检索存储在特性中的信息

  • 项目

检索自定义属性的过程非常简单。 首先，声明要检索的属性实例。 然后，使用 Attribute.GetCustomAttribute 方法，用要检索的属性的值初始化新属性。 在初始化新特性后，可使用它的属性来获取值。

重要

本文描述如何检索加载到执行上下文中的代码的特性。 若要为仅反射上下文中加载的代码检索属性，必须使用 CustomAttributeData 类，如如何：将程序集加载到仅反射上下文中所述。

此部分介绍了如何通过以下方式检索属性：

检索一个属性实例

在下面的示例中，DeveloperAttribute（如上一部分所述）在类一级适用于 MainApp 类。 GetAttribute 方法使用 GetCustomAttribute 在类级别检索 DeveloperAttribute 中存储的值，再在控制台中显示它们。

using namespace System;
using namespace System::Reflection;
using namespace CustomCodeAttributes;

[Developer("Joan Smith", "42", Reviewed = true)]
ref class MainApp
{
public:
    static void Main()
    {
        // Call function to get and display the attribute.
        GetAttribute(MainApp::typeid);
    }

    static void GetAttribute(Type^ t)
    {
        // Get instance of the attribute.
        DeveloperAttribute^ MyAttribute =
            (DeveloperAttribute^) Attribute::GetCustomAttribute(t, DeveloperAttribute::typeid);

        if (MyAttribute == nullptr)
        {
            Console::WriteLine("The attribute was not found.");
        }
        else
        {
            // Get the Name value.
            Console::WriteLine("The Name Attribute is: {0}." , MyAttribute->Name);
            // Get the Level value.
            Console::WriteLine("The Level Attribute is: {0}." , MyAttribute->Level);
            // Get the Reviewed value.
            Console::WriteLine("The Reviewed Attribute is: {0}." , MyAttribute->Reviewed);
        }
    }
};

using System;
using System.Reflection;
using CustomCodeAttributes;

[Developer("Joan Smith", "42", Reviewed = true)]
class MainApp
{
    public static void Main()
    {
        // Call function to get and display the attribute.
        GetAttribute(typeof(MainApp));
    }

    public static void GetAttribute(Type t)
    {
        // Get instance of the attribute.
        DeveloperAttribute MyAttribute =
            (DeveloperAttribute) Attribute.GetCustomAttribute(t, typeof (DeveloperAttribute));

        if (MyAttribute == null)
        {
            Console.WriteLine("The attribute was not found.");
        }
        else
        {
            // Get the Name value.
            Console.WriteLine("The Name Attribute is: {0}." , MyAttribute.Name);
            // Get the Level value.
            Console.WriteLine("The Level Attribute is: {0}." , MyAttribute.Level);
            // Get the Reviewed value.
            Console.WriteLine("The Reviewed Attribute is: {0}." , MyAttribute.Reviewed);
        }
    }
}

Imports System.Reflection
Imports CustomCodeAttributes

<Developer("Joan Smith", "42", Reviewed:=True)>
Class MainApp
    Public Shared Sub Main()
        ' Call function to get and display the attribute.
        GetAttribute(GetType(MainApp))
    End Sub

    Public Shared Sub GetAttribute(t As Type)
        ' Get instance of the attribute.
        Dim MyAttribute As DeveloperAttribute =
            CType(Attribute.GetCustomAttribute(t, GetType(DeveloperAttribute)), DeveloperAttribute)

        If MyAttribute Is Nothing Then
            Console.WriteLine("The attribute was not found.")
        Else
            ' Get the Name value.
            Console.WriteLine("The Name Attribute is: {0}.", MyAttribute.Name)
            ' Get the Level value.
            Console.WriteLine("The Level Attribute is: {0}.", MyAttribute.Level)
            ' Get the Reviewed value.
            Console.WriteLine("The Reviewed Attribute is: {0}.", MyAttribute.Reviewed)
        End If
    End Sub
End Class

执行前面的程序会显示以下文本：

The Name Attribute is: Joan Smith.  
The Level Attribute is: 42.  
The Reviewed Attribute is: True.

如果找不到特性，GetCustomAttribute 方法会将 MyAttribute 初始化为 null 值。 此示例在 MyAttribute 中查找此类实例，并在找不到特性时通知用户。 如果在类范围中找不到 DeveloperAttribute，控制台会显示以下消息：

The attribute was not found.

上述示例假定特性定义位于当前的命名空间中。 请注意，如果特性定义不在当前的命名空间中，请导入特性定义所在的命名空间。

检索应用于同一范围的多个属性实例

在上一示例中，要检查的类和要查找的特定特性都传递给 GetCustomAttribute 方法。 此代码非常适用于只有一个属性实例在类一级应用的情况。 不过，如果在相同类级别应用一个特性的多个实例，GetCustomAttribute 方法不会检索所有信息。 如果同一特性的多个实例应用于相同范围，可以使用 Attribute.GetCustomAttributes 方法将特性的所有实例都放入一个数组中。 例如，如果在相同的类一级应用两个 DeveloperAttribute 实例，可以将 GetAttribute 方法修改为显示在这两个属性中找到的信息。 请记住，在同一级别应用多个特性。 必须在 AttributeUsageAttribute 类中将 AllowMultiple 属性设置为 true 来定义该特性。

下面的代码示例展示了如何使用 GetCustomAttributes 方法来创建数组，以引用任何给定类中的所有 DeveloperAttribute 实例。 然后，代码会将所有特性的值输出到控制台。

public:
    static void GetAttribute(Type^ t)
    {
        array<DeveloperAttribute^>^ MyAttributes =
            (array<DeveloperAttribute^>^) Attribute::GetCustomAttributes(t, DeveloperAttribute::typeid);

        if (MyAttributes->Length == 0)
        {
            Console::WriteLine("The attribute was not found.");
        }
        else
        {
            for (int i = 0 ; i < MyAttributes->Length; i++)
            {
                // Get the Name value.
                Console::WriteLine("The Name Attribute is: {0}." , MyAttributes[i]->Name);
                // Get the Level value.
                Console::WriteLine("The Level Attribute is: {0}." , MyAttributes[i]->Level);
                // Get the Reviewed value.
                Console::WriteLine("The Reviewed Attribute is: {0}.", MyAttributes[i]->Reviewed);
            }
        }
    }

public static void GetAttribute(Type t)
{
    DeveloperAttribute[] MyAttributes =
        (DeveloperAttribute[]) Attribute.GetCustomAttributes(t, typeof (DeveloperAttribute));

    if (MyAttributes.Length == 0)
    {
        Console.WriteLine("The attribute was not found.");
    }
    else
    {
        for (int i = 0 ; i < MyAttributes.Length ; i++)
        {
            // Get the Name value.
            Console.WriteLine("The Name Attribute is: {0}." , MyAttributes[i].Name);
            // Get the Level value.
            Console.WriteLine("The Level Attribute is: {0}." , MyAttributes[i].Level);
            // Get the Reviewed value.
            Console.WriteLine("The Reviewed Attribute is: {0}.", MyAttributes[i].Reviewed);
        }
    }
}

Public Shared Sub GetAttribute(t As Type)
    Dim MyAttributes() As DeveloperAttribute =
        CType(Attribute.GetCustomAttributes(t, GetType(DeveloperAttribute)), DeveloperAttribute())

    If MyAttributes.Length = 0 Then
        Console.WriteLine("The attribute was not found.")
    Else
        For i As Integer = 0 To MyAttributes.Length - 1
            ' Get the Name value.
            Console.WriteLine("The Name Attribute is: {0}.", MyAttributes(i).Name)
            ' Get the Level value.
            Console.WriteLine("The Level Attribute is: {0}.", MyAttributes(i).Level)
            ' Get the Reviewed value.
            Console.WriteLine("The Reviewed Attribute is: {0}.", MyAttributes(i).Reviewed)
        Next i
    End If
End Sub

如果找不到任何属性，此代码会向用户发出警报。 如果找到，就会显示两个 DeveloperAttribute 实例中包含的信息。

检索应用于不同范围的多个属性实例

GetCustomAttributes 和 GetCustomAttribute 方法不会搜索整个类，也不会返回该类中特性的所有实例。 而是一次只搜索一个指定方法或成员。 如果你有一个类将同一特性应用于每个成员，并且你需要检索应用于这些成员的所有特性的值，则必须将各个方法或成员单独提供给 GetCustomAttributes 和 GetCustomAttribute。

下面的代码示例将一个类用作参数，并在类级别以及该类的每一个方法上搜索 DeveloperAttribute（如前面所定义）：

public:
    static void GetAttribute(Type^ t)
    {
        DeveloperAttribute^ att;

        // Get the class-level attributes.

        // Put the instance of the attribute on the class level in the att object.
        att = (DeveloperAttribute^) Attribute::GetCustomAttribute (t, DeveloperAttribute::typeid);

        if (att == nullptr)
        {
            Console::WriteLine("No attribute in class {0}.\n", t->ToString());
        }
        else
        {
            Console::WriteLine("The Name Attribute on the class level is: {0}.", att->Name);
            Console::WriteLine("The Level Attribute on the class level is: {0}.", att->Level);
            Console::WriteLine("The Reviewed Attribute on the class level is: {0}.\n", att->Reviewed);
        }

        // Get the method-level attributes.

        // Get all methods in this class, and put them
        // in an array of System.Reflection.MemberInfo objects.
        array<MemberInfo^>^ MyMemberInfo = t->GetMethods();

        // Loop through all methods in this class that are in the
        // MyMemberInfo array.
        for (int i = 0; i < MyMemberInfo->Length; i++)
        {
            att = (DeveloperAttribute^) Attribute::GetCustomAttribute(MyMemberInfo[i], DeveloperAttribute::typeid);
            if (att == nullptr)
            {
                Console::WriteLine("No attribute in member function {0}.\n" , MyMemberInfo[i]->ToString());
            }
            else
            {
                Console::WriteLine("The Name Attribute for the {0} member is: {1}.",
                    MyMemberInfo[i]->ToString(), att->Name);
                Console::WriteLine("The Level Attribute for the {0} member is: {1}.",
                    MyMemberInfo[i]->ToString(), att->Level);
                Console::WriteLine("The Reviewed Attribute for the {0} member is: {1}.\n",
                    MyMemberInfo[i]->ToString(), att->Reviewed);
            }
        }
    }

public static void GetAttribute(Type t)
{
    DeveloperAttribute att;

    // Get the class-level attributes.

    // Put the instance of the attribute on the class level in the att object.
    att = (DeveloperAttribute) Attribute.GetCustomAttribute (t, typeof (DeveloperAttribute));

    if (att == null)
    {
        Console.WriteLine("No attribute in class {0}.\n", t.ToString());
    }
    else
    {
        Console.WriteLine("The Name Attribute on the class level is: {0}.", att.Name);
        Console.WriteLine("The Level Attribute on the class level is: {0}.", att.Level);
        Console.WriteLine("The Reviewed Attribute on the class level is: {0}.\n", att.Reviewed);
    }

    // Get the method-level attributes.

    // Get all methods in this class, and put them
    // in an array of System.Reflection.MemberInfo objects.
    MemberInfo[] MyMemberInfo = t.GetMethods();

    // Loop through all methods in this class that are in the
    // MyMemberInfo array.
    for (int i = 0; i < MyMemberInfo.Length; i++)
    {
        att = (DeveloperAttribute) Attribute.GetCustomAttribute(MyMemberInfo[i], typeof (DeveloperAttribute));
        if (att == null)
        {
            Console.WriteLine("No attribute in member function {0}.\n" , MyMemberInfo[i].ToString());
        }
        else
        {
            Console.WriteLine("The Name Attribute for the {0} member is: {1}.",
                MyMemberInfo[i].ToString(), att.Name);
            Console.WriteLine("The Level Attribute for the {0} member is: {1}.",
                MyMemberInfo[i].ToString(), att.Level);
            Console.WriteLine("The Reviewed Attribute for the {0} member is: {1}.\n",
                MyMemberInfo[i].ToString(), att.Reviewed);
        }
    }
}

Public Shared Sub GetAttribute(t As Type)
    Dim att As DeveloperAttribute

    ' Get the class-level attributes.

    ' Put the instance of the attribute on the class level in the att object.
    att = CType(Attribute.GetCustomAttribute(t, GetType(DeveloperAttribute)), DeveloperAttribute)

    If att Is Nothing
        Console.WriteLine("No attribute in class {0}.\n", t.ToString())
    Else
        Console.WriteLine("The Name Attribute on the class level is: {0}.", att.Name)
        Console.WriteLine("The Level Attribute on the class level is: {0}.", att.Level)
        Console.WriteLine("The Reviewed Attribute on the class level is: {0}.\n", att.Reviewed)
    End If

    ' Get the method-level attributes.

    ' Get all methods in this class, and put them
    ' in an array of System.Reflection.MemberInfo objects.
    Dim MyMemberInfo() As MemberInfo = t.GetMethods()

    ' Loop through all methods in this class that are in the
    ' MyMemberInfo array.
    For i As Integer = 0 To MyMemberInfo.Length - 1
        att = CType(Attribute.GetCustomAttribute(MyMemberInfo(i), _
            GetType(DeveloperAttribute)), DeveloperAttribute)
        If att Is Nothing Then
            Console.WriteLine("No attribute in member function {0}.\n", MyMemberInfo(i).ToString())
        Else
            Console.WriteLine("The Name Attribute for the {0} member is: {1}.",
                MyMemberInfo(i).ToString(), att.Name)
            Console.WriteLine("The Level Attribute for the {0} member is: {1}.",
                MyMemberInfo(i).ToString(), att.Level)
            Console.WriteLine("The Reviewed Attribute for the {0} member is: {1}.\n",
                MyMemberInfo(i).ToString(), att.Reviewed)
        End If
    Next
End Sub

如果在方法级别或类级别找不到 DeveloperAttribute 实例，GetAttribute 方法会通知用户找不到特性，并显示不包含该特性的方法名称或类名称。 如果找到特性，控制台会显示 Name、Level 和 Reviewed 字段。

可以使用 Type 类的成员，在传递的类中获取各个方法和成员。 此示例先查询 Type 对象，以获取类级别的特性信息。 接下来，它使用 Type.GetMethods 将所有方法实例都放入 System.Reflection.MemberInfo 对象数组，以检索方法一级的属性信息。 还可以使用 Type.GetProperties 方法检查属性一级的属性，或使用 Type.GetConstructors方法检查构造函数一级的属性。

请参阅