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What is an attribute in C#, and how do you apply one to a class or method? 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 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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How do you use reflection to read a custom attribute's values off a property at runtime?

Intermediate

Answer

Use GetCustomAttribute<T>() (or GetCustomAttributes for multiple) on a PropertyInfo, MethodInfo, or Type object to retrieve an instance of the applied attribute, then read its properties normally — this is the core mechanism behind validation libraries, serializers, and ORMs that inspect your classes' decorations.
var properties = typeof(User).GetProperties();
foreach (var prop in properties) {
  var required = prop.GetCustomAttribute<RequiredAttribute>();
  if (required != null) {
    Console.WriteLine($"{prop.Name}: {required.ErrorMessage}");
  }
}
Real-world example Building a simple validation engine that scans a model's properties for [Required] attributes and reports errors.

Common follow-ups: How would you check whether an attribute exists WITHOUT retrieving its full instance, for a quick presence check?

Design Patterns in C#
All 11 Attributes & Reflection questions