Custom attribute classes in C Sharp (C#) - Mini Project: Build & Apply

Custom Attribute Classes in C#

📖 Scenario: Imagine you are building a simple library system. You want to mark some classes with special notes using custom tags called attributes. These tags will help you add extra information to your classes, like marking a class as "Important" or "Deprecated".

🎯 Goal: You will create a custom attribute class, apply it to a book class, and then read the attribute information using code.

📋 What You'll Learn

Create a custom attribute class called InfoAttribute with a string property Note.

Create a class called Book and apply the InfoAttribute with a note.

Write code to read and print the Note from the InfoAttribute applied to the Book class.

💡 Why This Matters

🌍 Real World

Custom attributes are used to add extra information to code elements, which can be read at runtime to change behavior or provide metadata.

💼 Career

Understanding custom attributes is useful for working with frameworks, libraries, and tools that use metadata, such as serialization, validation, or documentation generation.

1

Create the custom attribute class

Create a public class called InfoAttribute that inherits from System.Attribute. Add a public string property called Note with a getter and setter.

C Sharp (C#)

// Your code here

Hint

Remember, custom attributes must inherit from System.Attribute. Use public string Note { get; set; } to create the property.

2

Create the Book class and apply the attribute

Create a public class called Book. Apply the InfoAttribute to the Book class with the note set to "This is an important class".

C Sharp (C#)

using System;

public class InfoAttribute : Attribute
{
    public string Note { get; set; }
}

// Create the Book class below and apply InfoAttribute
// Your code here

Hint

Use square brackets [] to apply the attribute above the class. Set the Note property inside the parentheses.

3

Write code to read the attribute

Write code to get the InfoAttribute applied to the Book class using typeof(Book).GetCustomAttributes(typeof(InfoAttribute), false). Store the first attribute in a variable called infoAttr of type InfoAttribute.

C Sharp (C#)

using System;

public class InfoAttribute : Attribute
{
    public string Note { get; set; }
}

[InfoAttribute(Note = "This is an important class")]
public class Book
{
}

// Get the InfoAttribute from Book class
// Your code here

Hint

Use GetCustomAttributes on the Book type to get the attributes. Cast the first attribute to InfoAttribute.

4

Print the note from the attribute

Add a Console.WriteLine statement to print the Note property of infoAttr inside the Main method.

C Sharp (C#)

using System;

public class InfoAttribute : Attribute
{
    public string Note { get; set; }
}

[InfoAttribute(Note = "This is an important class")]
public class Book
{
}

class Program
{
    static void Main()
    {
        object[] attrs = typeof(Book).GetCustomAttributes(typeof(InfoAttribute), false);
        InfoAttribute infoAttr = (InfoAttribute)attrs[0];
        // Print the Note property below
        // Your code here
    }
}

Hint

Use Console.WriteLine(infoAttr.Note); to show the note on the screen.

Practice

1. What is the main purpose of creating a custom attribute class in C#?

easy

A. To create a new data type for variables

B. To replace the main class in a program

C. To add extra information to code elements like classes or methods

D. To handle exceptions automatically

Solution

1. Step 1: Understand what attributes do

   Attributes add metadata, or extra info, to code elements like classes or methods.

2. Step 2: Identify the role of custom attributes

   Custom attributes let you define your own extra info to attach to code elements.

3. Final Answer:

   To add extra information to code elements like classes or methods -> Option C

4. Quick Check:

   Custom attributes = extra info [OK]

Hint: Attributes add info to code elements, not change logic [OK]

Common Mistakes:

• Thinking attributes replace classes
• Confusing attributes with data types
• Assuming attributes handle errors

2. Which of the following is the correct way to declare a custom attribute class named InfoAttribute?

easy

A. public struct InfoAttribute : Attribute {}

B. public attribute InfoAttribute {}

C. public class InfoAttribute extends Attribute {}

D. public class InfoAttribute : System.Attribute {}

Solution

1. Step 1: Recall C# syntax for custom attributes

   Custom attribute classes must inherit from System.Attribute using a class declaration.

2. Step 2: Check each option

   public class InfoAttribute : System.Attribute {} correctly uses 'public class InfoAttribute : System.Attribute {}'. public attribute InfoAttribute {} uses invalid keyword 'attribute'. public class InfoAttribute extends Attribute {} uses 'extends' which is Java syntax. public struct InfoAttribute : Attribute {} uses 'struct' which is not correct for attributes.

3. Final Answer:

   public class InfoAttribute : System.Attribute {} -> Option D

4. Quick Check:

   Custom attribute = class + : System.Attribute [OK]

Hint: Custom attributes are classes inheriting System.Attribute [OK]

Common Mistakes:

• Using 'attribute' keyword instead of 'class'
• Using Java syntax 'extends' instead of ':'
• Declaring attribute as struct

3. What will be the output of the following code?

using System;
[Info("Version 1.0")]
class Program {}

public class InfoAttribute : Attribute
{
    public string Version { get; }
    public InfoAttribute(string version) => Version = version;
}

class Test
{
    static void Main()
    {
        var attrs = typeof(Program).GetCustomAttributes(typeof(InfoAttribute), false);
        if (attrs.Length > 0)
            Console.WriteLine(((InfoAttribute)attrs[0]).Version);
        else
            Console.WriteLine("No attribute");
    }
}

medium

A. Version 1.0

B. No attribute

C. Compilation error

D. Runtime exception

Solution

1. Step 1: Understand attribute application

   The Program class has the Info attribute with version "Version 1.0" applied.

2. Step 2: Check reflection code

   The code uses reflection to get InfoAttribute from Program. Since it exists, it prints the Version property.

3. Final Answer:

   Version 1.0 -> Option A

4. Quick Check:

   Reflection finds attribute version = Version 1.0 [OK]

Hint: Reflection reads attribute properties if applied [OK]

Common Mistakes:

• Assuming attribute is not found
• Confusing attribute class with target class
• Forgetting to cast attribute before accessing property

4. Identify the error in this custom attribute declaration:

public class MyAttribute
{
    public string Name { get; set; }
    public MyAttribute(string name) { Name = name; }
}

medium

A. Missing inheritance from System.Attribute

B. Property Name should be private

C. Constructor should not have parameters

D. Class should be declared as struct

Solution

1. Step 1: Check inheritance requirement

   Custom attribute classes must inherit from System.Attribute to be recognized as attributes.

2. Step 2: Analyze given code

   The class MyAttribute does not inherit from System.Attribute, so it is not a valid attribute class.

3. Final Answer:

   Missing inheritance from System.Attribute -> Option A

4. Quick Check:

   Attributes must inherit System.Attribute [OK]

Hint: Always inherit System.Attribute for custom attributes [OK]

Common Mistakes:

• Forgetting to inherit System.Attribute
• Thinking constructor must be parameterless
• Using struct instead of class

5. You want to create a custom attribute PriorityAttribute that accepts an integer priority level and can be applied only to methods. Which of the following is the correct way to declare it?

hard

A. [AttributeUsage(AttributeTargets.Class)]
public class PriorityAttribute : Attribute
{
public int Level;
public PriorityAttribute(int level) { Level = level; }
}

B. [AttributeUsage(AttributeTargets.Method)]
public class PriorityAttribute : Attribute
{
public int Level { get; }
public PriorityAttribute(int level) => Level = level;
}

C. public class PriorityAttribute : Attribute
{
public int Level { get; set; }
public PriorityAttribute() {}
}

D. [AttributeUsage(AttributeTargets.Method | AttributeTargets.Class)]
public struct PriorityAttribute
{
public int Level;
public PriorityAttribute(int level) { Level = level; }
}

Solution

1. Step 1: Check AttributeUsage target

   The attribute should apply only to methods, so AttributeTargets.Method is correct.

2. Step 2: Verify class declaration and constructor

   It must be a class inheriting Attribute, with a constructor accepting an int and a read-only property.

3. Step 3: Review options

   [AttributeUsage(AttributeTargets.Method)] public class PriorityAttribute : Attribute { public int Level { get; } public PriorityAttribute(int level) => Level = level; } matches all requirements. [AttributeUsage(AttributeTargets.Class)] public class PriorityAttribute : Attribute { public int Level; public PriorityAttribute(int level) { Level = level; } } targets classes, not methods. public class PriorityAttribute : Attribute { public int Level { get; set; } public PriorityAttribute() {} } lacks constructor parameter and target restriction. [AttributeUsage(AttributeTargets.Method | AttributeTargets.Class)] public struct PriorityAttribute { public int Level; public PriorityAttribute(int level) { Level = level; } } uses struct, which is invalid for attributes.

4. Final Answer:

   [AttributeUsage(AttributeTargets.Method)] public class PriorityAttribute : Attribute { public int Level { get; } public PriorityAttribute(int level) => Level = level; } -> Option B

5. Quick Check:

   AttributeUsage + class + constructor with param = [AttributeUsage(AttributeTargets.Method)] public class PriorityAttribute : Attribute { public int Level { get; } public PriorityAttribute(int level) => Level = level; } [OK]

Hint: Use AttributeUsage to limit targets and inherit Attribute [OK]

Common Mistakes:

• Using struct instead of class
• Wrong AttributeTargets in AttributeUsage
• Missing constructor parameter for priority