擷取儲存於屬性中的資訊
擷取自訂屬性是一個簡單的程序。 首先，對想要擷取的屬性宣告執行個體。 然後，使用 Attribute.GetCustomAttribute 方法將新屬性初始化為所要擷取之屬性的值。 新屬性 (attribute) 初始化之後，您就能使用其屬性 (poperty) 取得值。
重要
本文說明如何為載入到執行內容的程式碼擷取屬性。 若要針對載入僅限反映的內容中的程式碼擷取屬性，您必須使用 CustomAttributeData 類別，如如何：將組件載入僅限反映的內容中所示。
本節說明下列擷取屬性的方式：
擷取單一屬性執行個體
在下列範例中，DeveloperAttribute (上節所述) 會在類別層級上套用至 MainApp 類別。 GetAttribute 方法會先從類別層級使用 GetCustomAttribute 來擷取儲存在 DeveloperAttribute 的值，再顯示於主控台。
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
執行上述程式後，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。 此範例會檢查 MyAttribute 是否有此類執行個體，並在找不到屬性時通知使用者。 如果在 DeveloperAttribute 類別範圍找不到 ，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在目前的命名空間中。 如果屬性定義不在目前的命名空間中，請記得匯入屬性定義所在的命名空間。
擷取套用至相同範圍的多個屬性執行個體
在上述範例中，要檢查的類別和要尋找的特定屬性都會傳遞至 GetCustomAttribute 方法。 如果類別層級上只套用一個屬性執行個體，該程式碼可以運作良好。 不過，如果在相同的類別層級上套用多個屬性執行個體，GetCustomAttribute 方法不會擷取所有資訊。 在相同屬性的多個執行個體都套用至相同範圍的情況下，您可以使用 Attribute.GetCustomAttributes 方法將屬性的所有執行個體放入陣列。 例如，如果在相同類別的類別層級上套用 DeveloperAttribute 的兩個執行個體，可修改 GetAttribute 方法以顯示在兩個屬性中找到的資訊。 請務必將多個屬性套用在相同的層級。 定義屬性時，必須在 AttributeUsageAttribute 類別中將 AllowMultiple 屬性設為 true。
下列程式碼範例示範如何使用 GetCustomAttributes 方法來建立陣列，讓它參考任何指定類別中 DeveloperAttribute 的所有執行個體。 該程式碼接著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，此程式碼會警示使用者。 否則，DeveloperAttribute 的兩個執行個體中所包含的資訊隨即顯示。
擷取套用至不同範圍的多個屬性執行個體
GetCustomAttributes 和 GetCustomAttribute 方法不會搜尋整個類別，然後傳回該類別中屬性的所有執行個體。 而會一次只搜尋一個指定的方法或成員。 如果您的類別中每個成員都套用相同屬性，且您想要從套用至這些成員的所有屬性中擷取值，就必須將每個方法或成員個別提供給 GetCustomAttributes 和 GetCustomAttribute。
下列程式碼範例將類別視為參數，並在類別層級和該類別的每一個方法搜尋 DeveloperAttribute (先前所定義)：
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
如果方法層級或類別層級找不到任何 DeveloperAttribute 的執行個體，GetAttribute 方法會向使用者通知找不到任何屬性，並顯示不包含該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 欄位。
您可以使用 Type 類別的成員來取得所傳遞類別中的個別方法和成員。 此範例會先查詢 Type 物件以取得類別層級的屬性資訊。 接著，它會使用 Type.GetMethods 以將所有方法的執行個體置入 System.Reflection.MemberInfo 物件的陣列，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上的屬性，或使用 Type.GetConstructors 來檢查建構函式層級上的屬性。
