Monday, 21 January 2013

Entity Framework Interview Questions

What is Entity Framework?
Entity Framework is an additional layer between application and database that enables the developers to program against the conceptual application model instead of programming directly against the relational storage schema.

Will there be any issues adding a table without primary keys to a data model?
Every entity must have a key, even in the case where the entity maps to a view. When you use the Entity Designer to create or update a model, the classes that are generated inherit from EntityObject, which requires EntityKey. So, we have to have a primary key in the table to add it to the data model.

How do you truncate a table using entity data model?
Unfortunately Entity Framework doesn’t include anything straight forward to handle this. But we can still call a T-SQL statement using entity framework that will still minimizes the developers work. We can call ExecuteStoreCommand() method on ObjectContext as shown below.
using (var context = new MyTestDbEntities())
    context.ExecuteStoreCommand("TRUNCATE table Dummy");

How do you query in entity model when the result has a join from different database other than the entity model? E.g.: SELECT t1.c1, t2.c2 FROM table1 AS t1 JOIN table2 AS t2 ON t1.c1 = t2.c1
As the entity model doesn’t support querying from any entity other
than the entities defined in Entity Data Model, we have to query against the data base using ExecuteStoredQuery of the context.
Following code snippet shows how to query when other databases are joined.
string query = "SELECT t1.c1, t2.c2 FROM table1 AS t1 JOIN table2 AS t2 ON t1.c1 = t2.c1";
using (var context = new SampleEntities())
  ObjectResult<DbDataRecord> records = context.ExecuteStoreQuery<DbDataRecord>(query);
  foreach (DbDataRecord record in records)
    //Do whatever you want

What is minimum requirement for Entity Framework applications to run?
The Entity Framework is a component of the .NET Framework so Entity
Framework applications can run on any computer on which the .NET
Framework starting with version 3.5 SP1 is installed.

What is CSDL?
Conceptual schema definition language (CSDL) is an XML-based language
that describes the entities, relationships, and functions that make up a conceptual model of a data-driven application. This conceptual model can be used by the Entity Framework or WCF Data Services.
The metadata that is described with CSDL is used by the Entity
Framework to map entities and relationships that are defined in a
conceptual model to a data source.

What is SSDL?
Store schema definition language (SSDL) is an XML-based language that
describes the storage model of an Entity Framework application.
In an Entity Framework application, storage model metadata is loaded from a .ssdl file (written in SSDL) into an instance of the System.Data.Metadata.Edm.StoreItemCollection and is accessible by using methods in the System.Data.Metadata.Edm.MetadataWorkspace class. The Entity Framework uses storage model metadata to translate queries against the conceptual model to store-specific commands.

What is MSL?
Mapping specification language (MSL) is an XML-based language that describes the mapping between the conceptual model and storage model of an Entity Framework application.
In an Entity Framework application, mapping metadata is loaded from an .msl file (written in MSL) at build time. The Entity Framework uses mapping metadata at runtime to translate queries against the conceptual model to store-specific commands.

What is Entity Data Model?
The Entity Data Model (EDM) is a set of concepts that describe the structure of data, regardless of its stored form. The EDM borrows from the Entity-Relationship Model described by Peter Chen in 1976, but it also builds on the Entity-Relationship Model and extends its traditional uses.
The EDM addresses the challenges that arise from having data stored in many forms. For example, consider a business that stores data in relational databases, text files, XML files, spreadsheets, and reports. This presents significant challenges in data modeling, application design, and data access. When designing a data-oriented application, the challenge is to write efficient and maintainable code without sacrificing efficient data access, storage, and scalability. When data has a relational structure, data access, storage, and scalability are very efficient, but writing efficient and maintainable code becomes more difficult. When data has an object structure, the trade-offs are reversed: Writing efficient and maintainable code comes at the cost of efficient data access, storage, and scalability. Even if the right balance between these trade-offs can be found, new challenges arise when data is moved from one form to another. The Entity Data Model addresses these challenges by describing the structure of data in terms of entities and relationships that are independent of any storage schema. This makes the stored form of data irrelevant to application design and development. And, because entities and relationships describe the structure of data as it is used in an application (not its stored form), they can evolve as an application evolves.

Which are the key concepts of Entity Data Model?
The Entity Data Model (EDM) uses three key concepts to describe the structure of data: entity type, association type, and property. These are the most important concepts in describing the structure of data in any implementation of the EDM.
1. Entity Type: The entity type is the fundamental building block for describing the structure of data with the Entity Data Model. In a conceptual model, entity types are constructed from properties and describe the structure of top-level concepts, such as a customers and orders in a business application.
2. Association Type: An association type (also called an association) is the fundamental building block for describing relationships in the Entity Data Model. In a conceptual model, an association represents a relationship between two entity types (such as Customer and Order).
3. Property: Entity types contain properties that define their structure and characteristics. For example, a Customer entity type may have properties such as CustomerId, Name, and Address.

What is .edmx file and what it contains?
An .edmx file is an XML file that defines a conceptual model, a storage model, and the mapping between these models. An .edmx file also contains information that is used by the ADO.NET Entity Data Model Designer (Entity Designer) to render a model graphically.

How can you tell EF to have a different table or column name than that defined for the class?
By convention, EF defines the table and column names based on your class and property names.
You can use the [Table] and [Column] annotations to tell EF to use different names.

How do you mark a property as required? For example, For a Project, the Name is a required field.
You use the [Required] attribute to mark a property as required.

What is use of EntityDataSource Control?
The ADO.NET EntityDataSource control supports data binding scenarios in Web applications that use the ADO.NET Entity Framework. Like the Entity Framework, the control is available as part of the .NET Framework 3.5, beginning with SP1. Like the other Web server data source controls, the EntityDataSource control manages create, read, update, and delete operations against a data source on behalf of data-bound controls on the same page. The EntityDataSource works with editable grids, forms with user-controlled sorting and filtering, dually bound drop-down list controls, and master-detail pages.

What is Model First Approach?
A new Model First approach was supported in Visual Studio 2010, which was released together with the second Entity Framework version (Entity Framework v4). In Model First approach the development starts from scratch. At first, the conceptual model is created with Entity Data Model Designer, entities and relations are added to the model, but mapping is not created.
After this Generate Database Wizard is used to generate storage (SSDL) and mapping (MSL) parts from the conceptual part of the model and save them to the edmx file. Then the wizard generates DDL script for creating database (tables and foreign keys).
If the model was modified, the Generate Database Wizard should be used again to keep the model and the database consistent. In such case, the generated DDL script contains DROP statements for tables, corresponding to old SSDL from the .edmx file, and CREATE statements for tables, corresponding to new SSDL, generated by the wizard from the conceptual part. In Model First approach developer should not edit storage part or customize mapping, because they will be re-generated each time when Generate Database Wizard is launched.

What is Code First Approach?
Code First allows you to define your model using C# or VB.Net classes, optionally additional configuration can be performed using attributes on your classes and properties or by using a Fluent API. Your model can be used to generate a database schema or to map to an existing database.

What is Entity SQL?
Entity SQL is a SQL-like storage-independent language, designed to query and manipulate rich object graphs of objects based on the Entity Data Model (EDM).

What is LINQ To Entities?
LINQ to Entities provides Language-Integrated Query (LINQ) support for querying entities.
LINQ to Entities enables developers to write queries against the database using one of the supported .NET Framework programming languages such as Visual Basic or Visual C#.

What is EntityClient?
System.Data.EntityClient is a storage-independent ADO.NET data provider that contains classes such as EntityConnection, EntityCommand, and EntityDataReader. Works with Entity SQL and connects to storage specific ADO.NET data providers, such as SqlClient.

What is Deferred Loading(Lazy Loading)?
When objects are returned by a query, related objects are not loaded at the same time.
Instead they are loaded automatically when the navigation property is accessed. Also known as “lazy loading,”

What is Eager Loading?
The process of loading a specific set of related objects along with the objects that were explicitly requested in the query.

What is Complex Type?
A .NET Framework class that represents a complex property as defined in the conceptual model. Complex types enable scalar properties to be organized within entities. Complex objects are instances of complex types.

What is Conceptual Model?
An implementation of the Entity Data Model (EDM), specific to the Entity Framework, which represents an abstract specification for the data structures that define an entity-relationship representation of data in the domain of an application.

What is use of Entity Container?
Specifies entity sets and association sets that will be implemented in a specified namespace.

What is Explicit Loading?
When objects are returned by a query, related objects are not loaded at the same time. By default, they are not loaded until explicitly requested using the Load method on a navigation property.

What do you mean by Navigation Property?
A property of an entity type that represents a relationship to another entity type, as defined by an association. Navigation properties are used to return related objects as an EntityCollection or an EntityReference, depending on the multiplicity at the other end of the association.

What is scalar property?
A property of an entity that maps to a single field in the storage model.

What is split entity?
An entity type that is mapped to two separate types in the storage model.

What do you mean by table-per-hierarchy?
A method of modeling a type hierarchy in a database that includes the attributes of all the types in the hierarchy in one table.

What do you mean by table-per-type?
A method of modeling a type hierarchy in a database that uses multiple tables with one-to-one relationships to model the various types.

What is Model First Approach?
This allows you to model the entities (Eg: Employee,Student etc) in Visual Studio, write code against the model, test the complete application without having a physical database. Once you are satisfied with the application, you can then generate the schema for the underlying database right from the model.

What is Code First Approach?
Entity Framework supported the development of applications using Schema first and Model First approach. Now, the new Entity Framework introduced the Code First approach. In Code First approach, we are creating the code first and the Entity framework will automatically generate the corresponding Database as per the code.

What are the pros & cons of using Entity Framework Code-first over Model first/Database-first approaches? or Differences between code-first Vs Model-first Vs DB First approaches?
Code-First Approach:
  • It is especially popular for brand new project. The primary focus is getting the data and logic right for new projects.
  • Full control over the code (no auto generated code which is hard to modify).
 Database first Approach:
  • Very popular if you have DB designed by DBAs, developed separately or if you have existing DB.
  • You will let EF create entities for you and after modification of mapping you will generate POCO entities.
  • If you want additional features in POCO entities you must either T4 modify template or use partial classes.
 Model first Approach:
  • You will "draw" your model and let workflow to generate your database script and T4 template to generate your POCO entities. You will lose part of control on both your entities and database but for small easy projects you will be very productive.
  • Manual changes to database will be most probably lost because your model defines the database. This works better if you have Database generation power pack installed. It will allow you updating database schema (instead of recreating) or updating database projects in VS.

What is POCO in EF?
  • POCO Stands for Plain Old CLR Object. POCO's are Persistence Ignorant.  POCOs contain only data and domain logic, no persistence logic of any kind.  When your classes are persistence ignorant, they don't know anything about the data layer they are attached to and carry no dependency to said data layer.
  • A POCO is your Business Object.  It has data, validation, and any other business logic that you want to put in there. That means that if you have an Customer class it will never contain any methods that are used to get data from the db or save data to the db. You will not have an Customer.SaveCustomer() or an Customer.FetchCustomerDetailsByID() methods on a POCO.  
  • It provides separation of business logic and persistence logic, which is more in line with the Single Responsibility Principle.  The persistence logic will be implemented in repository or a data controller.

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