System.ComponentModel.DataAnnotations.Schema not found

问题

I am running into an issue in Visual Studio 2012 that involves the System.ComponentModel.DataAnnotations.Schema namespace. It tells me that the ForeignKeyAttribute cannot be resolved, the solution in the past was to add the using statement that is commented out below. VS2012 can't resolve the Schema namespace as VS2010 was able to. Has anything changed in recent .Net releases that would be causing this problem? If so, how do I work around them?

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.ComponentModel.DataAnnotations;
// using System.ComponentModel.DataAnnotations.Schema;

namespace Data
{
    public class AffiliateUser
    {
        [Key, ForeignKey("User")]
        public int UserId { get; set; }

            [StringLength(50)]
            public string AffiliateUserKey { get; set; }

            public Guid ApiKey { get; set; }
            public string PasswordHash { get; set; }
            public bool IsDeleted { get; set; }
        }
}

回答1:

Are you sure you are targeting .NET 4.5 Framework. ForeignKeyAttribute is only available in .NET 4.5

回答2:

I reinstalled the Entity Framework and it works!

回答3:

Your code and the System.ComponentModel.DataAnnotations.Schema namespace are correct.

So check your references. This one should be in Assembly System.ComponentModel.DataAnnotations.dll, v4.0.0.0

Did you upgrade the project from Fx 4?

You can fix the version with NuGet.

回答4:

You can find the assembly in the References section of the project. Click on the framework tab and search for the Assembly. This is a default assembly from the Microsoft .Net framework.

回答5:

Delete the already added reference System.ComponentModel.DataAnnotations and include it again.

If it still doesn't work, uninstall entity framework completely and reinstall it via nuget manager.

来源：https://stackoverflow.com/questions/12981617/system-componentmodel-dataannotations-schema-not-found