Figure 7-8: Error message shown when a load fails in C#.NET

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Figure 7-8: Error message shown when a load fails
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This message presents the problem to the user but doesn t indicate any good way to x the problem They have no option but to choose OK, at which point they will be returned to Visual Studio as though they had never asked to open the le What s generally required is to point the user in the direction of xing the problem; here, a decision has to be made Can this violation be xed using the modeling tool itself, or is it serious enough that the XML must be edited by hand to x the problem Typically, this comes down to one of two possibilities First, is the presentation model of the designer capable of rendering the malformed model In this case, the presentation doesn t care about cyclical loops, so the answer is yes Second, does any of the other custom code in the designer make assumptions that this rule is an invariant As you add code to your designer, it s very easy to start to assume that your own rules will not be broken Indeed, if you are fairly con dent that the rules are unlikely to change much, this may be a good, pragmatic approach to keeping your code free from reams of unnecessary precondition checks However, it does tend to mean that you will have to intercept problems in malformed XML les before your designer is
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launched; custom code will likely crash otherwise For example, you may have custom code in several places that walks the inheritance class hierarchy using a simple loop If a le is loaded that contains a cycle, then this code will be stuck in an endless loop The starting point for handling either of these situations more gracefully is to make the rule class previously described initially disabled and only turn it on once the designer is fully running This will allow you to handle the error yourself at a time of your choosing To do this, modify the RuleOn attribute on the rule by adding the InitiallyDisabled parameter:
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[RuleOn(typeof(Generalization), FireTime = TimeToFireTopLevelCommit, InitiallyDisabled=true)]
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You ll also then need to create a partial DocData class to switch the rule on after the initial load has nished The DocData class lives in the DslPackage project Typically, we add DocData and other Visual Studio integration customizations in new partial class les under a directory called Shell in the DslPackage project
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internal partial class NoLoopClassDocData : NoLoopClassDocDataBase { protected override void Load(string fileName, bool isReload) { baseLoad(fileName, isReload); RuleManager ruleManager = thisStoreRuleManager; ruleManagerEnableRule( typeof(CJKWExamplesNoLoopClassCyclicInheritanceAddRule)); } }
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You ll notice that this code in the DslPackage project is referring to the rule that is de ned as being internal to the Dsl project This wouldn t normally be allowed, but in order for the two packages to work together as one while still presenting an uncluttered public API, it is typical to make the DslPackage project a friend of the Dsl project To do this, add an attribute like the following to your Dsl project s Properties\AssemblyInfocs file:
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[assembly: InternalsVisibleTo("CJKWExamplesNoLoopClass DslPackage, PublicKey=00240000048000009400000006020000002400005253413100 040000010001009b498f24fcd75c6a243ae1831202e2da959d2c51662c94 8c0491e96bb4e924522f583e5149366919102d3c7b2b64fe6e70c282b065 a99cd0a79f30ad02a12e266aa2375b0d912408ec11ea924c1c617d1cd7f0 1e8cf56943fef1227c6d02568767b21a669e12de23f89c350faf638c04fa a20131e7b4436d9ffc31ccd0c0fe9d")]
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You can retrieve the very unwieldy PublicKey entry for this attribute by using the sn -Tp <assembly> command on your designer s assemblies from a Visual Studio command prompt
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This code now lets diagrams containing cycles load happily, allowing them to be corrected using the modeling tool itself However, there is now no indication that there is any problem to correct, so we need to introduce yet another mechanism to detect this error on loading a validation method will prove very effective:
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[ValidationState(ValidationStateEnabled)] public partial class Generalization { [ValidationMethod(ValidationCategoriesOpen)] private void ValidateNonCyclical(ValidationContext context) { string message=stringEmpty; if (!CyclicInheritanceAddRule TestValidInheritance( thisSubclass, thisSuperclass, ref message)) { contextLogError(message, "Err01", this); } } }
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Note again that this method reuses the same validation logic test, so only infrastructure has to be added We now have a designer that does not allow cycles to be created via the properties window, does not allow them to be created via the toolbox, and provides a nice experience to prevent it It also
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