Reading attributes with reflection in C Sharp (C#) - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of reading a custom attribute on a class

What is the output of this C# program that reads a custom attribute from a class using reflection?

C Sharp (C#)

using System;

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

[Info("Sample class for testing")]
public class SampleClass {}

public class Program
{
    public static void Main()
    {
        var type = typeof(SampleClass);
        var attr = (InfoAttribute)Attribute.GetCustomAttribute(type, typeof(InfoAttribute));
        Console.WriteLine(attr?.Description ?? "No attribute found");
    }
}

A Sample class for testing

B No attribute found

C System.InfoAttribute

D Compilation error

💡 Hint

Look at how the attribute is applied and how reflection retrieves it.

The class SampleClass has the InfoAttribute with Description set to "Sample class for testing". Reflection reads this attribute and prints its Description property.

❓ Predict Output

intermediate

Output when attribute is missing

What will be the output when trying to read a custom attribute that is not applied to the class?

C Sharp (C#)

using System;

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

public class NoAttributeClass {}

public class Program
{
    public static void Main()
    {
        var type = typeof(NoAttributeClass);
        var attr = (InfoAttribute)Attribute.GetCustomAttribute(type, typeof(InfoAttribute));
        Console.WriteLine(attr?.Description ?? "No attribute found");
    }
}

A Empty line

B null

C No attribute found

D Compilation error

💡 Hint

Check what happens when the attribute is not present and null is returned.

Since NoAttributeClass does not have the InfoAttribute, GetCustomAttribute returns null, so the null-coalescing operator prints "No attribute found".

🔧 Debug

advanced

Identify the error when reading multiple attributes

This code tries to read multiple custom attributes from a class but throws an exception. What is the cause?

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("alpha")]
[Tag("beta")]
public class MultiTagClass {}

public class Program
{
    public static void Main()
    {
        var type = typeof(MultiTagClass);
        var attr = (TagAttribute)Attribute.GetCustomAttributes(type, typeof(TagAttribute));
        Console.WriteLine(attr.Name);
    }
}

A Invalid cast exception because GetCustomAttributes returns an array

B Outputs 'alpha'

C Compilation error due to missing using directive

D NullReferenceException because no attribute found

💡 Hint

Check the return type of GetCustomAttributes when multiple attributes are allowed.

Attribute.GetCustomAttributes returns an array of matching attributes. Since AllowMultiple = true and two attributes are applied, it returns TagAttribute[], but the code casts directly to TagAttribute, causing InvalidCastException.

📝 Syntax

advanced

Correct syntax to read all attributes of a type

Which option correctly reads all custom attributes of type TagAttribute applied to a class?

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("one")]
[Tag("two")]
public class TestClass {}

public class Program
{
    public static void Main()
    {
        var type = typeof(TestClass);
        // Which option correctly reads all TagAttribute instances?
    }
}

A var attrs = Attribute.GetCustomAttribute(type, typeof(TagAttribute));

B var attrs = (TagAttribute)Attribute.GetCustomAttributes(type, typeof(TagAttribute));

C var attrs = type.GetCustomAttribute(typeof(TagAttribute));

D var attrs = (TagAttribute[])Attribute.GetCustomAttributes(type, typeof(TagAttribute));

💡 Hint

GetCustomAttributes returns an array of attributes, not a single attribute.

GetCustomAttributes returns an array of attributes matching the type. Casting it to TagAttribute[] is correct. Option D returns a single attribute, not all. Option D is invalid because GetCustomAttribute is not a method on Type. Option D tries to cast an array to a single attribute causing an error.

🚀 Application

expert

Count how many properties have a specific attribute

Given a class with properties decorated with a custom attribute, what is the value of count after running this code?

C Sharp (C#)

using System;
using System.Reflection;

[AttributeUsage(AttributeTargets.Property)]
public class ImportantAttribute : Attribute {}

public class Data
{
    [Important]
    public int Id { get; set; }

    public string Name { get; set; }

    [Important]
    public double Value { get; set; }
}

public class Program
{
    public static void Main()
    {
        var type = typeof(Data);
        int count = 0;
        foreach (var prop in type.GetProperties())
        {
            if (Attribute.IsDefined(prop, typeof(ImportantAttribute)))
                count++;
        }
        Console.WriteLine(count);
    }
}

A 3

B 2

C 1

D 0

💡 Hint

Count only properties with the ImportantAttribute applied.

Only the properties Id and Value have the ImportantAttribute. Name does not. So the count is 2.

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