Custom attribute classes in C Sharp (C#) - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to declare a custom attribute class named MyAttribute.

C Sharp (C#)

public class [1] : System.Attribute {}

Drag options to blanks, or click blank then click option'

AMyAttribute

BCustomAttr

CAttributeClass

DMyClass

Attempts:

3 left

💡 Hint

Common Mistakes

Naming the class without 'Attribute' suffix.

Not inheriting from System.Attribute.

The custom attribute class must be named MyAttribute and inherit from System.Attribute.

2fill in blank

medium

Complete the code to apply the custom attribute MyAttribute to the class Sample.

C Sharp (C#)

[[1]]
public class Sample {}

Drag options to blanks, or click blank then click option'

AMyClass

BAttribute

CCustomAttribute

DMyAttribute

Attempts:

3 left

💡 Hint

Common Mistakes

Using a wrong attribute name.

Missing the square brackets.

To apply the attribute, use its class name without the 'Attribute' suffix if desired, but here the full name MyAttribute is used.

3fill in blank

hard

Fix the error in the attribute constructor to accept a string parameter named description.

C Sharp (C#)

public class MyAttribute : System.Attribute {
    public string Description { get; }
    public MyAttribute([1]) {
        Description = description;
    }
}

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Adouble description

Bint description

Cstring description

Dbool description

Attempts:

3 left

💡 Hint

Common Mistakes

Using a wrong parameter type.

Mismatching parameter name and property.

The constructor must accept a string parameter named description to assign it to the property.

4fill in blank

hard

Fill both blanks to create a dictionary using ToDictionary that maps property names to their types for properties with type string.

C Sharp (C#)

var stringProps = typeof(MyAttribute).GetProperties()
    .Where(prop => prop.PropertyType == typeof(string))
    .ToDictionary(prop => [1], prop => [2]);

Drag options to blanks, or click blank then click option'

Aprop.Name

Bprop.PropertyType

Cprop.GetValue(this)

Dprop.ToString()

Attempts:

3 left

💡 Hint

Common Mistakes

Using property values instead of names or types.

Using incorrect property methods.

The dictionary keys are property names (prop.Name) and values are their types (prop.PropertyType).

5fill in blank

hard

Fill all three blanks to create a method that returns the description from the MyAttribute applied to a class.

C Sharp (C#)

public static string GetDescription(Type t) {
    var attr = (MyAttribute)Attribute.GetCustomAttribute(t, typeof([1]));
    return attr == null ? [2] : attr.[3];
}

Drag options to blanks, or click blank then click option'

AMyAttribute

Bstring.Empty

CDescription

DAttribute

Attempts:

3 left

💡 Hint

Common Mistakes

Using wrong attribute type in GetCustomAttribute.

Returning null instead of empty string.

Accessing wrong property name.

The method gets the MyAttribute from the type, returns empty string if none, else returns the Description property.

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