| title | description | ms.date | ms.custom | dev_langs | helpviewer_keywords | ms.topic | ||||||
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Retrieving Information Stored in Attributes |
Learn to retrieve information stored in attributes, such as for an attribute instance, many instances for the same scope, & many instances for different scopes. |
08/05/2022 |
devdivchpfy22 |
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how-to |
Retrieving a custom attribute is a simple process. First, declare an instance of the attribute you want to retrieve. Then, use the xref:System.Attribute.GetCustomAttribute%2A?displayProperty=nameWithType method to initialize the new attribute to the value of the attribute you want to retrieve. Once the new attribute is initialized, you can use its properties to get the values.
Important
This article describes how to retrieve attributes for code loaded into the execution context. To retrieve attributes for code loaded into the reflection-only context, you must use the xref:System.Reflection.CustomAttributeData class, as shown in How to: Load Assemblies into the Reflection-Only Context.
This section describes the following ways to retrieve attributes:
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Retrieving multiple instances of an attribute applied to the same scope
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Retrieving multiple instances of an attribute applied to different scopes
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.
[!code-cppConceptual.Attributes.Usage#18] [!code-csharpConceptual.Attributes.Usage#18] [!code-vbConceptual.Attributes.Usage#18]
The execution of the preceding program displays the following text:
The Name Attribute is: Joan Smith.
The Level Attribute is: 42.
The Reviewed Attribute is: True. If the attribute isn't found, the GetCustomAttribute method initializes MyAttribute to a null value. This example checks MyAttribute for such an instance and notifies the user if the attribute isn't found. If DeveloperAttribute isn't found in the class scope, the console displays the following message:
The attribute was not found.The preceding example assumes that the attribute definition is in the current namespace. Remember to import the namespace in which the attribute definition resides if it isn't in the current namespace.
In the preceding example, the class to inspect and the specific attribute to find are passed to the xref:System.Attribute.GetCustomAttribute%2A method. 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 doesn't retrieve all the information. In cases where multiple instances of the same attribute are applied to the same scope, you can use xref:System.Attribute.GetCustomAttributes%2A?displayProperty=nameWithType method 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 xref:System.AttributeUsageAttribute class.
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 code then outputs the values of all the attributes to the console.
[!code-cppConceptual.Attributes.Usage#19] [!code-csharpConceptual.Attributes.Usage#19] [!code-vbConceptual.Attributes.Usage#19]
If no attributes are found, this code alerts the user. Otherwise, the information contained in both instances of DeveloperAttribute is displayed.
The xref:System.Attribute.GetCustomAttributes%2A and xref:System.Attribute.GetCustomAttribute%2A methods don't 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:
[!code-cppConceptual.Attributes.Usage#20] [!code-csharpConceptual.Attributes.Usage#20] [!code-vbConceptual.Attributes.Usage#20]
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 doesn't contain the attribute. If an attribute is found, the console displays the Name, Level, and Reviewed fields.
You can use the members of the xref:System.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 xref:System.Type.GetMethods%2A?displayProperty=nameWithType to place instances of all methods into an array of xref:System.Reflection.MemberInfo?displayProperty=nameWithType objects to retrieve attribute information for the method level. You can also use the xref:System.Type.GetProperties%2A?displayProperty=nameWithType method to check for attributes on the property level or xref:System.Type.GetConstructors%2A?displayProperty=nameWithType to check for attributes on the constructor level.
- xref:System.Type?displayProperty=nameWithType
- xref:System.Attribute.GetCustomAttribute%2A?displayProperty=nameWithType
- xref:System.Attribute.GetCustomAttributes%2A?displayProperty=nameWithType
- Attributes