Custom attribute classes in C Sharp (C#) - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this code using a custom attribute?

Consider the following C# code that defines a custom attribute and uses it on a class. What will be printed when running the Main method?

C Sharp (C#)

using System;

[AttributeUsage(AttributeTargets.Class)]
public class InfoAttribute : Attribute
{
    public string Description { get; }
    public InfoAttribute(string description) => Description = description;
}

[Info("This is a sample class")]
public class SampleClass {}

public class Program
{
    public static void Main()
    {
        var attr = (InfoAttribute)Attribute.GetCustomAttribute(typeof(SampleClass), typeof(InfoAttribute));
        Console.WriteLine(attr.Description);
    }
}

A Compilation error

B System.InfoAttribute

C null

D This is a sample class

💡 Hint

Look at how the attribute is retrieved and what property is printed.

The InfoAttribute is applied to SampleClass with the description "This is a sample class". The code retrieves this attribute instance and prints its Description property, so the output is exactly that string.

🧠 Conceptual

intermediate

Which attribute usage is valid for this custom attribute?

Given a custom attribute defined as below, which AttributeUsage setting correctly restricts it to be used only on methods?

C Sharp (C#)

using System;

[AttributeUsage(AttributeTargets.Method)]
public class MyMethodAttribute : Attribute
{
}

A [AttributeUsage(AttributeTargets.Method)]

B [AttributeUsage(AttributeTargets.Class)]

C [AttributeUsage(AttributeTargets.Property)]

D [AttributeUsage(AttributeTargets.Field)]

💡 Hint

Think about which AttributeTargets value limits usage to methods only.

The AttributeTargets.Method restricts the attribute usage to methods only. Other options restrict usage to classes, properties, or fields, which are not correct for this attribute.

🔧 Debug

advanced

Why does this code throw an exception when retrieving the attribute?

Examine the code below. It throws a NullReferenceException at runtime. What is the cause?

C Sharp (C#)

using System;

[AttributeUsage(AttributeTargets.Class)]
public class TagAttribute : Attribute
{
    public string Name { get; }
    public TagAttribute(string name) => Name = name;
}

public class Product {}

public class Program
{
    public static void Main()
    {
        var attr = (TagAttribute)Attribute.GetCustomAttribute(typeof(Product), typeof(TagAttribute));
        Console.WriteLine(attr.Name.ToUpper());
    }
}

A The Product class does not have the TagAttribute applied, so attr is null causing NullReferenceException.

B The TagAttribute constructor is missing a parameterless constructor causing the error.

C AttributeUsage is set incorrectly causing the attribute not to be found.

D The cast to TagAttribute is invalid causing an InvalidCastException.

💡 Hint

Check if the attribute is actually applied to the class before retrieving it.

The Product class does not have the TagAttribute applied. So GetCustomAttribute returns null. Then calling attr.Name causes a NullReferenceException.

📝 Syntax

advanced

Which option correctly defines a custom attribute with a named property?

Which of the following code snippets correctly defines a custom attribute with a named property Version that can be set optionally?

A public class VersionAttribute : Attribute { public int Version; }

B public class VersionAttribute : Attribute { public int Version { get; set; } }

C public class VersionAttribute : Attribute { public int Version() { get; set; } }

D public class VersionAttribute : Attribute { public int GetVersion() => 1; }

💡 Hint

Remember named properties in attributes must be public properties with get and set accessors.

Option B correctly defines a public property Version with get and set accessors, allowing it to be set optionally when using the attribute. Option B uses a field which is not allowed for named parameters. Option B has invalid syntax. Option B defines a method, not a property.

🚀 Application

expert

What is the output of this code using multiple custom attributes and reflection?

Given the code below, what will be printed when running the Main method?

C Sharp (C#)

using System;
using System.Reflection;

[AttributeUsage(AttributeTargets.Class, AllowMultiple = true)]
public class TagAttribute : Attribute
{
    public string Name { get; }
    public TagAttribute(string name) => Name = name;
}

[Tag("Utility")]
[Tag("Core")]
public class Helper {}

public class Program
{
    public static void Main()
    {
        var attrs = (TagAttribute[])Attribute.GetCustomAttributes(typeof(Helper), typeof(TagAttribute));
        foreach (var attr in attrs)
            Console.Write(attr.Name + " ");
    }
}

A Compilation error due to multiple attributes

B Core Utility

C Utility Core

D UtilityCore

💡 Hint

Check the order attributes are declared and how they are retrieved.

The Helper class has two TagAttribute instances applied: first "Utility", then "Core". GetCustomAttributes returns them in declaration order. The code prints each Name followed by a space, so output is "Utility Core " exactly.

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