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RuleDescription Class
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As I discussed earlier, rules are described by use of a URI. A rule URI looks like this: rule://ruleClass/ruleMethod/targetProperty arg1=value&arg2=value The RuleDescription class understands how to parse a rule:// URI for easier use. Rather than manually writing code to parse the URI, or using the more generic System.Uri class, you can use the RuleDescription class to easily get at the parts of the URI. Csla.Validation.RuleDescription desc = new Csla.Validation.RuleDescription( "rule://typeName/methodName/propertyName arg1=value&arg2=value"); string scheme = desc.Scheme; string methodTypeName = desc.MethodTypeName; string methodName = desc.MethodName; string propertyName = desc.PropertyName; List<string> args = new List<string>(); foreach (var item in desc.Arguments) args.Add(item.Key + ", " + item.Value); Table 11-1 lists the properties available from RuleDescription. This class is particularly useful to UI framework authors, as it allows a UI framework to easily parse the rules associated with a business object.
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CHAPTER 11 BUS INE SS AND V ALIDA TION RULES
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Table 11-1. Properties Available from RuleDescription
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Property
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Scheme MethodTypeName MethodName PropertyName Arguments
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Description
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Returns the URI scheme, which is always rule:// Returns the type that implements the rule method Returns the rule method name specified in the URI Returns the name of the business object property with which this rule is associated Returns a Dictionary of name/value pairs representing the arguments passed to the rule method
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ValidationRules Class
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The ValidationRules class is the primary class in the Csla.Validation namespace. Every business object that uses the validation rules functionality will contain its own ValidationRules object. ValidationRules relies on the other classes in Csla.Validation to do its work. Together, these classes maintain the list of rules for each property and the list of currently broken rules.
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ValidationRulesManager and SharedValidationRulesManager Classes
You ve already seen how a business rule is defined based on the RuleHandler delegate. A key part of what ValidationRules does is to keep a list of such rule methods for each of the business object s properties. To do this, it relies on two other classes: ValidationRulesManager and SharedValidationRulesManager. The ValidationRulesManager keeps a list of rules for each property. One way that ValidationRulesManager is used is to keep a list of rules that is unique for each instance of a business object. This means that each time a business object is created, a set of rules can be associated with the properties of that particular instance. This has a pretty big impact on performance and memory consumption, and it s usually not the right approach. However, sometimes objects really do need unique rules, and this class enables that scenario. Another way ValidationRulesManager is used is by the SharedValidationRulesManager, as it keeps a list of rules for each business type. The SharedValidationRulesManager keeps a list of rules for each property that is common across all instances of a business object type. This means that the first time an instance of a given business class is created, the rules are associated with the properties of that type. The resulting associations are cached for the lifetime of the application and are shared by all instances of that business class. This has good performance and minimal memory consumption, and it s the recommended approach.
Note
CLSA .NET version 2.1 introduced the shared rules concept in response to performance and memory issues caused by the original per-instance rule technique.
As you ll see in 17 when I create the example business library, I typically only use shared rules.
Shared Rules and Threading
The big challenge with sharing a list of validation rules across all instances of a business type is that those instances could be running in parallel on different threads. In particular, on a web or application
CHAPTER 11 BUS IN ESS AN D V ALIDA TION RULES
server, many client requests may be running simultaneously, and they all need access to that same set of cached RuleMethod objects. Within SharedValidationRules, a Dictionary is used to cache all the rules for all the business object types. private static Dictionary<Type, ValidationRulesManager> _managers = new Dictionary<Type, ValidationRulesManager>(); It is indexed by Type, which is the type of the business objects used by the application. Each business object type has its own ValidationRulesManager, which stores the rules for that particular business type. To safely gain access to the ValidationRulesManager object for a type, the GetManager() method is used. internal static ValidationRulesManager GetManager( Type objectType, bool create) { ValidationRulesManager result = null; if (!_managers.TryGetValue(objectType, out result) && create) { lock (_managers) { if (!_managers.TryGetValue(objectType, out result)) { result = new ValidationRulesManager(); _managers.Add(objectType, result); } } } return result; } This method implements a simple but effective locking scheme. Remember that multiple threads may be executing this code at exactly the same time, and those threads may all attempt to get a value from the Dictionary. if (!_managers.TryGetValue(objectType, out result) && create) If this succeeds, then there s no problem. Multiple threads can read from the Dictionary at once without causing an issue. Things get more complex if the result field comes back as null, because that means a new ValidationRulesManager must be added to the Dictionary. Only one thread can be allowed to do this, so the lock statement is used to ensure that only one thread can run the next bit of code at a time. Note my careful choice of words: only one at a time. Many threads may run the code inside the lock statement, because many threads may have gotten a null value in result. So the code in the lock statement must ensure that only the first thread does any real work. It does this by rechecking to see if the value is in the Dictionary. if (!_managers.TryGetValue(objectType, out result)) { result = new ValidationRulesManager(); _managers.Add(objectType, result); } Only the first thread gets a null value for result here, and then it add a new value to the Dictionary. Every subsequent thread gets a non-null value for result, which is the desired outcome.
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