Creating instances dynamically in C Sharp (C#) - Performance & Efficiency

Time Complexity: Creating instances dynamically

O(n)

Understanding Time Complexity

When we create objects dynamically in C#, it's important to know how the time it takes grows as we create more objects.

We want to understand how the number of objects affects the total work done by the program.

Scenario Under Consideration

Analyze the time complexity of the following code snippet.

public class MyClass { }

public void CreateObjects(int n) {
    for (int i = 0; i < n; i++) {
        MyClass obj = new MyClass();
    }
}
    

This code creates n new instances of MyClass one after another in a loop.

Identify Repeating Operations

Identify the loops, recursion, array traversals that repeat.

• Primary operation: Creating a new instance of MyClass inside the loop.
• How many times: Exactly n times, once per loop iteration.

How Execution Grows With Input

As n grows, the number of objects created grows the same way.

Input Size (n)	Approx. Operations
10	10 creations
100	100 creations
1000	1000 creations

Pattern observation: The work grows directly in proportion to the number of objects created.

Final Time Complexity

Time Complexity: O(n)

This means the time to create objects grows linearly with how many you want to create.

Common Mistake

[X] Wrong: "Creating objects inside a loop is constant time because each creation is fast."

[OK] Correct: Even if one creation is fast, doing it many times adds up, so total time grows with the number of objects.

Interview Connect

Understanding how creating many objects affects time helps you reason about program speed and resource use in real projects.

Self-Check

"What if we create objects inside a nested loop instead of a single loop? How would the time complexity change?"

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