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What is reflection, and what basic information can you retrieve from a Type object at runtime? How do you create a custom attribute class, and what does AttributeUsage control? How do you use reflection to read a custom attribute's values off a property at runtime? How do you dynamically create an instance of a type and invoke a method on it using reflection, when the type isn't known at compile time? What is reflection in C#, and what are its most common practical uses and performance trade-offs? How does the .NET dependency injection container and ASP.NET Core's model binding use reflection and attributes together under the hood? How would you write a simple reflection-based object mapper that copies matching property values from one object to another of a different type? How do you use reflection to discover all types in an assembly that implement a specific interface, useful for plugin discovery? Why is reflection generally slower than direct code, and what techniques (like caching or compiled expressions) mitigate this cost? How would you use System.Reflection.Emit or source generators as faster, more modern alternatives to runtime reflection for code generation scenarios?
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C# › Attributes & Reflection

What is an attribute in C#, and how do you apply one to a class or method?

Beginner

Answer

An attribute is metadata attached to a code element (class, method, property, etc.) using square-bracket syntax placed directly above it — attributes don't affect runtime behavior on their own, but tools, frameworks, and your own reflection code can read them to alter behavior or generate documentation.
[Obsolete("Use NewMethod instead")]
public void OldMethod() { }

[Serializable]
public class Product { public string Name { get; set; } = ""; }
Real-world example Marking a deprecated API method with [Obsolete] so callers get a compiler warning, or marking a class [Serializable] for legacy binary serialization.

Common follow-ups: Can you apply multiple attributes to the same code element?

Reflection
All 11 Attributes & Reflection questions