MSDN Magazine - January 2008 - (Page 36)

Figure 3 DisplayGenericTypeInfo Procedure Private Sub DisplayGenericTypeInfo(ByVal t As Type) AddToResults(“{0}”, t) Dim indent As String = vbTab AddToResults(“{0}Generic type definition: {1}”, _ indent, t.IsGenericTypeDefinition) AddToResults(“{0}Generic type: {1} “, _ indent, t.IsGenericType) AddToResults(“{0}Unassigned generic parameters: {1}”, _ indent, t.ContainsGenericParameters) Dim args() As Type = t.GetGenericArguments() AddToResults(“{0}List type arguments ({1}):”, _ indent, args.Length) For Each argType As Type In args AddToResults(“{0}{0}{1}”, indent, argType) Next AddToResults(“{0}{1}”, indent, _ IIf(t.ContainsGenericParameters, “OPEN”, “CLOSED”)) End Sub generic parameters and can’t be instantiated). The code in Figure 3 then calls the GetGenericArguments method to retrieve a list of all the generic arguments and displays each generic argument in the output. Finally, the code uses the ContainsGenericParameters property to indicate whether the type is open or closed. (Note that you can apply the same sort of logic to MethodInfo instances in order to determine if a particular MethodInfo represents an open or closed generic method.) The TestForOpenOrClosedGeneric method includes the following code, which tests several of the sample methods, both as generic definitions and as constructed types. See if you can predict the output for each: Dim baseType As Type = GetType(BaseClass(Of ,)) DisplayGenericTypeInfo(baseType) Dim derivedType As Type = GetType(DerivedClass(Of )) DisplayGenericTypeInfo(derivedType) DisplayGenericTypeInfo(derivedType.BaseType) Dim constructedType As Type = GetType(DerivedClass(Of String)) DisplayGenericTypeInfo(constructedType) DisplayGenericTypeInfo(constructedType.BaseType) Figure 4 Comparing Constructed Types Dim type1 As Type = GetType(BaseClass(Of ,)) DisplayGenericTypeInfo(type1) ‘ Create an array of types to substitute for the type ‘ parameters. Dim args() As Type = {GetType(String), GetType(DateTime)} ‘ Create a Type object representing the constructed generic ‘ type. Dim constructed As Type = type1.MakeGenericType(args) DisplayGenericTypeInfo(constructed) Dim type2 As Type = GetType(BaseClass(Of String, DateTime)) AddToResults(“Constructed types equal: {0}”, _ constructed Is type2) AddToResults(“Generic types equal: {0}”, _ type2.GetGenericTypeDefinition Is type1) Figure 5 CallGenericMethodGivenGenericMethodDefinition Dim type1 As Type = GetType(GenericClass(Of Integer)) Dim mi As MethodInfo = type1.GetMethod(“GenericMethod”) DisplayGenericMethodInfo(mi) Dim argTypes() As Type = {GetType(String)} Dim miConstructed As MethodInfo = mi.MakeGenericMethod(argTypes) DisplayGenericMethodInfo(miConstructed) ‘ Invoke the method, creating an instance of the ‘ generic class first. Dim host As New GenericClass(Of Integer) ‘ Supply the method parameters, and then invoke the method: Dim args() As Object = {“test value”} AddToResults(“{0}”, miConstructed.Invoke(host, args)) AddToResults() ‘ Construct another version of the same ‘ generic method, this time taking an integer ‘ as the parameter: argTypes(0) = GetType(Integer) miConstructed = mi.MakeGenericMethod(argTypes) DisplayGenericMethodInfo(miConstructed) ‘ Supply the method parameters, and then invoke the method: args(0) = 13 AddToResults(“{0}”, miConstructed.Invoke(host, args)) The CreateGenericTypeGivenGenericTypeDefinition sample procedure shows how you can create a closed generic type given a generic type definition. You’ve already seen this technique applied to a method definition: just as you called the MethodInfo.MakeGenericMethod method to construct a generic method from its definition, you can call the Type.MakeGenericType method to construct a generic type from its definition. The sample code creates a constructed type at run time and compares it to a constructed type it created at design time, as you can see in Figure 4. In order to call a generic method given its generic method definition, you must call the MethodInfo.MakeGenericMethod method, passing an array containing Type objects corresponding to each of the method’s type placeholders. Given the new constructed method, you can call the MethodInfo.Invoke method, supplying a host object instance and an array of parameters for the method. Of course, the type of the parameters must correctly match the type of the placeholder type you specified when you constructed the generic method instance. The CallGenericMethodGivenGenericMethodDefinition sample procedure in Figure 5 demonstrates this technique. This sample starts by retrieving a MethodInfo instance corresponding to the GenericMethod method in GenericClass. This method accepts a single type placeholder and simply returns a string containing the value you supply at run time: Public Function GenericMethod(Of M)(ByVal item1 As M) As String Return String.Format(“You passed in: {0}”, item1) End Function The code does its work by constructing a specific generic class, supplying a type for the class’s type placeholder: Dim type1 As Type = GetType(GenericClass(Of Integer)) Dim mi As MethodInfo = type1.GetMethod(“GenericMethod”) DisplayGenericMethodInfo(mi) As you learned earlier, you won’t be able to call a generic method if there are any unsatisfied type placeholders in its ancestry (that is, if it’s not a closed generic method). To prove that point, run the procedure once as is and then modify it so that you specify a ge- 36 msdnmagazine Advanced Basics

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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