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.