MSDN Magazine - January 2008 - (Page 40)

To retrieve information about variance and other special constraint types, you must work with two separate bit masks. For variance constraints, you retrieve the type’s GenericParameterAttributes property. Then you must apply the GenericParameterAttributes.VarianceMask bit mask to the returned value. Compare the result to the GenericParameterAttributes.Covariant or Contravariant enumerated value to determine if the type argument has a variance constraint. To retrieve information about the special constraints, apply the GenericParameterAttributes.SpecialConstraintMask bit mask. This time, compare the results against the ReferenceTypeConstraint, NotNullableValueTypeConstraint, and the DefaultConstructorConstraint enumerated values to determine the status of each special constraint. The ExamineGenericParameterAttributes method does all this work for you, using the IsBitSet helper procedure to determine if a particular bit has been set: Private Function IsBitSet( _ ByVal value As Integer, ByVal bitValue As Integer) As Boolean Return (value And bitValue) = bitValue End Function Figure 8 Display Constraint Info Private Sub DisplayConstraintInfo(ByVal typ As Type) AddToResults(“{0}”, typ) Dim indent As String = vbTab Dim params() As Type = typ.GetGenericArguments() For Each paramType As Type In params ‘ Handle special constraints: ExamineGenericParameterAttributes(paramType) ‘ Loop through type constraints: Dim constraints As Type() = _ paramType.GetGenericParameterConstraints() For Each constraintType As Type In constraints AddToResults(“{0}{1}”, indent, constraintType) Next constraintType AddToResults() Next End Sub Figure 9 Sample Classes Public Class TestClass1( _ Of T As {IComparable, IDisposable, Class, New}) ‘ T must implement IComparable and IDisposable. ‘ T must be a reference type. ‘ T must supply a default constructor. End Class Public Class TestClass2(Of T As {NonGenericClass}) ‘ T must inherit from NonGenericClass. End Class Public Class TestClass3(Of T As {Structure}) ‘ T must be a value type. End Class The ExamineGenericParameterAttributes procedure includes the code in Figure 7, which follows the preceding steps. The sample DisplayConstraintInfo procedure includes the code in Figure 8, which displays parameter constraint information for the type it’s handed. As mentioned earliGeneric type argument er, this procedure loops constraints enforce rules on through all the parameters and, for each, first type arguments, ensuring calls the ExamineGethat generic types adhere to nericParameterAttributes application requirements. procedure (handling variance and other special constraints) and then loops through all the types that resulted from calling the GetGenericParameterConstraints method. The ExamineConstraints sample procedure is shown here: DisplayConstraintInto(GetType(TestClass1(Of ))) DisplayConstraintInto(GetType(TestClass2(Of ))) DisplayConstraintInto(GetType(TestClass3(Of ))) and generics. All you need to do now is download the sample application, follow through all the code and output, read the associated documentation topics, and you should be ready to tackle any problems related to reflection and generics. It examines the behavior of the DisplayConstraintInfo procedure for three different classes, all of which have different argument constraints. The sample classes look like Figure 9. Code download available at msdn.microsoft.com/msdnmag/code08.aspx. Send your questions and comments for Ken to basics@microsoft.com. Ken Getz is a Senior Consultant with MCW Technologies and a courseware author for AppDev (www.appdev.com). He is coauthor of ASP .NET Developers Jumpstart (Addison-Wesley, 2002), Access Developer’s Handbook (Sybex, 2002), and VBA Developer’s Handbook, 2nd Edition (Sybex, 2001). Reach him at keng@mcwtech.com. Your Turn At this point, you’ve seen just about every method and property exposed by the .NET Framework at the intersection of reflection 40 msdnmagazine Advanced Basics http://msdn.microsoft.com/msdnmag/code08.aspx http://www.appdev.com

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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