Top Entity Framework Interview Questions and Answers

What is Entity Framework (EF)?

Entity Framework (EF) is an open-source Object-Relational Mapper (ORM) framework developed by Microsoft for .NET applications. It simplifies database interactions by allowing you to work with data using objects and classes instead of writing raw SQL queries. EF maps your domain objects (classes) to database tables, automating much of the data access layer.

Reasons to Use Entity Framework

EF significantly reduces the amount of code required for data access, improving developer productivity. It handles many of the complexities associated with working directly with databases.

ADO.NET Entity Framework

ADO.NET Entity Framework is the ORM framework that enables you to work with relational databases using .NET objects and classes. It provides a higher-level abstraction for database interaction, simplifying data access and reducing the need for writing and maintaining extensive data access code.

ADO.NET vs. Entity Framework

Advantages and Disadvantages of Entity Framework

Entity Framework Architecture

Key components:

• Entity Data Model (EDM): Contains the conceptual model, storage model, and mapping between them.
• Entity SQL: A query language for EF.
• LINQ to Entities (L2E): Allows querying data using LINQ (Language Integrated Query).
• Entity Client Data Provider: Translates L2E queries to SQL.
• .NET Data Provider: Interacts with the specific database.
• Object Services: Manages object creation and data retrieval.

Primary Functions of Entity Framework

• Mapping of domain objects to database tables.
• Change tracking.
• LINQ query translation to SQL.
• Data persistence.

Migrations in Entity Framework

Migrations automate database updates when your model changes. Types include:

• Automated Migrations: EF automatically generates the database update script.
• Code-Based Migrations: You write the update scripts directly.

Loading Related Entities in EF

EF offers several ways to load related entities:

• Lazy Loading: Related entities are loaded only when accessed.
• Eager Loading: Related entities are loaded along with the parent entity.
• Explicit Loading: Manually load related entities using Include() or Load() methods.

Types of Inheritance Mapping in Entity Framework

• Table-per-Hierarchy (TPH): All entities are stored in a single table.
• Table-per-Type (TPT): Each entity has its own table.
• Table-per-Concrete-Type (TPC): Concrete classes have their own tables; abstract classes do not.

Parts of the Entity Data Model (EDM)

• Storage Model: Represents the database schema.
• Conceptual Model: Represents the application's objects and relationships.
• Mapping: Defines how the conceptual and storage models are related.

Models in Entity Framework

Models in EF are classes representing data entities. They often map directly to database tables.

public class Customer {
    public int ID { get; set; }
    public string Name { get; set; }
    // ... other properties
}
        

LINQ vs. Entity Framework

Conceptual Model

The conceptual model is a high-level, abstract representation of the data that is independent of the database's physical implementation.

Storage Model

The storage model is a representation of how data is stored in the physical database (tables, columns, indexes).

Creating an EDM (Entity Data Model)

1. Right-click on your project in Solution Explorer.
2. Select "Add" -> "New Item".
3. Choose the "ADO.NET Entity Data Model" template.
4. Give your model a name and click "Add".

Retrieving Data from a Database Using Entity Framework in an MVC Application

1. Create your MVC application.
2. Add the Entity Framework NuGet package to your project.
3. Create your model classes, mapping them to your database tables.
4. Use LINQ or Entity SQL to query your data using the DbContext (or ObjectContext).

Comparable and Comparator Interfaces

Both Comparable and Comparator are used for sorting in Java, but they differ in how they define the comparison criteria:

• Comparable defines the "natural ordering" of objects of a class; implemented within the class.
• Comparator defines a custom comparison for objects; implemented separately.

A class can only implement one Comparable, but you can have multiple Comparators for different sorting needs.

Static Methods in Java (Reiterated)

A static method belongs to the class itself, not to an instance. It's called using the class name (e.g., MyClass.myMethod()). Static methods cannot access instance variables directly.

The volatile Keyword (Reiterated)

The volatile keyword makes a variable visible across threads. All threads access the most up-to-date value directly from memory, preventing inconsistencies caused by thread caching.

The finalize() Method (Reiterated)

The finalize() method is called by the garbage collector before an object is destroyed. It's used for cleanup, but relying on it is discouraged; explicit resource management is preferred for reliable resource cleanup.

The `this` Keyword (Reiterated)

The this keyword refers to the current object instance. Key uses include:

• Differentiating between instance variables and local variables.
• Constructor chaining (calling another constructor within the same class).
• Passing the current object to a method.
• Returning the current object from a method.

Assigning to `this`

The this reference is immutable and cannot be reassigned.

Accessing Static Members with `this`

While possible, using this to access static members is not the recommended practice; static members are accessed directly using the class name.

Constructor Chaining with `this` (Reiterated)

Constructor chaining with this() allows a constructor to call another constructor within the same class, simplifying initialization.

Advantages of Passing `this` to a Method

• Immutability of this.
• Use in synchronized blocks.

Inheritance in Java (Reiterated)

Inheritance allows a class (subclass) to inherit properties and behaviors from another class (superclass). It supports code reuse and establishes IS-A relationships.

Reasons for Using Inheritance

• Code reusability.
• Extensibility (adding functionality to existing classes).
• Polymorphism (allowing objects of different classes to be treated as objects of a common type).

Superclass of All Classes in Java

All classes in Java implicitly extend the java.lang.Object class.

Why Java Doesn't Support Multiple Inheritance of Classes

Multiple inheritance can lead to ambiguity (if two parent classes have the same method). Java allows multiple inheritance of interfaces, avoiding this problem.

Aggregation

Aggregation represents a "has-a" relationship; one class contains a reference to another, but the contained object can exist independently.

Composition

Composition is a stronger "part-of" relationship; the contained object cannot exist without the container.

Aggregation vs. Composition

Why Java Doesn't Support Pointers Directly

Direct pointer support is avoided in Java to improve memory safety and reduce the risk of programming errors.

The super Keyword (Reiterated)

The super keyword refers to the immediate parent (super) class. It's used to access members of the superclass.

Constructor Chaining with super()

The super() keyword calls the superclass constructor from the subclass constructor, initializing inherited fields.

Uses of super

• Calling superclass constructors.
• Accessing hidden superclass members.

this vs. super (Reiterated)

Method Overriding

Method overriding allows a subclass to provide a specific implementation for a method that is already defined in its superclass. It's a key aspect of polymorphism.

Overriding Static Methods

Static methods cannot be overridden.

Overriding Overloaded Methods

Overloaded methods can be overridden, but the signature must exactly match the overridden method.

Changing the Access Modifier of an Overridden Method

You can change the access modifier when overriding, but you cannot reduce the accessibility (e.g., you can make a protected method public, but not a public method private).

Modifying the throws Clause

You can modify the throws clause of an overridden method, but you cannot add checked exceptions that weren't in the superclass method.

Method Overriding Example

// ... (code demonstrating method overriding would go here) ...
        

Virtual Functions in Java

All non-private, non-static methods in Java are virtual by default, enabling polymorphism.

Covariant Return Types

Since Java 5, overriding methods can have a return type that's a subtype of the superclass method's return type.

ObjectSets in Entity Framework

In Entity Framework, an ObjectSet represents a collection of entities that map to a database table or view. It provides methods for performing CRUD (Create, Read, Update, Delete) operations on the entities. ObjectSets are created using an ObjectContext instance and typically are not used in newer versions of Entity Framework.

Entity States in Entity Framework

Entities in Entity Framework have different states that reflect their lifecycle:

• Added: A new entity that hasn't been saved to the database.
• Deleted: An entity marked for deletion.
• Modified: An existing entity that has been changed.
• Unchanged: An entity that hasn't been modified.
• Detached: An entity that is no longer being tracked by the context.