Custom attribute classes in C Sharp (C#) - Step-by-Step Execution

Concept Flow - Custom attribute classes

Define attribute class

↓

Apply attribute to target

↓

Run program

↓

Use reflection to read attribute

↓

Access attribute data

↓

Use data in program

First, you create a custom attribute class. Then you put it on something like a class or method. When the program runs, you read that attribute using reflection to get its data.

Execution Sample

C Sharp (C#)

using System;

[AttributeUsage(AttributeTargets.Class)]
class MyInfoAttribute : Attribute {
  public string Info { get; }
  public MyInfoAttribute(string info) => Info = info;
}

[MyInfo("Hello World")]
class MyClass {}

class Program {
  static void Main() {
    var attr = (MyInfoAttribute)Attribute.GetCustomAttribute(typeof(MyClass), typeof(MyInfoAttribute));
    Console.WriteLine(attr.Info);
  }
}

This code defines a custom attribute with a string, applies it to a class, then reads and prints the string at runtime.

Execution Table

Step	Action	Evaluation	Result
1	Define MyInfoAttribute class	Class created	Ready to use attribute
2	Apply [MyInfo("Hello World")] to MyClass	Attribute attached to MyClass	MyClass has attribute data
3	Start Main method	Program runs	Ready to get attribute
4	Call Attribute.GetCustomAttribute for MyClass	Find MyInfoAttribute on MyClass	Returns MyInfoAttribute instance
5	Access attr.Info	Get string value	"Hello World"
6	Print attr.Info	Output to console	Hello World
7	Program ends	No more code	Execution stops

💡 Program ends after printing the attribute's Info string

Variable Tracker

Variable	Start	After Step 4	After Step 5	Final
attr	null	MyInfoAttribute instance	MyInfoAttribute instance	MyInfoAttribute instance
attr.Info	N/A	N/A	"Hello World"	"Hello World"

Key Moments - 3 Insights

Why do we cast the result of GetCustomAttribute to MyInfoAttribute?

GetCustomAttribute returns a general Attribute object, so we cast it to MyInfoAttribute to access the Info property, as shown in step 4 of the execution_table.

What happens if the attribute is not found on the class?

GetCustomAttribute returns null, so attr would be null and accessing attr.Info would cause an error. This is why checking for null is important in real code.

Why do we use [AttributeUsage(AttributeTargets.Class)] on the attribute class?

This limits the attribute to be used only on classes, preventing misuse on other targets like methods or properties.

Visual Quiz - 3 Questions

Look at the execution_table, what is the value of attr.Info at step 5?

A null

B "Hello World"

C An error

D "MyClass"

💡 Hint

Check the 'Evaluation' and 'Result' columns at step 5 in the execution_table.

At which step does the program find the attribute instance on MyClass?

A Step 4

B Step 6

C Step 3

D Step 2

💡 Hint

Look for when Attribute.GetCustomAttribute is called in the execution_table.

If we remove [AttributeUsage(AttributeTargets.Class)], what might happen?

A The attribute will not be found at runtime.

B The program will not compile.

C The attribute can be applied to any target, not just classes.

D The Info property will be empty.

💡 Hint

Refer to the key_moments explanation about AttributeUsage.

Concept Snapshot

Custom attribute classes let you add extra info to code elements.
Define a class inheriting Attribute.
Use [AttributeUsage] to limit targets.
Apply with [YourAttribute(...)] syntax.
Read at runtime with reflection (GetCustomAttribute).
Access your data from the attribute instance.

Full Transcript

This lesson shows how to create and use custom attribute classes in C#. First, you define a class that inherits from Attribute and add any properties you want. Then you apply this attribute to a class using square brackets. When the program runs, you use reflection to get the attribute instance from the class and read its data. The example prints the string stored in the attribute. Key points include casting the attribute to your custom type and using AttributeUsage to control where the attribute can be applied.