Attribute targets and usage in C Sharp (C#) - Step-by-Step Execution

Concept Flow - Attribute targets and usage

Define Attribute

↓

Specify Target(s)

↓

Apply Attribute to Target

↓

Compile and Use Reflection

↓

Access Attribute Info

This flow shows how to define an attribute, specify where it can be used, apply it, then access it at runtime.

Execution Sample

C Sharp (C#)

using System;

[AttributeUsage(AttributeTargets.Class | AttributeTargets.Method)]
class MyInfoAttribute : Attribute { }

[MyInfo]
class Sample {
  [MyInfo] public void Test() { }
}

Defines an attribute usable on classes and methods, then applies it to a class and a method.

Execution Table

Step	Action	Target	Attribute Applied	Result
1	Define MyInfoAttribute	N/A	N/A	Attribute class created
2	Specify AttributeUsage	Class, Method	MyInfoAttribute	Limits usage to classes and methods
3	Apply [MyInfo] to class Sample	Class Sample	MyInfoAttribute	Attribute attached to class
4	Apply [MyInfo] to method Test	Method Test	MyInfoAttribute	Attribute attached to method
5	Compile and run reflection	Sample class	MyInfoAttribute	Detects attribute on class and method
6	Exit	N/A	N/A	Execution complete

💡 All attribute targets applied and detected, execution ends

Variable Tracker

Variable	Start	After Step 2	After Step 4	Final
MyInfoAttribute	undefined	defined with targets Class, Method	applied to Sample and Test	available via reflection

Key Moments - 2 Insights

Why do we specify AttributeTargets in AttributeUsage?

Specifying AttributeTargets restricts where the attribute can be applied, as shown in step 2 of the execution_table. Without it, the attribute could be used anywhere, which may cause errors.

Can we apply the attribute to a property if AttributeTargets only includes Class and Method?

No. The attribute usage limits in step 2 mean applying it to a property would cause a compile error. This is why step 3 and 4 only show class and method targets.

Visual Quiz - 3 Questions

Look at the execution_table, at which step is the attribute usage restricted to classes and methods?

A Step 1

B Step 2

C Step 3

D Step 4

💡 Hint

Check the 'Specify AttributeUsage' action in the execution_table at step 2

According to variable_tracker, what is the state of MyInfoAttribute after step 4?

A Undefined

B Defined but not applied

C Applied to Sample and Test

D Removed

💡 Hint

Look at the 'After Step 4' column for MyInfoAttribute in variable_tracker

If we tried to apply [MyInfo] to a property, what would happen based on the execution flow?

A It would cause a compile error

B It would compile and run fine

C It would be ignored at runtime

D It would apply only to methods

💡 Hint

Refer to the AttributeTargets restriction in step 2 of execution_table

Concept Snapshot

Define an attribute class by inheriting Attribute.
Use [AttributeUsage] to specify valid targets (e.g., Class, Method).
Apply attribute with [YourAttribute] on allowed targets.
At runtime, use reflection to check attribute presence.
Restricting targets prevents incorrect usage and compile errors.

Full Transcript

This example shows how to create a custom attribute in C# and control where it can be applied using AttributeTargets. First, we define the attribute class MyInfoAttribute. Then, we specify that it can only be used on classes and methods with AttributeUsage. Next, we apply the attribute to a class named Sample and its method Test. Finally, at runtime, reflection can detect these attributes. This prevents applying the attribute to unsupported targets like properties, which would cause compile errors.