Entity Framework Core Add-Migration引发“已添加具有相同键的项。键：MyProject.Model.MyContext

Question

我的项目中有一个带有EF Core的SQL Server数据库，我想在另一个表中添加一个表和一个列。但是，当我在PM控制台中运行Add-Migration时，会出现此错误

An item with the same key has already been added. Key: MyProject.Model.MyContext

其中MyProject是我的asp.net核心项目，MyContext是我的DbContext类。

我的DbContext类看起来像

namespace MyProject.Model
{
    public class MyContext : DbContext
    {
        public MyWorker Worker { get; private set; }

        /// <summary>
        /// initialize the worker instance to get the crud methods of the database
        /// </summary>
        public MyContext()
        {
            Worker = new MyWorker(this);
        }

        public static string GetConnectionString()
        {
            return Startup.ConnectionString;
        }

        /// <summary>
        /// Our own context must override the OnConfiguring Method from Ef Core to set the right connection
        /// Connectionstring is set in appsettings.json
        /// </summary>
        /// <param name="_builder"></param>
        protected override void OnConfiguring(DbContextOptionsBuilder _builder)
        {
            _builder.UseSqlServer(GetConnectionString());
        }

        protected override void OnModelCreating(ModelBuilder _modelBuilder)        {
            _modelBuilder.Entity<User>().HasIndex(u => u.MailAdress).IsUnique();
        }

        public DbSet<Project>  Projects  { get; set; }
        public DbSet<Document> Documents { get; set; }
        public DbSet<DocData>  DocsData  { get; set; }
        public DbSet<User>     Users     { get; set; }
        public DbSet<UCLSystem>   Systems   { get; set; }
    }


    public class DesignTimeContextFactory : IDesignTimeDbContextFactory<MyContext>
    {
        public MyContext CreateDbContext(string[] args)
        {
            IConfiguration configuration = new ConfigurationBuilder()
                .SetBasePath(Directory.GetCurrentDirectory())
                .AddJsonFile("appsettings.json")
                .Build();

            var builder = new DbContextOptionsBuilder<MyContext>();

            var connectionString = configuration["Connectionstrings:MyConnection"];

            return new MyContext();
        }
    }
}

要添加的表是Systems表，要添加的两列在documents表中。

使用-verbose标志，我得到以下输出

PM> Add-Migration Update -verbose
Using project 'MyProject'.
Using startup project 'MyProject'.
Build started...
Build succeeded.
C:\Program Files\dotnet\dotnet.exe exec --depsfile D:\Development\Project\MyProject\MyProject\bin\Debug\netcoreapp2.2\MyProject.deps.json --additionalprobingpath C:\Users\chris\.nuget\packages --additionalprobingpath "C:\Program Files\dotnet\sdk\NuGetFallbackFolder" --runtimeconfig D:\Development\Project\MyProject\MyProject\bin\Debug\netcoreapp2.2\MyProject.runtimeconfig.json C:\Users\chris\.nuget\packages\microsoft.entityframeworkcore.tools\2.2.4\tools\netcoreapp2.0\any\ef.dll migrations add Update --json --verbose --no-color --prefix-output --assembly D:\Development\Project\MyProject\MyProject\bin\Debug\netcoreapp2.2\MyProject.dll --startup-assembly D:\Development\Project\MyProject\MyProject\bin\Debug\netcoreapp2.2\MyProject.dll --project-dir D:\Development\Project\MyProject\MyProject\ --language C# --working-dir D:\Development\Project\MyProject --root-namespace MyProject
Using assembly 'MyProject'.
Using startup assembly 'MyProject'.
Using application base 'D:\Development\Project\MyProject\MyProject\bin\Debug\netcoreapp2.2'.
Using working directory 'D:\Development\Project\MyProject\MyProject'.
Using root namespace 'MyProject'.
Using project directory 'D:\Development\Project\MyProject\MyProject\'.
Finding DbContext classes...
Finding IDesignTimeDbContextFactory implementations...
Found IDesignTimeDbContextFactory implementation 'DesignTimeContextFactory'.
Found DbContext 'MyContext'.
Found IDesignTimeDbContextFactory implementation 'DesignTimeContextFactory'.
Found DbContext 'MyContext'.
System.ArgumentException: An item with the same key has already been added. Key: MyProject.Model.MyContext
   at System.Collections.Generic.Dictionary`2.TryInsert(TKey key, TValue value, InsertionBehavior behavior)
   at Microsoft.EntityFrameworkCore.Design.Internal.DbContextOperations.FindContextTypes()
   at Microsoft.EntityFrameworkCore.Design.Internal.DbContextOperations.FindContextType(String name)
   at Microsoft.EntityFrameworkCore.Design.Internal.DbContextOperations.CreateContext(String contextType)
   at Microsoft.EntityFrameworkCore.Design.Internal.MigrationsOperations.AddMigration(String name, String outputDir, String contextType)
   at Microsoft.EntityFrameworkCore.Design.OperationExecutor.AddMigrationImpl(String name, String outputDir, String contextType)
   at Microsoft.EntityFrameworkCore.Design.OperationExecutor.OperationBase.<>c__DisplayClass3_0`1.<Execute>b__0()
   at Microsoft.EntityFrameworkCore.Design.OperationExecutor.OperationBase.Execute(Action action)
An item with the same key has already been added. Key: MyProject.Model.MyContext