entity-framework interview questions
Top entity-framework frequently asked interview questions
I have a one entity framework object and when I add it to my project, the connectionstring
is added to app.config in the connectionstring section, but when I want to create new entitycontext and use this connectionstring, this error appears
Source: (StackOverflow)
Now that .NET v3.5 SP1 has been released (along with VS2008 SP1), we now have access to the .NET entity framework.
My question is this. When trying to decide between using the Entity Framework and LINQ to SQL as an ORM, what's the difference?
The way I understand it, the Entity Framework (when used with LINQ to Entities) is a 'big brother' to LINQ to SQL? If this is the case - what advantages does it have? What can it do that LINQ to SQL can't do on its own?
Source: (StackOverflow)
I am having this error when seeding my database with code first approach.
Validation failed for one or more entities. See 'EntityValidationErrors' property for more details.
To be honest I don't know how to check the content of the validation errors, Visual Studio shows me that its an array with 8 objects. so 8 validation Errors.
This was working with my previous model, but I made a few changes that I explain below:
- I had an enum called Status, I changed it to a class called Status
- I changed the class ApplicantsPositionHistory to have 2 foreign key to the same table
Excuse me for the long code, but I have to paste it all. The exception is thrown in the last line of the following code.
namespace Data.Model
{
public class Position
{
[DatabaseGenerated(System.ComponentModel.DataAnnotations.DatabaseGeneratedOption.Identity)]
public int PositionID { get; set; }
[Required(ErrorMessage = "Position name is required.")]
[StringLength(20, MinimumLength = 3, ErrorMessage = "Name should not be longer than 20 characters.")]
[Display(Name = "Position name")]
public string name { get; set; }
[Required(ErrorMessage = "Number of years is required")]
[Display(Name = "Number of years")]
public int yearsExperienceRequired { get; set; }
public virtual ICollection<ApplicantPosition> applicantPosition { get; set; }
}
public class Applicant
{
[DatabaseGenerated(System.ComponentModel.DataAnnotations.DatabaseGeneratedOption.Identity)]
public int ApplicantID { get; set; }
[Required(ErrorMessage = "Name is required")]
[StringLength(20, MinimumLength = 3, ErrorMessage="Name should not be longer than 20 characters.")]
[Display(Name = "First and LastName")]
public string name { get; set; }
[Required(ErrorMessage = "Telephone number is required")]
[StringLength(10, MinimumLength = 3, ErrorMessage = "Telephone should not be longer than 20 characters.")]
[Display(Name = "Telephone Number")]
public string telephone { get; set; }
[Required(ErrorMessage = "Skype username is required")]
[StringLength(10, MinimumLength = 3, ErrorMessage = "Skype user should not be longer than 20 characters.")]
[Display(Name = "Skype Username")]
public string skypeuser { get; set; }
public byte[] photo { get; set; }
public virtual ICollection<ApplicantPosition> applicantPosition { get; set; }
}
public class ApplicantPosition
{
[Key]
[Column("ApplicantID", Order = 0)]
public int ApplicantID { get; set; }
[Key]
[Column("PositionID", Order = 1)]
public int PositionID { get; set; }
public virtual Position Position { get; set; }
public virtual Applicant Applicant { get; set; }
[Required(ErrorMessage = "Applied date is required")]
[DisplayFormat(DataFormatString = "{0:d}", ApplyFormatInEditMode = true)]
[Display(Name = "Date applied")]
public DateTime appliedDate { get; set; }
[Column("StatusID", Order = 0)]
public int StatusID { get; set; }
public Status CurrentStatus { get; set; }
//[NotMapped]
//public int numberOfApplicantsApplied
//{
// get
// {
// int query =
// (from ap in Position
// where ap.Status == (int)Status.Applied
// select ap
// ).Count();
// return query;
// }
//}
}
public class Address
{
[StringLength(20, MinimumLength = 3, ErrorMessage = "Country should not be longer than 20 characters.")]
public string Country { get; set; }
[StringLength(20, MinimumLength = 3, ErrorMessage = "City should not be longer than 20 characters.")]
public string City { get; set; }
[StringLength(50, MinimumLength = 3, ErrorMessage = "Address should not be longer than 50 characters.")]
[Display(Name = "Address Line 1")]
public string AddressLine1 { get; set; }
[Display(Name = "Address Line 2")]
public string AddressLine2 { get; set; }
}
public class ApplicationPositionHistory
{
[DatabaseGenerated(System.ComponentModel.DataAnnotations.DatabaseGeneratedOption.Identity)]
public int ApplicationPositionHistoryID { get; set; }
public ApplicantPosition applicantPosition { get; set; }
[Column("oldStatusID")]
public int oldStatusID { get; set; }
[Column("newStatusID")]
public int newStatusID { get; set; }
public Status oldStatus { get; set; }
public Status newStatus { get; set; }
[StringLength(500, MinimumLength = 3, ErrorMessage = "Comments should not be longer than 500 characters.")]
[Display(Name = "Comments")]
public string comments { get; set; }
[DisplayFormat(DataFormatString = "{0:d}", ApplyFormatInEditMode = true)]
[Display(Name = "Date")]
public DateTime dateModified { get; set; }
}
public class Status
{
[DatabaseGenerated(System.ComponentModel.DataAnnotations.DatabaseGeneratedOption.Identity)]
public int StatusID { get; set; }
[StringLength(20, MinimumLength = 3, ErrorMessage = "Status should not be longer than 20 characters.")]
[Display(Name = "Status")]
public string status { get; set; }
}
}
using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Data.Entity;
using System.IO;
namespace Data.Model
{
public class HRContextInitializer : DropCreateDatabaseAlways<HRContext>
{
protected override void Seed(HRContext context)
{
#region Status
Status applied = new Status() { status = "Applied" };
Status reviewedByHR = new Status() { status = "Reviewed By HR" };
Status approvedByHR = new Status() { status = "Approved by HR" };
Status rejectedByHR = new Status() { status = "Rejected by HR" };
Status assignedToTechnicalDepartment = new Status() { status = "Assigned to Technical Department" };
Status approvedByTechnicalDepartment = new Status() { status = "Approved by Technical Department" };
Status rejectedByTechnicalDepartment = new Status() { status = "Rejected by Technical Department" };
Status assignedToGeneralManager = new Status() { status = "Assigned to General Manager" };
Status approvedByGeneralManager = new Status() { status = "Approved by General Manager" };
Status rejectedByGeneralManager = new Status() { status = "Rejected by General Manager" };
context.Status.Add(applied);
context.Status.Add(reviewedByHR);
context.Status.Add(approvedByHR);
context.Status.Add(rejectedByHR);
context.Status.Add(assignedToTechnicalDepartment);
context.Status.Add(approvedByTechnicalDepartment);
context.Status.Add(rejectedByTechnicalDepartment);
context.Status.Add(assignedToGeneralManager);
context.Status.Add(approvedByGeneralManager);
context.Status.Add(rejectedByGeneralManager);
#endregion
#region Position
Position netdeveloper = new Position() { name = ".net developer", yearsExperienceRequired = 5 };
Position javadeveloper = new Position() { name = "java developer", yearsExperienceRequired = 5 };
context.Positions.Add(netdeveloper);
context.Positions.Add(javadeveloper);
#endregion
#region Applicants
Applicant luis = new Applicant()
{
name = "Luis",
skypeuser = "le.valencia",
telephone = "0491732825",
photo = File.ReadAllBytes(@"C:\Users\LUIS.SIMBIOS\Documents\Visual Studio 2010\Projects\SlnHR\HRRazorForms\Content\pictures\1.jpg")
};
Applicant john = new Applicant()
{
name = "John",
skypeuser = "jo.valencia",
telephone = "3435343543",
photo = File.ReadAllBytes(@"C:\Users\LUIS.SIMBIOS\Documents\Visual Studio 2010\Projects\SlnHR\HRRazorForms\Content\pictures\2.jpg")
};
context.Applicants.Add(luis);
context.Applicants.Add(john);
#endregion
#region ApplicantsPositions
ApplicantPosition appicantposition = new ApplicantPosition()
{
Applicant = luis,
Position = netdeveloper,
appliedDate = DateTime.Today,
StatusID = 1
};
ApplicantPosition appicantposition2 = new ApplicantPosition()
{
Applicant = john,
Position = javadeveloper,
appliedDate = DateTime.Today,
StatusID = 1
};
context.ApplicantsPositions.Add(appicantposition);
context.ApplicantsPositions.Add(appicantposition2);
#endregion
context.SaveChanges(); --->> Error here
}
}
}
Source: (StackOverflow)
I am currently getting this error:
System.Data.SqlClient.SqlException: New transaction is not allowed because there are other threads running in the session.
while running this code:
public class ProductManager : IProductManager
{
#region Declare Models
private RivWorks.Model.Negotiation.RIV_Entities _dbRiv = RivWorks.Model.Stores.RivEntities(AppSettings.RivWorkEntities_connString);
private RivWorks.Model.NegotiationAutos.RivFeedsEntities _dbFeed = RivWorks.Model.Stores.FeedEntities(AppSettings.FeedAutosEntities_connString);
#endregion
public IProduct GetProductById(Guid productId)
{
// Do a quick sync of the feeds...
SyncFeeds();
...
// get a product...
...
return product;
}
private void SyncFeeds()
{
bool found = false;
string feedSource = "AUTO";
switch (feedSource) // companyFeedDetail.FeedSourceTable.ToUpper())
{
case "AUTO":
var clientList = from a in _dbFeed.Client.Include("Auto") select a;
foreach (RivWorks.Model.NegotiationAutos.Client client in clientList)
{
var companyFeedDetailList = from a in _dbRiv.AutoNegotiationDetails where a.ClientID == client.ClientID select a;
foreach (RivWorks.Model.Negotiation.AutoNegotiationDetails companyFeedDetail in companyFeedDetailList)
{
if (companyFeedDetail.FeedSourceTable.ToUpper() == "AUTO")
{
var company = (from a in _dbRiv.Company.Include("Product") where a.CompanyId == companyFeedDetail.CompanyId select a).First();
foreach (RivWorks.Model.NegotiationAutos.Auto sourceProduct in client.Auto)
{
foreach (RivWorks.Model.Negotiation.Product targetProduct in company.Product)
{
if (targetProduct.alternateProductID == sourceProduct.AutoID)
{
found = true;
break;
}
}
if (!found)
{
var newProduct = new RivWorks.Model.Negotiation.Product();
newProduct.alternateProductID = sourceProduct.AutoID;
newProduct.isFromFeed = true;
newProduct.isDeleted = false;
newProduct.SKU = sourceProduct.StockNumber;
company.Product.Add(newProduct);
}
}
_dbRiv.SaveChanges(); // ### THIS BREAKS ### //
}
}
}
break;
}
}
}
Model #1 - This model sits in a database on our Dev Server.
Model #2 - This model sits in a database on our Prod Server and is updated each day by automatic feeds. 
Note - The red circled items in Model #1 are the fields I use to "map" to Model #2. Please ignore the red circles in Model #2: that is from another question I had which is now answered.
Note: I still need to put in an isDeleted check so I can soft delete it from DB1 if it has gone out of our client's inventory.
All I want to do, with this particular code, is connect a company in DB1 with a client in DB2, get their product list from DB2 and INSERT it in DB1 if it is not already there. First time through should be a full pull of inventory. Each time it is run there after nothing should happen unless new inventory came in on the feed over night.
So the big question - how to I solve the transaction error I am getting? Do I need to drop and recreate my context each time through the loops (does not make sense to me)?
Source: (StackOverflow)
I have a problem with Entity Framework in Asp.net. I want to get the Id value whenever I add an object to database. How can I do this?
Source: (StackOverflow)
I am trying to run my tests on TeamCity which is currently installed on my machine.
System.InvalidOperationException: System.InvalidOperationException:
The Entity Framework provider type 'System.Data.Entity.SqlServer.SqlProviderServices, EntityFramework.SqlServer, Version=6.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089' for the 'System.Data.SqlClient' ADO.NET provider could not be loaded.
Make sure the provider assembly is available to the running application.
See http://go.microsoft.com/fwlink/?LinkId=260882 for more information..
I have no reference to System.Data.Entity in any of my projects as was suggested on codeplex for upgrading to EF6.
So, I am not sure why am I getting this exception.
I do not get any such exception when I run the tests from VS.
I did try to set CopyLocal to false then again to true.. but that does not seem to work either.
Update
My app.config has the following . Does this cause some behavior that I don't understand ?
<?xml version="1.0" encoding="utf-8"?>
<configuration>
<configSections>
<section name="entityFramework" type="System.Data.Entity.Internal.ConfigFile.EntityFrameworkSection, EntityFramework, Version=6.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089" requirePermission="false" />
<!-- For more information on Entity Framework configuration, visit http://go.microsoft.com/fwlink/?LinkID=237468 -->
</configSections>
<entityFramework>
<defaultConnectionFactory type="System.Data.Entity.Infrastructure.SqlConnectionFactory, EntityFramework" />
</entityFramework>
</configuration>
I get the following stacktrace in teamcity .
[MSTest] IntegrationTests.CrudTest+QuestionTest.Create
[03:59:11][IntegrationTests.CrudTest+QuestionTest.Create] Initialization method IntegrationTests.CrudTest+QuestionTest.Initialize threw exception. System.InvalidOperationException: System.InvalidOperationException: The Entity Framework provider type 'System.Data.Entity.SqlServer.SqlProviderServices, EntityFramework.SqlServer, Version=6.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089' for the 'System.Data.SqlClient' ADO.NET provider could not be loaded. Make sure the provider assembly is available to the running application. See http://go.microsoft.com/fwlink/?LinkId=260882 for more information..
[03:59:11]
[IntegrationTests.CrudTest+QuestionTest.Create] at System.Data.Entity.Config.ProviderServicesFactory.GetInstance(String providerTypeName, String providerInvariantName)
at System.Data.Entity.Config.ProviderServicesFactory.GetInstanceByConvention(String providerInvariantName)
at System.Data.Entity.Config.DefaultProviderServicesResolver.GetService(Type type, Object key)
at System.Data.Entity.Config.CachingDependencyResolver.<>c__DisplayClass1.<GetService>b__0(Tuple`2 k)
at System.Collections.Concurrent.ConcurrentDictionary`2.GetOrAdd(TKey key, Func`2 valueFactory)
at System.Data.Entity.Config.CachingDependencyResolver.GetService(Type type, Object key)
at System.Data.Entity.Config.ResolverChain.<>c__DisplayClass3.<GetService>b__0(IDbDependencyResolver r)
at System.Linq.Enumerable.WhereSelectArrayIterator`2.MoveNext()
at System.Linq.Enumerable.FirstOrDefault[TSource](IEnumerable`1 source, Func`2 predicate)
at System.Data.Entity.Config.ResolverChain.GetService(Type type, Object key)
at System.Data.Entity.Config.RootDependencyResolver.GetService(Type type, Object key)
at System.Data.Entity.Config.ResolverChain.<>c__DisplayClass3.<GetService>b__0(IDbDependencyResolver r)
at System.Linq.Enumerable.WhereSelectArrayIterator`2.MoveNext()
at System.Linq.Enumerable.FirstOrDefault[TSource](IEnumerable`1 source, Func`2 predicate)
at System.Data.Entity.Config.ResolverChain.GetService(Type type, Object key)
at System.Data.Entity.Config.CompositeResolver`2.GetService(Type type, Object key)
at System.Data.Entity.Config.IDbDependencyResolverExtensions.GetService[T](IDbDependencyResolver resolver, Object key)
at System.Data.Entity.Config.InternalConfiguration.GetService[TService](Object key)
at System.Data.Entity.Config.DbConfiguration.GetService[TService](Object key)
at System.Data.Entity.Utilities.DbProviderFactoryExtensions.GetProviderServices(DbProviderFactory factory)
at System.Data.Entity.Infrastructure.DefaultManifestTokenService.GetProviderManifestToken(DbConnection connection)
at System.Data.Entity.Utilities.DbConnectionExtensions.GetProviderInfo(DbConnection connection, DbProviderManifest& providerManifest)
at System.Data.Entity.DbModelBuilder.Build(DbConnection providerConnection)
at System.Data.Entity.Internal.LazyInternalContext.CreateModel(LazyInternalContext internalContext)
at System.Data.Entity.Internal.RetryLazy`2.GetValue(TInput input)
at System.Data.Entity.Internal.LazyInternalContext.InitializeContext()
at System.Data.Entity.Internal.InternalContext.GetEntitySetAndBaseTypeForType(Type entityType)
at System.Data.Entity.Internal.Linq.InternalSet`1.Initialize()
at System.Data.Entity.Internal.Linq.InternalSet`1.get_InternalContext()
at System.Data.Entity.Internal.Linq.InternalSet`1.ActOnSet(Action action, EntityState newState, Object entity, String methodName)
at System.Data.Entity.Internal.Linq.InternalSet`1.Add(Object entity)
at System.Data.Entity.DbSet`1.Add(TEntity entity)
at EFRepository.Infrastructure.EFRepository`1.Add(T item) in c:\TeamCity\buildAgent\work\da2ea4e72c0e77f0\Repository\Infrastructure\EFRepository.cs:line 22
at IntegrationTests.CrudTest.Initialize() in c:\TeamCity\buildAgent\work\da2ea4e72c0e77f0\IntegrationTests\CrudTest.cs:line 34
Source: (StackOverflow)
I've mucked up my migrations, I used IgnoreChanges on the initial migration, but now I want to delete all my migrations and start with an initial migration with all of the logic.
When I delete the migrations in the folder and try and Add-Migration it doesn't generate a full file (it's empty - because I haven't made any changes since my last, but now deleted, migration).
Is there a Disable-Migrations command, so I can rerun Enable-Migrations?
Source: (StackOverflow)
How do I view the SQL generated by entity framework ?
(In my particular case I'm using the mysql provider - if it matters)
Source: (StackOverflow)
All of a sudden I keep getting a MetadataException on instantiating my generated ObjectContext class. The connection string in App.Config looks correct - hasn't changed since last it worked - and I've tried regenerating a new model (edmx-file) from the underlying database with no change.
Anyone have any ideas?
Further details: I haven't changed any properties, I haven't changed the name of any output assemblies, I haven't tried to embed the EDMX in the assembly. I've merely waited 10 hours from leaving work until I got back. And then it wasn't working anymore.
I've tried recreating the EDMX. I've tried recreating the project. I've even tried recreating the database, from scratch. No luck, whatsoever.
Source: (StackOverflow)
What are the pros & cons of using Entity Framework 4.1 Code-first over Model/Database-first with EDMX diagram?
I'm trying to fully understand all the approaches to building data access layer using EF 4.1. I'm using Repository pattern and IoC.
I know I can use code-first approach: define my entities and context by hand and use ModelBuilder to fine-tune the schema.
I can also create an EDMX diagram and choose a code generation step that uses T4 templates to generate the same POCO classes.
In both cases I end up with POCO object which are ORM agnostic and context that derives from DbContext.
Database-first seems to be most appealing since I can design database in Enterprise Manager, quickly synch the model and fine-tune it using the designer.
So what is the difference between those two approaches? Is it just about the preference VS2010 vs Enterprise Manager?
Source: (StackOverflow)
How would you rate each of them in terms of:
- Performance
- Speed of development
- Neat, intuitive, maintainable code
- Flexibility
- Overall
I like my SQL and so have always been a die-hard fan of ADO.NET and stored procedures but I recently had a play with Linq to SQL and was blown away by how quickly I was writing out my DataAccess layer and have decided to spend some time really understanding either Linq to SQL or EF... or neither?
I just want to check, that there isn't a great flaw in any of these technologies that would render my research time useless. E.g. performance is terrible, it's cool for simple apps but can only take you so far.
Update:
Can you concentrate on EF VS L2S VS SPs rather than ORM VS SPs. I'm mainly interested by EF VS L2S. But am keen to have them compared against stored procs too since plain SQl is something I know a lot about.
Source: (StackOverflow)
I am using Entity Framework 1 with .net 3.5.
I am doing something simple like this:
var RoomDetails = context.Rooms.ToList();
foreach (var Room in Rooms)
{
Room.LastUpdated = DateTime.Now;
}
I am getting this error when I try to do:
context.SaveChanges();
I get the error:
Unable to update the EntitySet - because it has a DefiningQuery and no <UpdateFunction> element exists in the <ModificationFunctionMapping> element to support the current operation.
I am doing lots of updates on the context and not having any issues, it's only when I try to update this particular entity.
All my searching shows up the same thing, that there is no primary key declared on the entity that I'm trying to update. But alas, I do have a Primary key declared...
Source: (StackOverflow)
I want to save my Edit to Database and I am using Entity FrameWork Code-First in ASP.NET MVC 3 / C# but I am getting errors. In my Event class, I have DateTime and TimeSpan datatypes but in my database, I've got Date and time respectively. Could this be the reason? How can I cast to the appropriate datatype in the code before saving changes to database.
public class Event
{
public int EventId { get; set; }
public int CategoryId { get; set; }
public int PlaceId { get; set; }
public string Title { get; set; }
public decimal Price { get; set; }
public DateTime EventDate { get; set; }
public TimeSpan StartTime { get; set; }
public TimeSpan EndTime { get; set; }
public string Description { get; set; }
public string EventPlaceUrl { get; set; }
public Category Category { get; set; }
public Place Place { get; set; }
}
Method in the controller >>>> Problem at storeDB.SaveChanges();
// POST: /EventManager/Edit/386
[HttpPost]
public ActionResult Edit(int id, FormCollection collection)
{
var theEvent = storeDB.Events.Find(id);
if (TryUpdateModel(theEvent))
{
storeDB.SaveChanges();
return RedirectToAction("Index");
}
else
{
ViewBag.Categories = storeDB.Categories.OrderBy(g => g.Name).ToList();
ViewBag.Places = storeDB.Places.OrderBy(a => a.Name).ToList();
return View(theEvent);
}
}
with
public class EventCalendarEntities : DbContext
{
public DbSet<Event> Events { get; set; }
public DbSet<Category> Categories { get; set; }
public DbSet<Place> Places { get; set; }
}
SQL Server 2008 R2 Database / T-SQL
EventDate (Datatype = date)
StartTime (Datatype = time)
EndTime (Datatype = time)
Http Form
EventDate (Datatype = DateTime) e.g. 4/8/2011 12:00:00 AM
StartTime (Datatype = Timespan/time not sure) e.g. 08:30:00
EndTime (Datatype = Timespan/time not sure) e.g. 09:00:00
Server Error in '/' Application.
Validation failed for one or more entities. See 'EntityValidationErrors' property for more details.
Description: An unhandled exception occurred during the execution of the current web request. Please review the stack trace for more information about the error and where it originated in the code.
Exception Details: System.Data.Entity.Validation.DbEntityValidationException: Validation failed for one or more entities. See 'EntityValidationErrors' property for more details.
Source Error:
Line 75: if (TryUpdateModel(theEvent))
Line 76: {
Line 77: storeDB.SaveChanges();
Line 78: return RedirectToAction("Index");
Line 79: }
Source File: C:\sep\MvcEventCalendar\MvcEventCalendar\Controllers\EventManagerController.cs Line: 77
Stack Trace:
[DbEntityValidationException: Validation failed for one or more entities. See 'EntityValidationErrors' property for more details.]
Source: (StackOverflow)
I have been investigating transactions and it appears that they take care of themselves in EF as long as I pass false to SaveChanges() and then call AcceptAllChanges() if there are no errors:
SaveChanges(false);
// ...
AcceptAllChanges();
What if something goes bad? don't I have to rollback or, as soon as my method goes out of scope, is the transaction ended?
What happens to any indentiy columns that were assigned half way through the transaction? I presume if somebody else added a record after mine before mine went bad then this means there will be a missing Identity value.
Is there any reason to use the standard TransactionScope class in my code?
Source: (StackOverflow)
Can't find anything relevant about Entity Framework/MySQL on Google so I'm hoping someone knows about it.
Source: (StackOverflow)