Understanding and Using .NET Attributes

[FileIOPermission(System.Security.Permissions.SecurityAction.Demand, Read= @"C:helloworld.txt")]
public class Form1 : System.Windows.Forms.Form
{
//...
}

Creating Custom Attributes

Custom attributes are classes that are used to provide additional descriptive information about properties, methods, etc. used in your classes. The information from the attribute can be obtained programmatically using our friend Reflection.

A custom attribute begins with the AttributeUsage attribute, which defines the way your attribute class can be used. Since a custom attribute uses an attribute it provides one example of how attributes can be used just by creating a custom attribute. There are three parameters that can be applied to your custom attributes. The AttributeTargets defines where the attribute can be used, for example, properties, methods, classes, etc. The Inherited parameter defines whether or not classes that are derived from classes that use your attribute inherit your attribute as well. The AllowMultiple parameter defines whether or not multiple instances of your parameter can exist on an item.

Here is an example of a custom attribute created to map an object property to a configuration file entry. The attribute consists of a FieldName that contains the name of the configuration file element that stores the setting.

/// 
/// Attribute used to indicate if a property should be configured
/// by the Configurator object. It is valid only on Properties and
/// only one attribute is allowed per property.
/// 
[AttributeUsage(AttributeTargets.Property, AllowMultiple = false)]
public class ConfiguratorFieldAttribute : Attribute
{
/// Name of the data field
public string FieldName { get; set; }
/// 
/// Constructor
/// 
/// Name of the data field
public ConfiguratorFieldAttribute(string FieldName)
{
this._FieldName = FieldName;
}
}

Reading Attributes with Reflection

One of the common ways that attributes are used at runtime is through reflection. The following sample code will demonstrate using reflection to read all of the attributes assigned to properties of an instance of an object. The attribute that will be read is based on the ConfiguratorFieldAttribute we defined above. The following code snippet defines our test object and shows the properties decorated with our custom attribute:

public class TestObject
{
[ConfiguratorField("Parameter1")]
public string Property1 { get; set; }
[ConfiguratorField("Parameter2")]
public string Property2 { get; set; }
[ConfiguratorField("Parameter3")]
public string Property3 { get; set; }
}

The following code snippet demonstrates a method that would take an instance of our TestObject or other object and traverse the properties for those with our custom attribute. When found it will call the method to set the properties value based on some answer retrieved from the GetSetting method.

public void ConfigureObject(object ObjectToConfig)
{
ConfiguratorFieldAttribute attr; // Custom ConfigurationFieldAttribute
object[] attributes; // Attributes assigned to the object
PropertyInfo property; // Object property
// List of object properties
PropertyInfo[] itemTypeProps = ObjectToConfig.GetType().GetProperties();
// Loop through all of the properties of the object
for( int i = 0; i < itemTypeProps.Length; i++ )
{
// Get the current ConfiguratorField attribute
property = itemTypeProps [i];
attributes = property.GetCustomAttributes(typeof(ConfiguratorFieldAttribute), true);
// Verify the ConfiguratorField attribute was found
if( attributes != null && attributes.Length == 1 )
{
// Set the value
attr = (ConfiguratorFieldAttribute) attributes[0];
if( property != null )
{
property.SetValue(ObjectToConfig, this.GetSetting(attr.FieldName), null);
}
}
}
}

About the Author:

Mark Strawmyer is a Senior Architect of .NET applications for large and mid-size organizations. Mark is a technology leader with Crowe Horwath LLP in Indianapolis, Indiana. He specializes in architecture, design and development of Microsoft-based solutions. Mark was honored to be named a Microsoft MVP for application development with C# again. You can reach Mark through http://markstrawmyer.com.