MSDN Magazine - January 2008 - (Page 39)

Figure 6 Displaying Generic Parameter Information AddToResults(“{0}”, t) Dim indent1 As String = vbTab Dim indent2 As String = vbTab & vbTab AddToResults(“{0}Generic type definition: {1}”, _ indent1, t.IsGenericTypeDefinition) AddToResults(“{0}Generic type: {1}”, _ indent1, t.IsGenericType) AddToResults(“{0}Unassigned generic parameters: {1} “, _ indent1, t.ContainsGenericParameters) If t.IsGenericType Then ‘ If a generic type, display the arguments: Dim args As Type() = t.GetGenericArguments() AddToResults(“{0}List type arguments ({1}):”, _ indent1, args.Length) For Each argType As Type In args If argType.IsGenericParameter Then AddToResults(“{0}{1} at position {2}”, _ indent2, argType, argType.GenericParameterPosition) Else AddToResults(“{0}{1}”, indent2, argType) End If Next argType End If check the type’s DeclaringMethod property. If the value isn’t Nothing, it will be a MethodInfo instance, and you will know that the type parameter came from a generic method, and the MethodInfo’s IsGenericMethodDefinition property will return True. If the source isn’t a generic method, the DeclaringMethod property returns Nothing, and you can then retrieve the DeclaringType property. This type will always be a generic type definition. You can only retrieve the DeclaringMethod property if the IsGenericParameter property of the type returns True; otherwise, an exception will be raised. Generic Type Argument Constraints The RetrieveTypeParameterInfo procedure contains within it the following code: Dim type1 As Type = GetType(DerivedClass(Of String)) DisplayGenericParameterInfo(type1) Dim type2 As Type = type1.GetGenericTypeDefinition DisplayGenericParameterInfo(type2) Dim type3 As Type = type2.BaseType DisplayGenericParameterInfo(type3) Finally, you may need to investigate generic type argument constraints. These constraints enforce rules on type arguments, ensuring that generic types adhere to the application requirements. You can add constraints that force a type parameter to inherit from a specific class, implement one or more specific interfaces, be a value type, be a reference type, and/or provide a default constructor. You might want to determine programmatically which constraints, if any, apply to a type argument, so that you can construct a type dynamically that meets the constraint requirements. In order to examine constraints, you must take two sets of steps. In order to retrieve information about a single class or one or more interfaces that a type argument must inherit or implement, you need to call the Type.GetGenericParameterConstraints method. This method returns an array of Type objects, each of which indicates one class or interface. Figure 7 Examine Generic Parameter Attributes ‘ Select the variance flags. Dim variance As GenericParameterAttributes = _ parameterAttributes And _ GenericParameterAttributes.VarianceMask Dim indent As String = vbTab If variance = GenericParameterAttributes.None Then AddToResults(“{0}No variance flag”, indent) Else If IsBitSet(variance, GenericParameterAttributes.Covariant) Then AddToResults(“{0}Variance flag = Covariant”, indent) Else AddToResults(“{0}Variance flag = Contravariant”, indent) End If End If ‘ Select the constraint flags. Dim constraints As GenericParameterAttributes = _ parameterAttributes And _ GenericParameterAttributes.SpecialConstraintMask If constraints = GenericParameterAttributes.None Then AddToResults(“{0}No special constraints”, indent) Else If IsBitSet(constraints, _ GenericParameterAttributes.ReferenceTypeConstraint) Then AddToResults(“{0}ReferenceTypeConstraint”, indent) End If If IsBitSet(constraints, _ GenericParameterAttributes.NotNullableValueTypeConstraint) Then AddToResults(“{0}NotNullableValueTypeConstraint”, indent) End If If IsBitSet(constraints, _ GenericParameterAttributes.DefaultConstructorConstraint) Then AddToResults(“{0}DefaultConstructorConstraint”, indent) End If End If What this code displays is the parameter information for a constructed generic type, for the corresponding generic type definition, and for the base type of the generic type definition (the BaseClass sample class). DisplayGenericParameterInfo does most of the work in this sample and includes the code in Figure 6. After displaying the type name, the code displays indications of whether the type is a generic type definition, whether it’s a generic type, and whether it contains generic parameters. If it is a generic type, the code retrieves the array of generic arguments: Dim args As Type() = t.GetGenericArguments() Then, for each Type instance in the array, the code determines if it has a generic parameter (or a Type, from a constructed type). If so, it displays the GenericParameterPosition property value; if not, it displays the type name. (Retrieving the GenericParameterPosition property for types that aren’t generic parameters will trigger a runtime exception.) Determining the Source of a Generic Parameter If some code hands you a generic parameter, you may need to determine if it was for a generic type or a generic method. It might come from a type you are examining, from an enclosing type, or from a generic method. How can you determine the source? First, Advanced Basics january2008 39

Table of Contents Feed for the Digital Edition of MSDN Magazine - January 2008

MSDN Magazine - January 2008 Contents Toolbox CLR Inside Out Data Points Advanced Basics Cutting Edge IIS 7.0 - Enhance Your Apps with the Integrated ASP.NET Pipeline World Ready - Around the World with ASP.NET AJAX Applications WCF Syndication - HTTP Programming with WCF and the .NET Framework 3.5 Wicked Code Foundations Extreme ASP.NET {End Bracket}

MSDN Magazine - January 2008

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