如何通过反射调用方法？

如何调用方法？

本示例阐释如何通过反射调用各种方法。由于所调用方法的名称存储在字符串中，因此该机制提供在运行时（而不是在设计时）指定要调用的方法的功能，提供了使您的用户可以控制调用哪个特定方法的余地。尽管本演示集中于调用方法，如果需要您还可以设置和获取属性和字段。有关本主题的另一个实例示教，请参阅如何使用数学函数主题下的示例。

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在许多代码方案中，在执行任务以前您知道要实现的任务。因此，您可以指定需要调用的方法以及需要传递给它们的参数。但是，还有一些情况下您可能希望根据特定方案或用户操作动态调用方法。该功能可通过 Reflection 命名空间使用，方法是使用 Type 对象上的 InvokeMember 方法。

您还可以进行其他操作，如获取或设置指定属性的值。这些操作可通过 BindingFlags 枚举使用。InvokeMethod 的第二个参数是您指定的 BindingFlags 操作的组合。例如，如果想调用某个类上的静态方法，可以在 BindingFlags 和 InvokeMethod BindingFlag 中包括该静态元素。下面的示例展示如何调用名为 SayHello 的假想方法，其中 SayHello 是静态方法。

// calling a static method, receiving no arguments // don't forget that we are using object in the reflection namespace... using System; using System.Reflection; public class Invoke { public static void Main (String [] cmdargs) { // Declare a type object, used to call our InvokeMember method... Type t = typeof (TestClass); // BindingFlags has three bitor'ed elements. Default indicates // that default binding rules should be applied. t.InvokeMember ("SayHello", BindingFlags.Default | BindingFlags.InvokeMethod | BindingFlags.Static, null, null, new object [] {}); } } ' calling a static method, receiving no arguments ' don't forget that we are using object in the reflection namespace... Imports System Imports System.Reflection Public Class Invoke Public Shared Sub Main () ' Declare a type object, used to call our InvokeMember method... Dim t As Type = GetType (TestClass) ' BindingFlags has three bitor'ed elements. Default indicates that ' default binding rules should be applied. t.InvokeMember ("SayHello", _ BindingFlags.Default BitOr BindingFlags.InvokeMethod _ BitOr BindingFlags.Static, nothing, _ nothing, new object () {}) End Sub End Class
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快速查看一下传递给 Invoke 方法的其余参数。传递的第一个空参数请求使用默认联编程序绑定正在调用的方法。当调用默认联编程序时，请包含默认的 BindingFlags。第三个参数可以不为空，您可以指定一个 Binder 对象，它定义一组属性并启用绑定，这可能涉及选择重载方法或强制参数类型。第二个空参数是您在其上调用所选方法的对象。最后，传递由成员接收的参数对象数组。在本例中，SayHello 方法不接收任何参数，因此传递一个空数组。

下面的情况略有不同。调用名为 ComputeSum 的另一个静态方法，但是在此情况下，此方法需要两个参数。因此，用这些参数填充一个对象数组，并将它们作为最后一个参数传递到 InvokeMember 中。

// Calling a static method, which needs arguments object [] args = new object [] {100.09, 184.45}; // we know that this particular method returns a value, being the computed sum, // so we create a variable to hold the return // note the datatype of the return is object, the only datatype InvokeMethod returns... object result; // invoke the method. Note the change in the last parameter: the array we populated... result = t.InvokeMember ("ComputeSum", BindingFlags.Default | _ BindingFlags.InvokeMethod | BindingFlags.Static, null, null, args); // write the results to the user's console... Console.WriteLine ("{0} + {1} = {2}", args[0], args[1], result); ' Calling a static method, which needs arguments Dim args as object () args = new object () {100.09, 184.45} ' we know that this particular method returns a value, being the computed sum, ' so we create a variable to hold the return ' note the datatype of the return is object, the only datatype InvokeMethod returns... Dim result As object ' invoke the method. Note the change in the last parameter: the array we populated... result = t.InvokeMember ("ComputeSum", BindingFlags.Default BitOr _ BindingFlags.InvokeMethod BitOr BindingFlags.Static, _ nothing, nothing, args) ' write the results to the user's console... Console.WriteLine ("{0} + {1} = {2}", args(0), args(1), result)
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在前两个示例中，已调用了静态方法。还可以调用实例方法。若要这样做，将您要在其上调用方法的类型的对象作为第三个参数传递。本示例还展示为了使用 InvokeMember，您不必有实际的 Type 对象。在此情况下，通常将希望使用所拥有的类实例来调用 GetType，如下面的示例所示。注意由于未调用静态方法，所以 BindingFlags 已更改。

// Calling an instance method // we need an object reference to invoke an instance member TestClass c = new TestClass (); // use the instance of our class to call GetType // we no longer include the Static element in BindingFlags for our | // the fourth parameter is no longer null: we instead pass an instance // of the object we wish to invoke our method on c.GetType().InvokeMember ("AddUp", BindingFlags.Default | BindingFlags.InvokeMethod, null, c, new object [] {}); c.GetType().InvokeMember ("AddUp", BindingFlags.Default | BindingFlags.InvokeMethod, null, c, new object [] {}); ' Calling an instance method ' we need an object reference to invoke an instance member Dim c as TestClass c = new TestClass () ' use the instance of our class to call GetType ' we no longer include the Static element in BindingFlags for our bitor ' the fourth parameter is no longer null: we instead pass an instance ' of the object we wish to invoke our method on c.GetType().InvokeMember ("AddUp", BindingFlags.Default BitOr BindingFlags.InvokeMethod, _ nothing, c, new object () {}) c.GetType().InvokeMember ("AddUp", BindingFlags.Default BitOr BindingFlags.InvokeMethod, _ nothing, c, new object () {})
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有时不想调用方法，而需要调用其他成员，如属性或字段。若要实现它，只需更改 BindingFlags 组合（而不是 InvokeMethod）以包含适当元素即可。下面的示例展示获取和设置字段值。所讨论字段不是静态字段，因此需要创建一个对象实例来请求该字段。设置字段值时，需要将所设置的值作为对象数组参数的唯一元素传递。获取值时，需要将 InvokeMember 方法的返回类型分配给一个对象。

// Setting a field. Assume we are using the same Type and Class declared in the // previous examples (t and c). The field we are setting is the Name field // note the BindingFlags argument now includes SetField rather thanInvokeMember // Further, this is an instance field, so we pass the instance of our class t.InvokeMember ("Name", BindingFlags.Default | BindingFlags.SetField, null, c, new object [] {"NewName"}); // similar usage... result = t.InvokeMember ("Name", BindingFlags.Default | BindingFlags.GetField, null, c, new object [] {}); Console.WriteLine ("Name == {0}", result); ' Setting a field. Assume we are using the same Type and Class declared in the ' previous examples (t and c). The field we are setting is the Name field ' note the BindingFlags argument now includes SetField rather thanInvokeMember ' Further, this is an instance field, so we pass the instance of our class t.InvokeMember ("Name", BindingFlags.Default BitOr BindingFlags.SetField, _ nothing, c, new object () {"NewName"}) ' similar usage... result = t.InvokeMember ("Name", BindingFlags.Default BitOr BindingFlags.GetField, _ nothing, c, new object () {}) Console.WriteLine ("Name == {0}", result)
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还可以获取和设置属性，但在本示例中，假定所设置属性是一个具有多个元素的数组或集合。若要指定特定元素的设置，您需要指定索引。若要设置属性，请分配 BindingFlags.SetProperty。若要指定属性的集合索引或数组索引，请将要设置元素的索引值放在对象数组的第一个元素中，然后将要设置的值作为第二个元素。若要取回该属性，请将索引作为对象数组中的唯一元素传递，指定 BindingFlags.GetProperty。

// Set an indexed property value int index = 3; // specify BindingFlags.SetProperty, and because this is an instance property, // pass the object to call the property on (c). In the object array, make two elements, // the first being the index, and the second being the value to set t.InvokeMember ("Item", BindingFlags.Default |BindingFlags.SetProperty, null, c, new object [] {index, "NewValue"}); // Get an indexed property value // specify BindingFlags.GetProperty, and because this is an instance property, // pass the object to call the property on (c). In the object array, specify the index only result = t.InvokeMember ("Item", BindingFlags.Default |BindingFlags.GetProperty, null, c, new object [] {index}); Console.WriteLine ("Item[{0}] == {1}", index, result); ' Set an indexed property value Dim index As Int32 = 3 ' specify BindingFlags.SetProperty, and because this is an instance property, ' pass the object to call the property on (c). In the object array, make two elements, ' the first being the index, and the second being the value to set t.InvokeMember ("Item", BindingFlags.Default BitOr BindingFlags.SetProperty, _ nothing, c, new object () {index, "NewValue"}) ' Get an indexed property value ' specify BindingFlags.GetProperty, and because this is an instance property, ' pass the object to call the property on (c). In the object array, specify the index only result = t.InvokeMember ("Item", BindingFlags.Default BitOr BindingFlags.GetProperty, _ nothing, c, new object () {index}) Console.WriteLine ("Item[{0}] == {1}", index, result)
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还可以使用命名参数，在此情况下需要使用 InvokeMember 方法的另一个重载版本。像迄今一直进行的那样创建对象参数的数组，并创建所传递参数的名称的字符串数组。您要使用的重载方法接受参数名列表作为最后一个参数，并接受要设置的值的列表作为第五个参数。在本演示中，所有其他参数都可以为空（当然前两个除外）。

// Calling a method using named arguments // the argument array, and the parameter name array. Obviously, you will need // to determine the names of the parameters in advance object[] argValues = new object [] {"Mouse", "Micky"}; String [] argNames = new String [] {"lastName", "firstName"}; // the first five parameters for this overloaded method are the same as the // the five parameters we have used to this point. The final parameter needs to be // set to the names of the parameters t.InvokeMember ("PrintName", BindingFlags.Default | BindingFlags.InvokeMethod, null, null, argValues, null, null, argNames); ' Calling a method using named arguments ' the argument array, and the parameter name array. Obviously, you will need ' to determine the names of the parameters in advance object[] argValues = new object [] {"Mouse", "Micky"}; String [] argNames = new String [] {"lastName", "firstName"}; ' the first five parameters for this overloaded method are the same as the ' the five parameters we have used to this point. The final parameter needs to be ' set to the names of the parameters t.InvokeMember ("PrintName", BindingFlags.Default BitOr BindingFlags.InvokeMethod, _ nothing, nothing, argValues, nothing, nothing, argNames)
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下一个示例展示如何调用类上的默认成员。确保在其上进行调用的类指定有默认成员。然后在 InvokeMember 方法中，不要指定要调用成员的名称，如本示例所示。

// our class with it's default member specified, using the defaultmemeber attribute [DefaultMemberAttribute ("PrintTime")] public class TestClass2 { public void PrintTime () { Console.WriteLine (DateTime.Now); } } // the client code that uses the above class... Type t3 = typeof (TestClass2); t3.InvokeMember ("", BindingFlags.Default |BindingFlags.InvokeMethod, null, new TestClass2(), new object [] {}); ' our class with it's default member specified, using the defaultmemeber attribute public class TestClass2 public Sub PrintTime () Console.WriteLine (DateTime.Now) End Sub End Class ' the client code that uses the above class... Dim t3 As Type t3 = GetType (TestClass2) t3.InvokeMember ("", BindingFlags.Default BitOr BindingFlags.InvokeMethod, _ nothing, new TestClass2(), new object () {})
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最后一个示例使用略有不同的过程调用方法。不直接使用 Type 对象，而是直接创建一个单独的 MethodInfo 对象来表示将调用的方法。然后调用 MethodInfo 对象上的 Invoke 方法，传递需要在其上调用方法的对象的实例（在要调用实例方法的情况下，但是，如果方法是静态的，则为空）。像以前一样，需要参数的对象数组。如果需要，该特定示例允许您通过引用传递参数。

// Invoking a ByRef member MethodInfo m = t.GetMethod("Swap"); args = new object[2]; args[0] = 1; args[1] = 2; m.Invoke(new TestClass(),args); Console.WriteLine ("{0}, {1}", args[0], args[1]); ' Invoking a ByRef member Dim m as MethodInfo = m = t.GetMethod("Swap") args = new object() {CObj(1), CObj(2)} m.Invoke(new TestClass(),args) Console.WriteLine ("{0}, {1}", args(0), args(1))
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