Reading attributes with reflection in C Sharp (C#) - Mini Project: Build & Apply

Reading attributes with reflection

📖 Scenario: Imagine you have a simple program where you want to label some classes with extra information using attributes. Later, you want to read these labels to decide what to do.

🎯 Goal: You will create a custom attribute, apply it to a class, and then use reflection to read and display the attribute's value.

📋 What You'll Learn

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

Create a class called Product and apply the InfoAttribute with the description "This is a product class".

Use reflection to get the InfoAttribute from the Product class.

Print the Description value of the InfoAttribute.

💡 Why This Matters

🌍 Real World

Attributes let you add extra information to your code that can be read later. This is useful for marking classes with metadata, like marking which classes need special handling.

💼 Career

Many C# jobs require understanding attributes and reflection to work with frameworks, libraries, or custom tools that use metadata for configuration or behavior.

1

Create the custom attribute class

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

C Sharp (C#)

# Your code here

Hint

Remember, custom attributes inherit from System.Attribute. Use a public property to hold the description.

2

Create the Product class with the attribute

Create a public class called Product. Apply the InfoAttribute to it with the description set to "This is a product class".

C Sharp (C#)

using System;

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

# Your code here

Hint

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

3

Use reflection to read the attribute

Write code to get the InfoAttribute from the Product class using reflection. Store the attribute in a variable called infoAttr.

C Sharp (C#)

using System;

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

[InfoAttribute(Description = "This is a product class")]
public class Product
{
}

# Your code here

Hint

Use typeof(Product) to get the type. Use Attribute.GetCustomAttribute to get the attribute.

4

Print the attribute description

Add a Console.WriteLine statement to print the Description property of infoAttr.

C Sharp (C#)

using System;
using System.Reflection;

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

[InfoAttribute(Description = "This is a product class")]
public class Product
{
}

class Program
{
    static void Main()
    {
        Type productType = typeof(Product);
        InfoAttribute infoAttr = (InfoAttribute)Attribute.GetCustomAttribute(productType, typeof(InfoAttribute));
        # Your code here
    }
}

Hint

Use Console.WriteLine(infoAttr.Description); to print the description.

Practice

1. What does reading attributes with reflection in C# allow you to do?

easy

A. Access extra information added to classes or methods at runtime

B. Change the source code of a program while it runs

C. Compile code faster by skipping attribute checks

D. Automatically generate user interfaces without coding

Solution

1. Step 1: Understand what attributes are

   Attributes are extra notes or metadata added to code elements like classes or methods.

2. Step 2: Understand reflection's role

   Reflection lets you look at these attributes while the program runs, to read their information.

3. Final Answer:

   Access extra information added to classes or methods at runtime -> Option A

4. Quick Check:

   Reflection reads attributes = Access extra info [OK]

Hint: Reflection reads metadata added to code elements [OK]

Common Mistakes:

• Thinking reflection changes code instead of reading it
• Confusing attributes with variables or methods
• Believing attributes speed up compilation

2. Which of the following is the correct way to get custom attributes of a class using reflection in C#?

easy

A. GetAttributes(typeof(MyClass))

B. typeof(MyClass).GetCustomAttributes()

C. MyClass.GetAttributes()

D. MyClass.Attributes()

Solution

1. Step 1: Recall the reflection syntax for attributes

   In C#, to get attributes of a class, use typeof(ClassName).GetCustomAttributes().

2. Step 2: Check each option

   Only typeof(MyClass).GetCustomAttributes() uses the correct syntax with typeof and GetCustomAttributes method.

3. Final Answer:

   typeof(MyClass).GetCustomAttributes() -> Option B

4. Quick Check:

   Correct syntax uses typeof and GetCustomAttributes() [OK]

Hint: Use typeof(Class).GetCustomAttributes() to read attributes [OK]

Common Mistakes:

• Calling GetAttributes() directly on class name
• Using instance methods instead of typeof
• Confusing method names like Attributes()

3. Given this code snippet, what will be the output?

[System.Obsolete("Use NewMethod instead")]
class OldClass {}

var attrs = typeof(OldClass).GetCustomAttributes(typeof(System.ObsoleteAttribute), false);
Console.WriteLine(attrs.Length);

medium

A. 0

B. null

C. Throws an exception

D. 1

Solution

1. Step 1: Identify the attribute applied

   The class OldClass has the Obsolete attribute applied once.

2. Step 2: Check GetCustomAttributes call

   GetCustomAttributes is called with ObsoleteAttribute type, so it returns an array with that attribute instance.

3. Final Answer:

   1 -> Option D

4. Quick Check:

   One Obsolete attribute found = 1 [OK]

Hint: Count attributes returned by GetCustomAttributes to find output length [OK]

Common Mistakes:

• Assuming zero because attribute is ignored
• Expecting null instead of empty array
• Thinking it throws exception without reason

4. What is wrong with this code snippet that tries to read attributes?

class MyClass {}

var attrs = MyClass.GetCustomAttributes();
Console.WriteLine(attrs.Length);

medium

A. MyClass does not have any attributes

B. Console.WriteLine cannot print Length property

C. GetCustomAttributes is not a method of the class itself

D. Missing semicolon after class declaration

Solution

1. Step 1: Check method call on class

   GetCustomAttributes is a method of Type, not the class itself, so calling MyClass.GetCustomAttributes() is invalid.

2. Step 2: Correct usage

   You must use typeof(MyClass).GetCustomAttributes() to get attributes via reflection.

3. Final Answer:

   GetCustomAttributes is not a method of the class itself -> Option C

4. Quick Check:

   Use typeof(Class).GetCustomAttributes() not Class.GetCustomAttributes() [OK]

Hint: Call GetCustomAttributes on typeof(Class), not class name [OK]

Common Mistakes:

• Calling GetCustomAttributes directly on class name
• Ignoring that GetCustomAttributes is a Type method
• Assuming attributes exist without checking

5. You want to read a custom attribute InfoAttribute applied to multiple methods in a class and print their descriptions. Which approach correctly reads and prints these descriptions?

hard

A. Use reflection to get MethodInfo array, then for each method call GetCustomAttributes(typeof(InfoAttribute), false) and print the Description property

B. Call GetCustomAttributes on the class type directly and print Description from the first attribute found

C. Use a foreach loop on class fields and call GetCustomAttributes on each field

D. Use reflection to get properties of the class and print their names

Solution

1. Step 1: Identify target elements

   The attribute InfoAttribute is applied to methods, so we must get methods via reflection.

2. Step 2: Read attributes per method

   For each MethodInfo, call GetCustomAttributes with InfoAttribute type to get the attribute instance and access its Description property.

3. Step 3: Print descriptions

   Print the Description property for each method's attribute found.

4. Final Answer:

   Use reflection to get MethodInfo array, then for each method call GetCustomAttributes(typeof(InfoAttribute), false) and print the Description property -> Option A

5. Quick Check:

   Read attributes per method, not class or fields [OK]

Hint: Get methods, then read attributes on each method [OK]

Common Mistakes:

• Reading attributes only on class instead of methods
• Checking fields instead of methods
• Assuming one attribute for whole class