擷取儲存在屬性中的資訊
擷取自定義屬性是一個簡單的過程。 首先，宣告您要擷取的屬性實例。 然後，使用 Attribute.GetCustomAttribute 方法，將新屬性初始化為您想要擷取的屬性值。 初始化新屬性之後，您可以使用其屬性來取得值。
重要
本文說明如何擷取載入執行內容之程式代碼的屬性。 若要擷取載入僅供反射用之內容的程式碼的屬性，您必須使用 CustomAttributeData 類別，如 所述：如何將組件載入 Reflection-Only 上下文。
本節說明下列擷取屬性的方式：
擷取單一屬性實例
在下列範例中，DeveloperAttribute（如上一節所述）會套用至類別層級上的 MainApp 類別。
GetAttribute 方法會使用 GetCustomAttribute，先擷取類別層級上 DeveloperAttribute 中儲存的值，再向控制台顯示這些值。
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
此程式執行會顯示下列文字：
The Name Attribute is: Joan Smith.
The Level Attribute is: 42.
The Reviewed Attribute is: True.
如果找不到屬性，GetCustomAttribute 方法會將 MyAttribute 初始化為 null 值。 本範例會檢查 MyAttribute 這類實例，並在找不到屬性時通知使用者。 如果在類別範圍中找不到 DeveloperAttribute，主控台會顯示下列訊息：
The attribute was not found.
上述範例假設屬性定義位於目前的命名空間中。 請記得匯入屬性定義所在的命名空間，如果它不在目前的命名空間中。
擷取套用至相同範圍之屬性的多個實例
在上述範例中，要檢查的 類別，以及要尋找的特定屬性會傳遞至 GetCustomAttribute 方法。 如果類別層級上只套用了一個屬性的實例，該程式代碼會正常運作。 不過，如果屬性的多個實例套用在相同的類別層級，則 GetCustomAttribute 方法不會擷取所有資訊。 如果相同屬性的多個實例套用至相同的範圍，您可以使用 Attribute.GetCustomAttributes 方法將所有屬性的實例放入陣列中。 例如，如果在相同類別的類別層級套用兩個 DeveloperAttribute 實例，則可以修改 GetAttribute 方法以顯示這兩個屬性中找到的資訊。 請記住，要在相同的層級上套用多個屬性。 屬性必須在 AttributeUsageAttribute 類別中，將 AllowMultiple 屬性設定為 true 來定義。
下列程式代碼範例示範如何使用 GetCustomAttributes 方法來建立數位列，參考任何指定類別中 DeveloperAttribute 的所有實例。 然後，程式代碼會將所有屬性的值輸出至主控台。
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
如果找不到任何屬性，此程式代碼會警示使用者。 否則，會顯示這兩個實例 DeveloperAttribute 中包含的資訊。
擷取套用至不同範圍之屬性的多個實例
GetCustomAttributes 和 GetCustomAttribute 方法不會搜尋整個類別，並傳回該類別中屬性的所有實例。 相反地，它們一次只搜尋一個指定的方法或成員。 如果您有一個應用在每個成員的相同屬性的類別，並且您想要擷取這些成員所有屬性的值，那麼必須個別將每個方法或成員提供給 GetCustomAttributes 和 GetCustomAttribute。
下列程式代碼範例會採用 類別做為參數，並在類別層級和該類別的每個個別方法上搜尋 DeveloperAttribute （先前定義） ：
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
如果在方法層級或類別層級上找不到 DeveloperAttribute 實例，GetAttribute 方法會通知使用者找不到任何屬性，並顯示不包含屬性的方法或類別名稱。 如果找到屬性，主控台會顯示 Name、Level和 Reviewed 字段。
您可以使用 Type 類別的成員來取得被傳遞類別中的單獨方法和成員。 此範例會先查詢 Type 物件，以取得類別層級的屬性資訊。 接下來，它會使用 Type.GetMethods 將所有方法的實例放入 System.Reflection.MemberInfo 對象的陣列，以擷取方法層級的屬性資訊。 您也可以使用 Type.GetProperties 方法來檢查屬性層級的特性，或使用 Type.GetConstructors 來檢查建構函式層級的特性。
從類別成員擷取屬性
除了在類別層級擷取屬性之外，屬性也可以套用至個別成員，例如方法、屬性和字段。
GetCustomAttribute 和 GetCustomAttributes 方法可用來擷取這些屬性。
例
下列範例示範如何擷取套用至 方法的屬性：
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.");
}
}
}
