擷取儲存於屬性中的資訊

發行項
05/22/2023

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
擷取套用至相同範圍的多個屬性執行個體
擷取套用至不同範圍的多個屬性執行個體

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。

擷取儲存於屬性中的資訊

擷取單一屬性執行個體

擷取套用至相同範圍的多個屬性執行個體

擷取套用至不同範圍的多個屬性執行個體

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