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Retrieving Information Stored in Attributes

  • Articolo

Retrieving a custom attribute is a simple process. First, declare an instance of the attribute you want to retrieve. Then, use the Attribute.GetCustomAttribute method to initialize the new attribute to the value of the attribute you want to retrieve. Once the new attribute is initialized, you simply use its properties to get the values.

This section describes the following ways to retrieve attributes:

  • Retrieving a single instance of an attribute
  • Retrieving multiple instances of an attribute applied to the same scope
  • Retrieving multiple instances of an attribute applied to different scopes

Retrieving a Single Instance of an Attribute

In the following example, the DeveloperAttribute (described in the previous section) is applied to the MainApp class on the class level. The GetAttribute method uses GetCustomAttribute to retrieve the values stored in DeveloperAttribute on the class level before displaying them to the console.

using System;

[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(null == MyAttribute)
            {
                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);
            }
    }
}
[Visual Basic]Imports System

<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

This program displays the following text when executed.

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

If the attribute is not found, the GetCustomAttribute method initializes MyAttribute to a null value. This example checks MyAttribute for such an instance and notifies the user if no attribute is found. If the DeveloperAttribute is not found in the class scope, the following message displays to the console.

The attribute was not found.

This example assumes that the attribute definition is in the current namespace. Remember to import the namespace in which the attribute definition resides if it is not in the current namespace.

Retrieving Multiple Instances of an Attribute Applied to the Same Scope

In the previous example, the class to inspect and the specific attribute to find are passed to GetCustomAttribute. That code works well if only one instance of an attribute is applied on the class level. However, if multiple instances of an attribute are applied on the same class level, the GetCustomAttribute method does not retrieve all the information. In cases where multiple instances of the same attribute are applied to the same scope, you can use Attribute.GetCustomAttributes to place all instances of an attribute into an array. For example, if two instances of DeveloperAttribute are applied on the class level of the same class, the GetAttribute method can be modified to display the information found in both attributes. Remember, to apply multiple attributes on the same level, the attribute must be defined with the AllowMultiple property set to true in the AttributeUsageAttribute.

The following code example shows how to use the GetCustomAttributes method to create an array that references all instances of DeveloperAttribute in any given class. The values of all attributes are then displayed to the console.

public static void GetAttribute(Type t)
{
    DeveloperAttribute[] MyAttribute =
    (DeveloperAttribute[]) Attribute.GetCustomAttributes(t, typeof (DeveloperAttribute));
    
    if(null == MyAttribute)
    {
        Console.WriteLine("The attribute was not found.");
    }
    else
    {
        for(int i = 0 ; i < MyAttribute.Length ; i++)
        {
            //Get the Name value.    
            Console.WriteLine("The Name Attribute is: {0}." , MyAttribute[i].Name);
            //Get the Level value.  
            Console.WriteLine("The Level Attribute is: {0}." , MyAttribute[i].Level);
            //Get the Reviewed value.
            Console.WriteLine("The Reviewed Attribute is: {0}.", MyAttribute[i].Reviewed);
        }
    }
}
[Visual Basic]Public Shared Sub GetAttribute(t As Type)
    Dim MyAttribute As DeveloperAttribute() = _
    CType(Attribute.GetCustomAttributes(t, GetType(DeveloperAttribute)), DeveloperAttribute())
       
    If MyAttribute Is Nothing Then
        Console.WriteLine("The attribute was not found.")
    Else
        Dim i As Integer
        For i = 0 To MyAttribute.Length - 1
            'Get the Name value. 
            Console.WriteLine("The Name Attribute is: {0}.", MyAttribute(i).Name)
            'Get the Level value.   
            Console.WriteLine("The Level Attribute is: {0}.", MyAttribute(i).Level)
            'Get the Reviewed value.
            Console.WriteLine("The Reviewed Attribute is: {0}.", MyAttribute(i).Reviewed)
        Next i
    End If
End Sub

If no attributes are found, this code alerts the user. Otherwise, the information contained in both instances of DeveloperAttribute is displayed.

Retrieving Multiple Instances of an Attribute Applied to Different Scopes

The GetCustomAttributes and GetCustomAttribute methods do not search an entire class and return all instances of an attribute in that class. Rather, they search only one specified method or member at a time. If you have a class with the same attribute applied to every member and you want to retrieve the values in all the attributes applied to those members, you must supply every method or member individually to GetCustomAttributes and GetCustomAttribute.

The following code example takes a class as a parameter and searches for the DeveloperAttribute (defined previously) on the class level and on every individual method of that class.

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(null == att)
    {
        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(null == att)
        {
            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);
        }        
    }
}
[Visual Basic]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 Then
        Console.WriteLine("No attribute in class {0}.", t.ToString())
        Console.WriteLine()
    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}.", att.Reviewed)
        Console.WriteLine()
    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.
    Dim i As Integer
    For i = 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}.", MyMemberInfo(i).ToString())
            Console.WriteLine()
        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}.", MyMemberInfo(i).ToString(), att.Reviewed)
            Console.WriteLine()
        End If
    Next i
End Sub

If no instances of the DeveloperAttribute are found on the method level or class level, the GetAttribute method notifies the user that no attributes were found and displays the name of the method or class that does not contain the attribute. If an attribute is found, the Name, Level, and Reviewed fields are displayed to the console.

You can use the members of the Type class to get the individual methods and members in the passed class. This example first queries the Type object to get attribute information for the class level. Next, it uses Type.GetMethods to place instances of all methods into an array of System.Reflection.MemberInfo objects to retrieve attribute information for the method level. You can also use the Type.GetProperties method to check for attributes on the property level or Type.GetConstructors to check for attributes on the constructor level.

See Also

Extending Metadata Using Attributes | System.Type | Attribute.GetCustomAttribute | Attribute.GetCustomAttributes