Creating instances dynamically in C Sharp (C#) - Practice Exercises

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of dynamic instance creation with Activator.CreateInstance

What is the output of this C# code that creates an instance dynamically?

C Sharp (C#)

using System;

class Person {
    public string Name { get; set; } = "Unknown";
    public override string ToString() => $"Person: {Name}";
}

class Program {
    static void Main() {
        Type t = typeof(Person);
        var obj = Activator.CreateInstance(t);
        Console.WriteLine(obj.ToString());
    }
}

A System.Object

B Person: Unknown

C NullReferenceException

D Person:

💡 Hint

Activator.CreateInstance creates an object using the default constructor.

The code uses Activator.CreateInstance to create a Person object. The default Name is "Unknown", so ToString returns "Person: Unknown".

❓ Predict Output

intermediate

Output when creating instance with parameterized constructor dynamically

What will this C# code print when creating an instance dynamically with parameters?

C Sharp (C#)

using System;
using System.Reflection;

class Car {
    public string Model { get; }
    public Car(string model) {
        Model = model;
    }
    public override string ToString() => $"Car model: {Model}";
}

class Program {
    static void Main() {
        Type t = typeof(Car);
        object obj = Activator.CreateInstance(t, new object[] { "Tesla" });
        Console.WriteLine(obj.ToString());
    }
}

A System.Object

B Car model:

C MissingMethodException

D Car model: Tesla

💡 Hint

Activator.CreateInstance can pass constructor arguments as an object array.

The code calls the constructor with "Tesla" as argument, so the Model property is set to "Tesla" and printed.

🔧 Debug

advanced

Identify the error when creating instance dynamically without default constructor

What error does this code produce when trying to create an instance dynamically?

C Sharp (C#)

using System;

class Book {
    public string Title { get; }
    public Book(string title) {
        Title = title;
    }
}

class Program {
    static void Main() {
        Type t = typeof(Book);
        var obj = Activator.CreateInstance(t);
        Console.WriteLine(obj);
    }
}

A MissingMethodException

B NullReferenceException

C InvalidCastException

D No error, prints Book object

💡 Hint

Activator.CreateInstance without parameters requires a default constructor.

Book has only a constructor with a string parameter. Activator.CreateInstance(t) tries to call a default constructor which does not exist, causing MissingMethodException.

📝 Syntax

advanced

Which code snippet correctly creates an instance dynamically using reflection?

Choose the code snippet that correctly creates an instance of class Animal dynamically with a default constructor.

A var obj = Activator.CreateInstance(typeof(Animal));

B var obj = typeof(Animal).CreateInstance();

C var obj = Activator.CreateInstance<Animal>();

D var obj = new Activator(typeof(Animal));

💡 Hint

Activator.CreateInstance(Type) is the correct method to create an instance dynamically.

Option A uses Activator.CreateInstance with a Type argument correctly. Option A is invalid syntax. Option A uses a generic overload that requires compile-time type information. Option A is invalid because Type does not have CreateInstance method.

🚀 Application

expert

How many instances are created dynamically in this code?

Given this code, how many objects are created dynamically using Activator.CreateInstance?

C Sharp (C#)

using System;
using System.Collections.Generic;

class Shape {
    public string Name { get; set; }
    public Shape() { Name = "Shape"; }
}

class Program {
    static void Main() {
        List<object> shapes = new List<object>();
        Type shapeType = typeof(Shape);
        for (int i = 0; i < 3; i++) {
            var shape = Activator.CreateInstance(shapeType);
            shapes.Add(shape);
        }
        Console.WriteLine(shapes.Count);
    }
}

A 0

B 1

C 3

D 4

💡 Hint

Activator.CreateInstance is called inside a loop 3 times.

The loop runs 3 times, each time creating a new Shape instance dynamically and adding it to the list. So 3 instances are created.

Practice

1. What does Activator.CreateInstance(typeof(MyClass)) do in C#?

easy

A. Creates a new instance of the class MyClass at runtime.

B. Deletes an existing instance of MyClass.

C. Checks if MyClass has a default constructor.

D. Converts MyClass to a string representation.

Solution

1. Step 1: Understand Activator.CreateInstance usage

   Activator.CreateInstance is used to create objects dynamically when the type is known only at runtime.

2. Step 2: Analyze the given code

   The code Activator.CreateInstance(typeof(MyClass)) creates a new instance of MyClass using its default constructor.

3. Final Answer:

   Creates a new instance of the class MyClass at runtime. -> Option A

4. Quick Check:

   Activator.CreateInstance creates objects dynamically [OK]

Hint: Activator.CreateInstance makes objects when type is known at runtime [OK]

Common Mistakes:

• Thinking it deletes or modifies existing objects
• Confusing it with type checking or conversion
• Assuming it calls any constructor without matching parameters

2. Which of the following is the correct syntax to create an instance of Person dynamically using Activator.CreateInstance?

easy

A. var obj = Activator.CreateInstance(typeof(Person));

B. var obj = new Activator.CreateInstance(typeof(Person));

C. var obj = Activator.CreateInstance<Person>(typeof(Person));

D. var obj = Person.CreateInstance();

Solution

1. Step 1: Recall correct Activator.CreateInstance syntax

   The method Activator.CreateInstance takes a Type object as parameter, e.g., typeof(Person).

2. Step 2: Evaluate each option

   var obj = Activator.CreateInstance<Person>(typeof(Person)); uses generic syntax incorrectly by passing a Type argument. var obj = new Activator.CreateInstance(typeof(Person)); tries to use new with a method call, which is wrong. var obj = Activator.CreateInstance(typeof(Person)); correctly calls Activator.CreateInstance(typeof(Person)). var obj = Person.CreateInstance(); calls a non-existent method.

3. Final Answer:

   var obj = Activator.CreateInstance(typeof(Person)); -> Option A

4. Quick Check:

   Use typeof(Type) inside Activator.CreateInstance [OK]

Hint: Use typeof(Type) inside Activator.CreateInstance() [OK]

Common Mistakes:

• Using new keyword with Activator.CreateInstance
• Trying to call generic version incorrectly
• Calling non-existent CreateInstance on the class

3. What will be the output of this code?

class Animal { public string Name; public Animal() { Name = "Dog"; } }
var obj = Activator.CreateInstance(typeof(Animal));
Console.WriteLine(((Animal)obj).Name);

medium

A. null

B. Compilation error

C. Animal

D. Dog

Solution

1. Step 1: Understand the class and constructor

   The class Animal has a default constructor that sets Name to "Dog".

2. Step 2: Analyze Activator.CreateInstance and output

   Activator.CreateInstance(typeof(Animal)) creates a new Animal object, calling the default constructor. So Name is "Dog". The Console.WriteLine prints this value.

3. Final Answer:

   Dog -> Option D

4. Quick Check:

   Default constructor sets Name to Dog [OK]

Hint: Activator calls default constructor, so fields are initialized [OK]

Common Mistakes:

• Assuming Name is null because of dynamic creation
• Thinking output is class name string
• Expecting compile or runtime error

4. Identify the error in this code snippet:

class Car { public int Speed; public Car(int speed) { Speed = speed; } }
var obj = Activator.CreateInstance(typeof(Car));

medium

A. No error, instance created successfully.

B. Missing parameterless constructor causes runtime exception.

C. Syntax error in Activator.CreateInstance call.

D. Cannot create instance of class with fields.

Solution

1. Step 1: Check constructors in Car class

   The class Car has only a constructor with an int parameter. No default (parameterless) constructor exists.

2. Step 2: Understand Activator.CreateInstance behavior

   Activator.CreateInstance(typeof(Car)) tries to call the default constructor. Since none exists, it throws a runtime exception.

3. Final Answer:

   Missing parameterless constructor causes runtime exception. -> Option B

4. Quick Check:

   Activator needs parameterless constructor unless parameters passed [OK]

Hint: Activator.CreateInstance needs parameterless constructor unless parameters given [OK]

Common Mistakes:

• Assuming it creates instance without matching constructor
• Thinking it's a syntax error
• Believing fields prevent instance creation

5. You want to create an instance of a class dynamically and pass constructor parameters. Which is the correct way to do this?

class Book { public string Title; public Book(string title) { Title = title; } }

hard

A. var obj = new Book("C# Guide");

B. var obj = Activator.CreateInstance<Book>(typeof(Book));

C. var obj = Activator.CreateInstance(typeof(Book), "C# Guide");

D. var obj = Activator.CreateInstance(typeof(Book)); obj.Title = "C# Guide";

Solution

1. Step 1: Understand how to pass parameters with Activator.CreateInstance

   To call a constructor with parameters dynamically, pass them as additional arguments after the type.

2. Step 2: Evaluate options

   var obj = Activator.CreateInstance<Book>(typeof(Book)); uses invalid generic syntax by passing a Type argument. var obj = new Book("C# Guide"); uses normal instantiation, not dynamic. var obj = Activator.CreateInstance(typeof(Book), "C# Guide"); correctly passes the string parameter to the constructor. var obj = Activator.CreateInstance(typeof(Book)); obj.Title = "C# Guide"; creates default instance (which fails) and sets property manually, but no default constructor exists.

3. Final Answer:

   var obj = Activator.CreateInstance(typeof(Book), "C# Guide"); -> Option C

4. Quick Check:

   Pass constructor args after type in Activator.CreateInstance [OK]

Hint: Pass constructor arguments after type in Activator.CreateInstance [OK]

Common Mistakes:

• Using generic syntax incorrectly
• Trying to set properties after creation without default constructor
• Confusing normal new with dynamic creation