Custom Attributes in .NET

Custom attributes in .NET (Core) are a helpful mechanism to attach additional information to classes, structs, and even their members. In this article, we’re going to explain how to create, access, and get the information from custom attributes in .NET through some practical examples.

Let’s start.

Declaring Custom Attributes

We can define an attribute by creating a class. This class should inherit from the Attribute class.

Microsoft recommends appending the ‘Attribute’ suffix to the end of the class’s name. After that, each property of our derived class will be a parameter of the desired data type.

Customizing Custom Attribute Usage

The AttributeUsageAttribute class specifies the usage of another attribute class by defining some of the fundamental features.

This class has three members:

• AttributeTargets enum
• Inherited property (bool)
• AllowMultiple property (bool)

AttributeTargets Enum

The AttributeTargets enum specifies application elements we can apply our custom attribute to. 

To see how it works, let’s create a new class called TaskDescriptorAttribute:

[AttributeUsage(AttributeTargets.Class | AttributeTargets.Struct)]
public class TaskDescriptorAttribute : Attribute
{
    public string? Name { get; set; }
    public string? Description { get; set; }
    public bool NeedsManager { get; set; }
    public int DeveloperCount { get; set; }
}

We can apply this attribute (TaskDescriptorAttribute) just to the classes and structures because we have set its target to both using a bitwise combination. Of course, other than classes and structs, we can use methods, enums, and other application elements when creating attributes. Those attributes also apply to all of the application elements if we use AttributeTargets.All value (it’s the default).

Let’s use our attribute with the MyTasks class:

[TaskDescriptor(Name = "The task's name",
    Description = "Some descriptions for the task",
    NeedsManager = true,
    DeveloperCount = 5)]
public class MyTasks
{
}

When we apply our TaskDescriptorAttribute to a class, we just use the TaskDescriptor part because the compiler lets us use it without the ‘Attribute’ suffix.

AllowMultiple Property

The AllowMultiple property permits multiple instances of our attribute. This property can be either false (the default) or true.

Let’s create another attribute called DeveloperTaskAttribute:

[AttributeUsage(AttributeTargets.Method, AllowMultiple = true)]
public class DeveloperTaskAttribute : Attribute
{
    public Priorities Priority { get; set; }
    public string? Description { get; set; }
    public DeveloperTaskAttribute(Priorities priority)
    {
        Priority = priority;
    }
}

We can use this attribute only on methods. And we can apply multiple instances of it to them. It has the required Priority and the optional Description parameters.

To apply this attribute, we are going to create a new ScheduleMeeting() method inside the MyTasks class:

public class MyTasks
{
    [DeveloperTask(Priorities.Low)]
    [DeveloperTask(Priorities.High, Description = "High level description")]
    public void ScheduleMeeting()
    {
    }
}

The ScheduleMeeting() method has two DeveloperTask attributes now. We declare the first one with only the required parameter but the second one with both the required and optional parameters. We cannot define a DeveloperTask attribute without a Priorities parameter though, or we’ll get a compiler error.

Inherited Property

The Inherited property is another key feature that we can apply to a custom attribute. It indicates whether that attribute can be inherited. This property has a default value of true.

To see the usage of this property, let’s create the ManagerTaskAttribute attribute:

[AttributeUsage(AttributeTargets.Method, Inherited = false)]
public class ManagerTaskAttribute : Attribute
{
    public Priorities Priority { get; set; }
    public bool NeedsReport { get; set; }
}

This new attribute cannot be inherited, and both of its parameters are optional.

Now, we are going to create the ScheduleInterview method in the MyTasks class to utilize it:

public class MyTasks
{
    [ManagerTask(Priority = Priorities.Mid, NeedsReport = true)]
    [DeveloperTask(Priorities.High, Description = "High level description")]
    public virtual void ScheduleInterview()
    {
    }
}

This method has two attributes, one DeveloperTask and one ManagerTask attribute each. We also add the virtual keyword because we want to override it in another class.

So, let’s create the YourTasks class that inherits from the MyTasks class:

public class YourTasks : MyTasks
{
    [DeveloperTask(Priorities.Mid, Description = "Mid level description")]
    public override void ScheduleInterview()
    {
    }
}

The ScheduleInterview method inside the YourTasks inherited class overrides the previous ScheduleInterview method from the MyTasks base class. This method does not have the ManagerTask attribute because its Inherited property has a value of false.

But, the DeveloperTask attribute has a default Inherited value of true. So, the YourTasks.ScheduleInterview method has two DeveloperTask attributes. We have declared the first one inside the YourTasks class and the second one inside the MyTasks class.

Access an Instance of a Custom Attribute

Once we want to retrieve the values from our attributes, we can use the static GetCustomAttribute method from the Attribute class. So, let’s create the GetAttribute method for acquiring the information stored in the TaskDescriptor instance:

public static string? GetAttribute(Type desiredType, Type desiredAttribute)
{
    var attributeInstance = Attribute.GetCustomAttribute(desiredType, desiredAttribute);

    if (attributeInstance == null)
        Console.WriteLine($"The class {desiredType} does not have atributes.");
    else
        WriteOnTheConsole(attributeInstance);

    return attributeInstance?.ToString();
}

Our GetAttribute method expects a (class) type and an attribute type as input parameters and prints the information on the console.

Inside the Attribute base class, we can find different overloads of the GetCustomAttribute method. For our example, we use the GetCustomAttribute(MemberInfo element, Type attributeType) overload to get our desired information.

The Type inherits from the MemberInfo base class so, we can pass it to the method as the first argument. We also send our custom attribute’s type as the second argument. We print all information on the console using a bit of reflection inside the WriteOnTheConsole() method. (you can check the source code for implementation) 

Creating an Instance of a Custom Attribute

The GetCustomAttribute method returns either an instance of the attribute or a null value. So, the attributeInstance variable stores an instance of our custom attribute if it does exist. Now, we are going to retrieve the information of that instance.

Retrieving the Information of a Custom Attribute

To retrieve the information of our custom attribute, we are going to call the GetAttribute method and pass typeof(MyTasks) and typeof(TaskDescriptorAttribute) as its arguments:

GetAttribute(typeof(MyTasks), typeof(TaskDescriptorAttribute));

If the GetAttribute method finds an instance of the TaskDescriptorAttribute class, we should get all of its properties as a result:

The CustomAttributes.TaskDescriptorAttribute attribute:
The Name property is: The task's name
The Description property is: Some descriptions for the task
The NeedsManager property is: True
The DeveloperCount property is: 5

We retrieved the information of a custom attribute class successfully.

Getting the Instances of Different Custom Attributes

Sometimes, we need to access all attributes of the class’s members. The Attribute base class has another GetCustomAttributes method to return them as an array.

Let’s create the GetAttributesOfMethods method to access instances of all attributes and retrieve their information:

public static List<string> GetAttributesOfMethods(Type elementType)
{
    List<string> attributes = new List<string>();

    var methodInfoList = elementType.GetMethods(BindingFlags.Public |
        BindingFlags.Instance |
        BindingFlags.DeclaredOnly);

    if (methodInfoList == null || methodInfoList.Length == 0)
    {
        Console.WriteLine($"The type {elementType} does not have any methods.");
        return attributes;
    }

    foreach (var methodInfo in methodInfoList)
    {
        var attributeList = Attribute.GetCustomAttributes(methodInfo, true);

        if (attributeList.Length == 0)
        {
            Console.WriteLine($"The {elementType.Name}.{methodInfo.Name} method does not have attributes.");
            continue;
        }

        Console.WriteLine($"The {elementType.Name}.{methodInfo.Name} method's attribute:");

        foreach (var att in attributeList)
        {
            WriteOnTheConsole(att);
            attributes.Add(methodInfo.Name + "-" + att.ToString());
        }

        Console.WriteLine();
    }

    return attributes;
}

We want to obtain all of the attributes of every declared method. So, we call the GetMethods method using appropriated enums.

Inside the first foreach loop, we call the GetCustomAttributes method using the GetCustomAttribute(MemberInfo element, bool inherit) overload and send the fetched MethodInfo values, one by one. We also pass true as the second argument because we need the inherited attributes.

Inside the inner foreach loop, we sequentially fetch items of the attributeList array and print them on the console using the WriteOnTheConsole method.

Now, let’s call the GetAttributesOfMethods method for the MyTasks class:

GetAttributesOfMethods(typeof(MyTasks));

We send the type of the MyTasks class to this method and as a result, we expect to see all the attributes of its methods:

The MyTasks.ScheduleMeeting method's attribute:

The CustomAttributes.DeveloperTaskAttribute attribute:
The Description property is:
The Priority property is: Low

The CustomAttributes.DeveloperTaskAttribute attribute:
The Description property is: High level description
The Priority property is: High

The MyTasks.ScheduleInterview method's attribute:

The CustomAttributes.ManagerTaskAttribute attribute:
The Priority property is: Mid
The NeedsReport property is: True

The CustomAttributes.DeveloperTaskAttribute attribute:
The Description property is: High level description
The Priority property is: High

The GetAttributesOfMethods prints the attributes of ScheduleMeeting and ScheduleInterview methods respectively.

We can do the same for the YourTasks class:

GetAttributesOfMethods(typeof(YourTasks));

And find the results very similar:

The YourTasks.ScheduleInterview method's attribute:

The CustomAttributes.DeveloperTaskAttribute attribute:
The Description property is: Mid level description
The Priority property is: Mid

The CustomAttributes.DeveloperTaskAttribute attribute:
The Description property is: High level description
The Priority property is: High

We don’t see the ManagerTask attribute on the console because this attribute has an Inherited value of false. The reason we see two DeveloperTask attributes is because we’ve declared the first one inside the MyTasks class and the second inside the YourTasks class.

Conclusion

In this article, we have learned how to declare custom attributes in .NET. We’ve seen that we can utilize them for classes and their members. We also have touched upon the access ways to the single and multiple attribute instances. Finally, we have found out how to retrieve their information.