamed = value
        End Set
    End Property
End Class
'Here the RuleSearchObject is used as a base class
Public Class MyRule
    Inherits RuleSearchObject
    'A custom rule must have at least two constructors
    'The default constructor
    Public Sub New()
        MyBase.New()
    End Sub
    'A constructor which takes an IRuleBaseProperties-descendant interface type parameter
    Public Sub New(ByVal properties As IRuleSearchObjectProperties)
        MyBase.New(properties)
    End Sub
    Protected Overrides Function IsValidInternal(ByVal target As Object, _
    <System.Runtime.InteropServices.Out()> ByRef errorMessageTemplate As String) As Boolean
        'Check whether the rule is satisfied
    End Function
    Public Overrides ReadOnly Property UsedProperties() As ReadOnlyCollection(Of String)
        Get
            'Specify the properties that will be highlighted when the rule is broken
        End Get
    End Property
    'If you have a custom Properties type, return the Properties object
    'cast to the corresponding interface type
    Protected Shadows ReadOnly Property Properties() As IRuleMyProperties
        Get
            Return CType(MyBase.Properties, IRuleMyProperties)
        End Get
    End Property
    'If you have a custom Properties type, return it here
    Public Overrides ReadOnly Property PropertiesType() As Type
        Get
            Return GetType(MyRuleProperties)
        End Get
    End Property
End Class
'Here an abstract RuleBaseAttribute class is used as a base class
Public Class MyRuleAttribute
    Inherits RuleBaseAttribute
    'Return the type of the rule for which you implement this attribute
    Protected Overrides ReadOnly Property RuleType() As Type
        Get
            Return GetType(MyRule)
        End Get
    End Property
    'If you have a custom Properties type, return it
    Protected Overrides ReadOnly Property PropertiesType() As Type
        Get
            Return GetType(MyRuleProperties)
        End Get
    End Property
    Public Sub New(ByVal id As String, ByVal targetContextIDs As String, _
    ByVal additionalProperty As String)
        MyBase.New(id, targetContextIDs)
        Properties.AdditionalProperty = additionalProperty
    End Sub
    Public Sub New(ByVal id As String, ByVal targetContexts As DefaultContexts, _
    ByVal additionalProperty As String)
        MyBase.New(id, targetContexts)
        Properties.AdditionalProperty = additionalProperty
    End Sub
    'If you have a custom Properties type, return the Properties object
    'cast to the corresponding interface type
    Protected Shadows ReadOnly Property Properties() As IRuleMyProperties
        Get
            Return CType(MyBase.Properties, IRuleMyProperties)
        End Get
    End Property
    Public ReadOnly Property AdditionalProperty() As String
        Get
            Return Properties.AdditionalProperty
        End Get
    End Property
End Class

Note

Declare the RuleBaseAttribute descendant’s Properties property as non-public. Use the new modifier to hide the base class Properties property.

When implementing a custom Properties type, the following attributes can be useful:

• RulePropertiesRequired

  Applied to a property of a Properties class. Indicates that the rule node’s property that corresponds to this property cannot be empty.

• RulePropertiesMemberOf

  Applied to a Properties class’ property, specifying the name of a business class property that takes part in the rule’s logic. This attribute’s parameter passes the name of the Properties class’ property, specifying the type containing the property that uses this attribute. For instance:

  C#

  [DomainComponent]
  public class MyRuleProperties : RuleSearchObjectProperties, IRuleMyProperties {
      private Type someType;
      public Type SomeType {
          get { return someType; }
          set { someType = value; }
      }
      private string property1Name;
      [RulePropertiesMemberOf("SomeType ")]
      public string Property1Name{
          get { return property1Name; }
          set { property1Name = value; }
      }
      private string property2Name;
      [RulePropertiesMemberOf("SomeType ")]
      public string Property2Name{
          get { return property2Name; }
          set { property2Name= value; }
      }
  }
  

  VB.NET

  <DomainComponent> _
  Public Class MyRuleProperties
      Inherits RuleSearchObjectProperties
      Implements IRuleMyProperties
      Private fSomeType As Type
      Public Property SomeType() As Type
          Get
              Return fSomeType
          End Get
          Set
              fSomeType = Value
          End Set
      End Property
      Private fProperty1Name As String
      <RulePropertiesMemberOf("SomeType ")> _
      Public Property Property1Name() As String
          Get
              Return fProperty1Name
          End Get
          Set
              fProperty1Name = Value
          End Set
      End Property
      Private fProperty2Name As String
      <RulePropertiesMemberOf("SomeType ")> _
      Public Property Property2Name() As String
          Get
              Return fProperty2Name
          End Get
          Set
              fProperty2Name = Value
          End Set
      End Property
  End Class
  

• RulePropertiesIndex

  Applied to properties of a Properties class to specify the order in which the corresponding properties will be shown in the Model Editor.

To register a custom validation rule, override a module’s Setup method that takes an ApplicationModulesManager instance as a parameter. The following code snippet illustrates this.

C#

using DevExpress.ExpressApp;
using DevExpress.ExpressApp.Validation;
//...
public sealed partial class MyModule : ModuleBase {
    //...
    public override void Setup(ApplicationModulesManager moduleManager) {
        base.Setup(moduleManager);
        ValidationRulesRegistrator.RegisterRule(moduleManager, typeof(MyRule), typeof(IRuleMyProperties));
    }
}

VB.NET

Imports DevExpress.ExpressApp
Imports DevExpress.ExpressApp.Validation
'...
Public NotInheritable Partial Class MyModule
    Inherits ModuleBase
    '...
    Public Overrides Sub Setup(ByVal moduleManager As ApplicationModulesManager)
        MyBase.Setup(moduleManager)
        ValidationRulesRegistrator.RegisterRule(moduleManager, GetType(MyRule), GetType(IRuleMyProperties))
    End Sub
End Class

To see an example of implementing a custom rule and its attributes and properties, refer to the FeatureCenter demo installed with XAF that have the RuleStringLengthComparison, RuleStringLengthComparisonAttribute, and RuleStringLengthComparisonProperties implemented. This demo is in the %PUBLIC%\Documents\DevExpress Demos 24.1\Components\XAF\FeatureCenter.NETFramework.XPO folder.

Implement a Code Rule

With the Code Rule approach, you do not implement a custom attribute or apply it to business classes. The approach is useful when you need to apply a validation rule in code to a business class implemented in a referenced assembly without using the Model Editor. You can also use this approach to check objects using a highly tailored algorithm.

The main idea of the Code Rule approach is that you implement a custom rule, as described above, and decorate it with the CodeRuleAttribute. This approach differs from the approach described above to collect validation rules. Typically, information on validation rules is loaded into the Application Model based on the rule attributes applied to business classes. With this approach, information on validation rules is loaded into the Application Model based on validation rule classes decorated with the CodeRule attribute.

The rule’s target class can be specified in the rule’s declaration using one of the following techniques:

• The target type is specified as a generic parameter for the rule’s base class:

  C#

  [CodeRule]
  public class MyCodeRule : RuleBase<Contact> {
     protected override bool IsValidInternal(
        Contact target, out string errorMessageTemplate) {
        //Check whether the rule is satisfied
     }
      public MyCodeRule() : base("", "Save") { }
      public MyCodeRule(IRuleBaseProperties properties) : base(properties) { }
  }
  

  VB.NET

  <CodeRule> _
  Public Class MyCodeRule
      Inherits RuleBase(Of Contact)
      Protected Overrides Function IsValidInternal(ByVal target As Contact, _
      <System.Runtime.InteropServices.Out()> ByRef errorMessageTemplate As String) As Boolean
          'Check whether the rule is satisfied
      End Function
      Public Sub New()
          MyBase.New("", "Save")
      End Sub
      Public Sub New(ByVal properties As IRuleBaseProperties)
          MyBase.New(properties)
      End Sub
  End Class
  

• The target type is passed as a parameter of the base rule’s constructor:

  C#

  [CodeRule]
  public class MyCodeRule : RuleBase {
     protected override bool IsValidInternal(
        object target, out string errorMessageTemplate) {
        //Check whether the rule is satisfied
     }
      public MyCodeRule() : base("", "Save", typeof(Contact)) { }
      public MyCodeRule(IRuleBaseProperties properties) : base(properties) { }
  }
  

  VB.NET

  <CodeRule> _
  Public Class MyCodeRule
      Inherits RuleBase
      Protected Overrides Function IsValidInternal(ByVal target As Object, _
      <System.Runtime.InteropServices.Out()> ByRef errorMessageTemplate As String) As Boolean
          'Check whether the rule is satisfied
      End Function
      Private Function MyCodeRule() As [Public]
          MyBase.New("", "Save", GetType(Contact))
      End Function
      Private Function MyCodeRule(ByVal properties As IRuleBaseProperties) As [Public]
          MyBase.New(properties)
      End Function
  End Class
  

You can specify the rule’s target properties (properties that XAF highlights in the UI when the rule is broken) in the overridden IRule.UsedProperties property getter. In ASP.NET Core Blazor, the editors for the listed properties trigger the re-evaluation of the rule when they are changed. You should specify all the affected properties before the first evaluation because the subscription takes place when XAF creates the controls.

C#

using System.Collections.ObjectModel;
// ...
public override ReadOnlyCollection<string> UsedProperties {
    get {
        return new ReadOnlyCollection<string>(new List<string>() { "Amount" });
    }
}

VB.NET

Imports System.Collections.ObjectModel
' ...
Public Overrides ReadOnly Property UsedProperties() As ReadOnlyCollection(Of String)
    Get
        Return New ReadOnlyCollection(Of String)(New List(Of String)() From {"Amount"})
    End Get
End Property

To access the IObjectSpace instance in the RuleBase descendant, use the IObjectSpaceLink interface:

public class MyCodeRule : RuleBase<Contact> {
    protected override bool IsValidInternal(
        Contact target, out string errorMessageTemplate) {
        var os = ((IObjectSpaceLink)this).ObjectSpace;

Do not forget to register the rule by overriding a module’s Setup method that takes an ApplicationModulesManager instance as a parameter.

C#

using DevExpress.ExpressApp;
using DevExpress.ExpressApp.Validation;
//...
public sealed partial class MyModule : ModuleBase {
    //...
    public override void Setup(ApplicationModulesManager moduleManager) {
        base.Setup(moduleManager);
        ValidationRulesRegistrator.RegisterRule(moduleManager, typeof(MyCodeRule), typeof(IRuleBaseProperties));
    }
}

VB.NET

Imports DevExpress.ExpressApp
Imports DevExpress.ExpressApp.Validation
'...
Public NotInheritable Partial Class MyModule
    Inherits ModuleBase
    '...
    Public Overrides Sub Setup(ByVal moduleManager As ApplicationModulesManager)
        MyBase.Setup(moduleManager)
        ValidationRulesRegistrator.RegisterRule(moduleManager, GetType(MyCodeRule), GetType(IRuleBaseProperties))
    End Sub
End Class

To see an example of implementing a custom rule using the CodeRule attribute, refer to the FeatureCenter demo and the CodeRuleObjectIsValidRule implemented in this demo. This demo is located in the %PUBLIC%\Documents\DevExpress Demos 24.1\Components\XAF\FeatureCenter.NETFramework.XPO folder.