Figure 7-3: Validation messages displayed in the Visual Studio Error List Window in Visual C#.NET

Drawer QR Code JIS X 0510 in Visual C#.NET Figure 7-3: Validation messages displayed in the Visual Studio Error List Window
Figure 7-3: Validation messages displayed in the Visual Studio Error List Window
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When the end user of the designer double-clicks on these messages, the selection in the designer is updated to re ect the ModelElements passed to the LogError() method, and the designer attempts to navigate its UI to make these elements visible
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Using Validation outside the IDE It is very typical to want to use validation in a custom tool developed using the DSL Tools infrastructure For example, in a command-line tool to transform model les, you might want to ensure that a model is well-formed before embarking on processing it Happily, using validation in custom tools is very straightforward with the DSL Tools, although depending on your requirements you may need to write slightly more custom code than you do with validation in a designer The good news is that unless you have speci cally hard-coded a dependency on Visual Studio, all of your validation methods should simply work both inside and outside of the IDE However, you will have to set up the
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7: Constraints and Validation
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validation infrastructure of a ValidationController, associated ValidationContext and ValidationObservers yourself Typically, you will simply instantiate and use the ValidationController base class This will provide a basic ValidationContext, which creates simple ValidationMessage instances via its LogError() and related methods If, however, you need to pass richer information of some kind to your custom ValidationObserver when errors are raised, you ll need a custom ValidationMessage type and a way to construct it Because the ValidationContext is a factory for ValidationMessages, a custom ValidationContext provides this facility In turn, a ValidationController is a factory for ValidationContext objects, so you ll also need a custom ValidationController This is, in fact, what the VsValidationController that is used by default in designers does The ErrorListObserver class requires extra information about the lename where the error was reported, so the controller creates a custom VsValidationContext to handle the creation of TaskValidationMessage objects that carry the extra data
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Validation Against External Data Although the validation framework in the DSL Tools is tied to validating sets of ModelElement-derived objects as its subjects, it is also sometimes desirable to cross-validate these elements against external data sources For example, in the IssueStateModel designer, the connection to the external database is speci ed with the ServerName and DatabaseName properties on the IssueStateModel class The following validation method checks that the connection is valid and available
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[ValidationMethod(ValidationCategorySave | ValidationCategoryMenu)] protected void ValidateDbConnection(ValidationContext context) { bool ok = false; try { // IssueDBConnection is a specialized class // for talking to issue databases using (IssueDbConnection connection = new IssueDbConnection(this, thisServerName, thisDatabaseName))
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Hard Constraints in the DSL Tools
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{ if (connectionConnection != null) ok = true; } } catch (SystemDataSqlClientSqlException) { } if (!ok) { string description = StringFormat(CultureInfoCurrentCulture, "Failed to connect to database {0}/{1}", thisServerName, thisDatabaseName); contextLogError(description, "Err 04", this); } }
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This type of validation can be very valuable, especially before an operation that relies on external data, such as an import operation However, it can also be troublesome Here, the method is specified as running on load, save, and menu, which means that in order to save a model le when the database is unavailable, the user must wait for the network timeout of the database connection In general, this type of validation is often best used from a custom command whose usage the end user expects to be coupled to the availability of the external data
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Hard Constraints in the DSL Tools
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As we ve seen by example, hard constraints are required less often than soft constraints However, when author-speci ed hard constraints are required, they can be added by adding custom code to your designer Because hard constraints are sometimes intimately connected to the user experience that would cause them to be broken, there is not a single standardized method to add them Changes that would invalidate a hard constraint can often be disallowed at an API level by throwing an exception when some value changes in a model We ll look at how to use one such change-handling mechanism, rules, shortly Another store change-handling mechanism is to override the OnXxxChanging() method in the value property handler nested class for a given domain property For example, if the string property Name on a domain class NamedElement can never have an empty value, the following code might be employed:
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