The type or namespace name 'DbContext' cou

问题:

I am VERY new to ASP.NET MVC (3) and am having a hard time resolving a build error in Visual Studio:

The type or namespace name 'DbContext' could not be found (are you missing a using directive or an assembly reference?)

using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;
using System.ComponentModel.DataAnnotations;
using System.Data.Entity;

namespace MyProjectName.Models
{   
    public class MachineModel
    {
        // name
        [Required]
        [Display(Name = "Nom de la machine")]
        public string Name { get; set; }

        // IP
        [Required]
        [RegularExpression(@"(?:(?:25[0-5]|2[0-4][0-9]|[01]?[0-9][0-9]?)\.){3}(?:25[0-5]|2[0-4][0-9]|[01]?[0-9][0-9]?)",
           ErrorMessage = "Donnez une adresse IPv4 valide.")]
        [Display(Name = "Adresse IP de la machine")]
        public string IP { get; set; }
    }

    public class MachineDbContext : DbContext
    {
        public DbSet<MachineModel> Machines{ get; set; }
    }
}

The two errors I am getting are:

• The type or namespace name 'DbContext' could not be found (are you missing a using directive or an assembly reference?)
• The type or namespace name 'DbSet' could not be found (are you missing a using directive or an assembly reference?)

What am I missing?

回答1:

I had the same issue. Turns out, you need the EntityFramework.dll reference (and not System.Data.Entity).

I just pulled it from the MvcMusicStore application which you can download from: http://mvcmusicstore.codeplex.com/

It's also a useful example of how to use entity framework code-first with MVC.

回答2:

You need to reference the System.Data.Entity assebmly to your project or install the EntityFramework NuGet package which will setup everything for you.

回答3:

Just a quick note. It is DbContext, not DBContext. i.e. with a lowercase 'B'. I discovered this because I had the same problem while intelesense was not working until I tried typing the full name space System.Data.Entity... and name and finally it suggested the lowercase 'b' option:-

System.Data.Entity.DbContext

回答4:

I had the same problem using VS2010. I know this isn't really an answer. I just thought it might help someone. I resolved it by using the fully qualified name for DBContext.

Instead of

public class MachineDbContext : DbContext

I used

public class MachineDbContext : System.Data.Entity.DbContext

and rebuilt the project. Suddenly VS was happy, and I was even able to remove the fully qualified name, and just use DBContext.

回答5:

I had the same issue... Installing the EF from Package Manager Console worked for me

the command was: Install-Package EntityFramework

回答6:

I had the same problem..I have VS2010 express..

(Note: If you see this problem try checking references to EntityFramework.dll .. May be it is missing.)

The following resolved it for me.

I installed latest MVC 3 Tools Update
then I installed EntityFramework 4.1
or using NUGet ie. from with Visual Studio 2010 Express (Tools->Library Package Manager -> Add library Package reference -> Select Online -> EntityFramework)

Strangely that didnt work..So i had to manually add a reference to "EntityFramework.dll"
try doing a search for the dll ..may be here
"C:\Program Files (x86)\Reference Assemblies\Microsoft\Framework.NETFramework\v4.0\"

If you already have it..just add a '.net' reference.

Note: If you use NuGet ,it creates a folder "packages" along side your Solution directory. You will find the "EntityFramework.4.1.10331.0" folder inside it.Within "Libs" folder you will find "EntityFramework.dll" .
Add reference to it using Browse tab and select the above dll.

回答7:

For step-by-step instructions, see this new MVC / EF tutorial series: http://www.asp.net/entity-framework/tutorials/creating-an-entity-framework-data-model-for-an-asp-net-mvc-application The tutorial assumes you have installed the latest MVC 3 Tools Update and provides a link in case you haven't.

回答8:

If your compiler doesn't recognize

• System.Data.Entity.Infrastructure
• DbContext
• DbSet
• et alii,

make sure

1. you have Entity Framework 4.1 installed on your machine;

2. in your .csproj file, you have the following reference

   <Reference Include="EntityFramework, Version=4.1.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089, processorArchitecture=MSIL" />
   

   within the element

   <ItemGroup>
     ...
   </ItemGroup>
   

   that houses references to other assemblies.

回答9:

I am using Visual Studio 2010 express and adding a reference to C:\Program Files\Microsoft ADO.NET Entity Framework 4.1\Binaries\EntityFramework.dll solved the problem.

回答10:

This helps really handy:

1. Select the ProjectNAme project in Solution Explorer.
2. From the Tools Menu, choose Library Package Manager which has a sub-menu.
3. From the sub-menu choose Package Manager Console.
4. At the console’s PM prompt type install-package EntityFramework then hit enter.

回答11:

Visual Studio Express SP1 Right click in Solution Explorer > References > Add Library Package Reference > EntityFramework

回答12:

Use CTP5 instead it is newer version with some changes in API. You have to add reference to EntityFramework.dll which is installed in CTP directory - default is: c:\Program Files\Microsoft ADO.NET Entity Framework Feature CTP5\Binaries\EntityFramework.dll for 32bit system. On 64bit system it will be placed in Program Files (x86).

回答13:

This happened to me when I branched code. To fix it I right-clicked my project in Visual Studio, chose manage Nu-get packages, uninstalled EntityFramework, then re-installed it. Problem solved

回答14:

I just had this issue and none of the other solutions worked for me. I'm using Visual Studio 2010 and I have two projects in my solution: UIProject and DataAccessProject. The UIProject has a reference to the DataAccessProject.

My UIProject was targeting the .Net Framework 4 but my DataAccessProject was targeting the .Net Framework 3.5 for some reason.

• I changed both projects to target .Net Framework 4
• Uninstalled then re-installed the Entity Framework NuGet Package.

Problems solved. Now I have references to both EntityFramework and System.Data.Entity

回答15:

i had the same problem.I had to rewrite "DBContext" after adding reference to entityframework and it resolved the problem.
its means that Visual studio was not able to recognize the class automatically and waited for it to be re-typed. so its best to add reference first and then implement code. VS 2013 - ultimate used

回答16:

I had to first uninstall EntityFramework(Uninstall-package EntityFramework (not case sensitive apparently)), then install it again(Install-package EntityFramework), via the PM Console

回答17:

Your project unable to resolve EntityFramework classes until you not added it in your project. For adding EntityFramework support you have to follow this steps: Tools->Nuget Package Manager ->Manage Nuget package for solution browse EntityFramework It shows latest stable EntityFramework version. currently 6.1.3 is latest version Install it for the selected project.

回答18:

Download http://www.dll-found.com/download/e/EntityFramework.dll

Paste it in (for x86)

C:\Program Files (x86)\Reference Assemblies\Microsoft\Framework\.NETFramework\v4.0\

Then Right click on project -> add reference -> select EntityFramework

Bingo......

回答19:

I had this problem, read the above answer and download the entityframework.ddl but found that it is alreadt referenced. So I added the namespace and problem was solved

using System.Data.Entity;

I am using Visual Studio 2010, SP1 installed

回答20:

having referenced entityframework.dll both system.data.entity worked.

回答21:

I had the same error but the problem was just an accidental problem with my model.

I accidentaly put...

public class MyModelDBContext : Context
{
 public DBSet<MyModel> MyModels { get; set; }
}

...inside of the model class.

回答22:

As alternative way you can go HERE - instruction how to install any required dll.

Or you can download NuGet and manage it from VS

回答23:

Right click your reference and go to manage NuGet packages, then choose online all, then NuGet package source in the search textbox type Entity Framework and install it.

回答24:

1) Uninstalling Entity Framework from All projects

2) Restart Visual Studio

3) Reinstalling to all required projects

and it started working

回答25:

There might be a case where you reference everything you need to and you can even Go to Definition by pressing F12 on DbContext class which takes you to System.Data.Entity namespace but you still get this nasty compiler warning. Make sure that the Target Framework of your project and that of the Entity Framework version you are using match. Go to Project->Properties->Application Tab. Change the Target Framework(can't exactly say to which one, trial and error will help.). Just my two cents.

回答26:

I also get irritated from this issue many times and finally find the solution.Go to edmx file->Update Model from Database->Refresh->Finish. Clean Solution->Rebuild Solution

回答27:

you can try on package manager console

PM> EntityFrameWork\enable-migrations

回答28:

Like the others have suggested:

1. Add the correct references and directives. But it still doesn't work? Maybe you have the same problem I did:

Have a look below and see if you can tell me what is wrong:

public class PanelLengthContext : DBContext { } ??!

Make sure the class name is not misspelt - (case sensitivity)!

• DbContext is the correct spelling.
• this is how it should look:
• check the spelling. don't waste 20 min of your life like i did. public class PanelLengthContext : DbContext {}

HTH

回答29:

I have the same issue as you, I'm unable to implement it in the Controller class while it works when I put it in the model class. Add these codes on the top of your controller class

 using TimeSheetManagementSystem.Data;
 using Microsoft.Extensions.Configuration;
 using Microsoft.AspNetCore.Identity.EntityFrameworkCore;
 using Microsoft.EntityFrameworkCore;
 using Newtonsoft.Json;

回答30:

I have faced same issue and then i have add the namespace

using Practiceusingentityframework.Models;

After Adding it's Working fine

Try this may help you to solve your problem

and before that install the entity framework from Nuget Manager