MSDN Magazine - January 2008 - (Page 34)

Figure 2 Testing for Generic Type Definition Dim Dim Dim Dim type1 type2 type3 type4 As As As As Type = GetType(Dictionary(Of ,)) New Dictionary(Of String, DateTime) Type = type2.GetType() Type = type3.GetGenericTypeDefinition() AddToResults(“type1 is generic type definition: {0}”, _ type1.IsGenericTypeDefinition) AddToResults(“type3 is generic type definition: {0}”, _ type3.IsGenericTypeDefinition) AddToResults(“type4 is generic type definition: {0}”, _ type4.IsGenericTypeDefinition) AddToResults() AddToResults(“type1 is type4: {0}”, type1 Is type4) a generic type, it appends the number of generic parameters. That’s why the Dictionary class appears as Dictionary`2. You can repeat the same sort of experimentation for methods. When it comes time to execute a method dynamically, you’ll need to know if the method is generic so you can construct a version of it in which you’ve satisfied all its type placeholders. A method is considered generic if it includes two sets of type arIf you want to create an guments: one that describes instance of a type, you’ll the method as a whole, and the other that supplies the need to know if it’s a method’s parameters. In generic type definition or a GenericClass, for example, constructed generic type. you’ll find two overloaded versions of the Swap method. The first one isn’t generic—its parameter types are defined by the class’s type placeholder. The second one is generic, however. In this case, the compiler infers the type of the placeholder from the type of the first parameter to the procedure. (If you make an attempt to call the procedure, passing in two different types, the compiler will complain.) The TestForGenericMethod procedure uses the MethodInfo.IsGenericMethod property to determine if a method is generic and calls the following code to test the property: Dim typ As Type = GetType(GenericClass(Of Integer)) For Each method As MethodInfo In typ.GetMethods( _ BindingFlags.Public Or BindingFlags.Instance _ Or BindingFlags.DeclaredOnly) AddToResults(method.Name & “ is generic: {0}”, _ method.IsGenericMethod) Next neric type definition has one or more of its type placeholders unsatisfied—in other words, a generic type definition isn’t aware of its parameter types. A constructed generic type (as you’ve seen in the previous examples) has its type placeholders replaced with specific types. You can’t create an instance of a non-constructed generic type—you must supply its types before you attempt to create the instance. Use the Type.IsGenericTypeDefinition property to determine the status of the type, as in the code from the TestForGenericTypeDefinition procedure shown in Figure 2. You’ve already seen how to retrieve a Type object corresponding to a constructed generic type. You can also do the same with a generic type definition, as you saw in the first line of Figure 2 (make sure you include a comma separator between each of the required type placeholders): Dim type1 As Type = GetType(Dictionary(Of ,)) Examining Methods Given a constructed type, you can use the Type.GetGenericTypeDefinition method to retrieve the generic type’s definition. The following code creates a constructed instance of the generic Dictionary class, retrieves its type, and then retrieves the generic type definition of the constructed generic type: Dim type2 As New Dictionary(Of String, DateTime) Dim type3 As Type = type2.GetType() Dim type4 As Type = type3.GetGenericTypeDefinition() Finally, the procedure compares the references in type4 and type1; they should be the exact same reference (that is, the generic type definition of the Dictionary instance). The procedure’s output looks like the following, indicating that type1 and type4 are the same generic type definition: type1 is generic type definition: True type3 is generic type definition: False type4 is generic type definition: True type1 is type4: True Type3, a constructed generic type, isn’t a generic type definition. You can use a similar technique to determine if the type you’re inspecting is a standard type, a generic type, or a generic type definition. The following illustrates the MethodInfo.IsGenericMethod and MethodInfo.IsGenericMethodDefinition properties: Dim type1 As Type = GetType(GenericClass(Of )) Dim method As MethodInfo = type1.GetMethod(“GenericMethod”) If method.IsGenericMethodDefinition Then Dim miConstructed As MethodInfo = _ method.MakeGenericMethod(GetType(Integer)) AddToResults(“Is generic method definition: {0}”, _ miConstructed.IsGenericMethodDefinition) AddToResults(“Is generic method: {0}”, _ miConstructed.IsGenericMethod) End If This code uses the BindingFlags enumeration in its call to the Type.GetMethods method, so that it only retrieves public instance methods that are declared directly in the class. The output looks like this: Swap is generic: True GenericMethod is generic: True Swap is generic: False Creating an Instance of a Type If you want to create an instance of a type, you’ll need to know if it’s a generic type definition or a constructed generic type. A ge34 msdnmagazine Advanced Basics Every generic method definition is also a generic method, but the converse is not true; every generic method is not a generic method definition. If you’ve supplied the type placeholder values for a method, it’s a generic method but not a generic method definition. This sample also demonstrates the MethodInfo.MakeGenericMethod method, which allows you to take a generic method definition and supply types for its type placeholders, thereby constructing a generic method from the definition. The sample procedure displays the following output: Is generic method definition: False Is generic method: True

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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