Attribute targets and usage in C Sharp (C#) - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of attribute usage on method

What is the output when the following C# program runs?

C Sharp (C#)

using System;

[AttributeUsage(AttributeTargets.Method)]
public class InfoAttribute : Attribute
{
    public string Text { get; }
    public InfoAttribute(string text) => Text = text;
}

class Program
{
    [Info("Test method")]
    public static void Display() => Console.WriteLine("Hello World");

    static void Main()
    {
        var method = typeof(Program).GetMethod("Display");
        var attr = (InfoAttribute)Attribute.GetCustomAttribute(method, typeof(InfoAttribute));
        Console.WriteLine(attr.Text);
        Display();
    }
}

A Test method\nHello World

B Hello World\nTest method

C Hello World

D Compilation error

💡 Hint

Look at how the attribute is retrieved and printed before calling the method.

The attribute InfoAttribute is applied to the method Display. The program retrieves this attribute and prints its Text property first, then calls Display which prints "Hello World".

❓ Predict Output

intermediate

Attribute usage on class and property

What will be the output of this C# code?

C Sharp (C#)

using System;

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

[Label("Person Class")]
class Person
{
    [Label("Person Name")]
    public string Name { get; set; }
}

class Program
{
    static void Main()
    {
        var classAttr = (LabelAttribute)Attribute.GetCustomAttribute(typeof(Person), typeof(LabelAttribute));
        var propAttr = (LabelAttribute)Attribute.GetCustomAttribute(typeof(Person).GetProperty("Name"), typeof(LabelAttribute));
        Console.WriteLine(classAttr.Name);
        Console.WriteLine(propAttr.Name);
    }
}

A Compilation error

B Person Name\nPerson Class

C Person Class\nPerson Name

D Person Class

💡 Hint

Check which targets the attribute is allowed on and where it is applied.

The LabelAttribute is applied to both the Person class and its Name property. The program retrieves and prints both attribute names in order.

❓ Predict Output

advanced

Effect of AttributeUsage AllowMultiple = true

What is the output of this program?

C Sharp (C#)

using System;
using System.Linq;

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

[Tag("First")]
[Tag("Second")]
class Sample {}

class Program
{
    static void Main()
    {
        var attrs = Attribute.GetCustomAttributes(typeof(Sample), typeof(TagAttribute)).Cast<TagAttribute>();
        foreach(var attr in attrs)
            Console.WriteLine(attr.Name);
    }
}

A Compilation error due to multiple attributes

B First

C Second\nFirst

D First\nSecond

💡 Hint

AllowMultiple = true allows multiple attributes of the same type on one target.

Because AllowMultiple is true, both Tag attributes are applied and retrieved in the order they appear.

❓ Predict Output

advanced

Attribute usage with invalid target

What error does this code produce when compiled?

C Sharp (C#)

using System;

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

[Test]
class Demo
{
    public void Run() {}
}

A Compilation error: Attribute 'Test' is not valid on class

B No error, compiles successfully

C Warning but compiles

D Runtime error: Invalid attribute usage

💡 Hint

Check the AttributeTargets specified and where the attribute is applied.

TestAttribute is allowed only on methods but is applied to a class, causing a compilation error.

🧠 Conceptual

expert

Determining valid attribute targets

Given this attribute declaration, which of the following targets can it be applied to without error? [AttributeUsage(AttributeTargets.Field | AttributeTargets.Property | AttributeTargets.Parameter)] public class ValidateAttribute : Attribute {}

A Only a property and a class

B A field, a property, and a method parameter

C Only a field and a method

D A method, a property, and a class

💡 Hint

Look carefully at the AttributeTargets flags used.

The attribute can be applied to fields, properties, and parameters only. Methods and classes are not allowed targets.