检索存储在属性中的信息
检索自定义属性是一个简单的过程。 首先，声明要检索的属性的实例。 然后，使用 Attribute.GetCustomAttribute 方法将新属性初始化为要检索的属性的值。 初始化新属性后，可以使用其属性获取值。
重要
本文介绍如何检索加载到执行上下文中的代码的属性。 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。
本部分介绍检索属性的以下方法：
检索属性 的单个实例
检索应用于同一作用域 的属性的多个实例
检索属性的单个实例
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 检索类级别上 DeveloperAttribute 中存储的值，然后再将其显示到控制台。
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
上述程序的执行显示以下文本：
The Name Attribute is: Joan Smith.
The Level Attribute is: 42.
The Reviewed Attribute is: True.
如果未找到该属性，GetCustomAttribute 方法将 MyAttribute 初始化为 null 值。 此示例检查 MyAttribute 此类实例，并在找不到该属性时通知用户。 如果在类范围中找不到 DeveloperAttribute，控制台将显示以下消息：
The attribute was not found.
前面的示例假定属性定义位于当前命名空间中。 请记住导入属性定义所在的命名空间（如果它不在当前命名空间中）。
检索应用于同一作用域的属性的多个实例
在前面的示例中，要检查的类和要查找的特定属性将传递给 GetCustomAttribute 方法。 如果类级别上仅应用了一个属性的一个实例，则此代码有效。 但是，如果在同一类级别应用了属性的多个实例，则 GetCustomAttribute 方法不会检索所有信息。 如果同一属性的多个实例应用于同一范围，可以使用 Attribute.GetCustomAttributes 方法将所有属性实例放入数组中。 例如，如果在同一类的类级别应用了两个 DeveloperAttribute 实例，则可以修改 GetAttribute 方法以显示这两个属性中找到的信息。 请记住，在同一级别应用多个属性。 属性必须在 AttributeUsageAttribute 类中定义，其 AllowMultiple 属性设置为 true。
下面的代码示例演示如何使用 GetCustomAttributes 方法创建一个数组，该数组引用任何给定类中的所有 DeveloperAttribute 实例。 然后，代码会将所有属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
如果未找到任何属性，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。
检索应用于不同作用域的属性的多个实例
GetCustomAttributes 和 GetCustomAttribute 方法不会搜索整个类并返回该类中属性的所有实例。 相反，它们一次只搜索一个指定的方法或成员。 如果你有一个类，其中相同的属性应用于每个成员，并且你想要检索应用于那些成员的所有属性中的值，那么必须单独将每个方法或成员提供给 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
如果未在方法级别或类级别找到 DeveloperAttribute 实例，GetAttribute 方法会通知用户找不到任何属性，并显示不包含该属性的方法或类的名称。 如果找到属性，控制台将显示 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。 此示例首先查询 Type 对象以获取类级别的属性信息。 接下来，它使用 Type.GetMethods 将所有方法的实例放入 System.Reflection.MemberInfo 对象的数组中，以检索方法级别的属性信息。 你还可以使用 Type.GetProperties 方法来检查属性级别的特性，或使用 Type.GetConstructors 方法检查构造函数级别的特性。
从类成员检索属性
除了在类级别检索属性外，还可以将属性应用于各个成员，例如方法、属性和字段。 GetCustomAttribute 和 GetCustomAttributes 方法可用于检索这些属性。
例
以下示例演示如何检索应用于方法的属性：
using System;
using System.Reflection;
[AttributeUsage(AttributeTargets.Method)]
public class MyAttribute : Attribute
{
public string Description { get; }
public MyAttribute(string description) { Description = description; }
}
public class MyClass
{
[MyAttribute("This is a sample method.")]
public void MyMethod() { }
}
class AttributeRetrieval
{
public static void Main()
{
// Create an instance of MyClass
MyClass myClass = new MyClass();
// Retrieve the method information for MyMethod
MethodInfo methodInfo = typeof(MyClass).GetMethod("MyMethod");
MyAttribute attribute = (MyAttribute)Attribute.GetCustomAttribute(methodInfo, typeof(MyAttribute));
if (attribute != null)
{
// Print the description of the method attribute
Console.WriteLine("Method Attribute: {0}", attribute.Description);
}
else
{
Console.WriteLine("Attribute not found.");
}
}
}
