Custom Attributes in C#: Adding Metadata to Your Code

Learn about custom attributes in C#—a powerful way to add metadata to your code elements (classes, methods, properties). This guide explains how to create and apply custom attributes, illustrating their use in extending C#'s capabilities and influencing code behavior.

Custom Attributes in C#

Custom attributes in C# let you add metadata (extra information) to your code. This metadata isn't directly executed but can be used by the compiler, runtime environment, or other tools to modify how your code behaves or is processed. They're a powerful way to extend C#'s capabilities.

What are Custom Attributes?

Custom attributes are annotations you add to your code (classes, methods, properties, etc.). They provide extra information about those elements that isn't directly part of the code itself. Think of them as tags that add context and meaning. This metadata can influence various aspects of your code's behavior, including code generation, validation, and runtime operations.

Creating Custom Attributes

To create a custom attribute, you define a class that inherits from the `System.Attribute` class. You can then add properties and other members to this class to store the specific metadata you want to associate with your code elements.

[AttributeTargets(AttributeTargets.Method)] //Specify where it can be used
public class MyCustomAttribute : Attribute {
    public string Description { get; set; }
}

The `AttributeUsage` attribute specifies where this custom attribute can be applied (in this case, only to methods).

Applying Custom Attributes

You apply custom attributes using square brackets `[]` before the element you want to annotate:

[MyCustomAttribute(Description = "This is my method.")]
public void MyMethod() { }

Common Uses of Custom Attributes

• Code Analysis: Provide information for code analysis tools (e.g., the `Obsolete` attribute marks a method as deprecated).
• Code Generation: Guide code generation tools (e.g., the `DataContract` and `DataMember` attributes for data serialization).
• Runtime Behavior: Influence runtime behavior (e.g., the `Conditional` attribute controls whether code executes based on build configurations).
• Reflection: Retrieve metadata about types and members at runtime (e.g., for building dynamic systems or plugin architectures).

Example: A Custom `Deprecated` Attribute

[AttributeUsage(AttributeTargets.Method)]
public class DeprecatedAttribute : Attribute {
    public string Reason { get; set; }
}

[DeprecatedAttribute(Reason = "Use NewMethod instead.")]
public void OldMethod() { }