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When you read this question, you think it is a repeated question and that I did not do enough search for finding the solution of this issue. Unfortunately, I spent many hours searching for the solution for this issue. I am developing an ASP.NET Core 2.1 application, that consists mainly from ASP.NET Web APP and Entity Framework Core.

I am using the code first approach for adding migrations and updating database, and everything looks working fine, until I separated the asp.net core web app from the Entity Framework Core. Actually, I put the EntityFrameworkCore in separated class library.
When I tried to add new migration using

dotnet ef migrations add InitialCreate

I had this error

An error occurred while accessing the IWebHost on class 'Program'. Continuing without the application service provider. Error: Could not load type 'EF.EFCore.ApplicationDbContext' from assembly 'ef, Version=2.1.4.0, Culture=neutral, PublicKeyToken=adb9793829ddae60'.
System.TypeLoadException: Could not load type 'EF.EFCore.ApplicationDbContext' from assembly 'EF, Version=1.0.0.0, Culture=neutral, PublicKeyToken=null'.
at System.Reflection.CustomAttribute._CreateCaObject(RuntimeModule pModule, IRuntimeMethodInfo pCtor, Byte** ppBlob, Byte* pEndBlob, Int32* pcNamedArgs)
at System.Reflection.CustomAttribute.CreateCaObject(RuntimeModule module, IRuntimeMethodInfo ctor, IntPtr& blob, IntPtr blobEnd, Int32& namedArgs)
at System.Reflection.CustomAttribute.GetCustomAttributes(RuntimeModule decoratedModule, Int32 decoratedMetadataToken, Int32 pcaCount, RuntimeType attributeFilterType, Boolean mustBeInheritable, IList derivedAttributes)
at System.Reflection.CustomAttribute.GetCustomAttributes(RuntimeType type, RuntimeType caType, Boolean inherit)
at System.Attribute.GetCustomAttributes(MemberInfo element, Type type, Boolean inherit)
at System.Attribute.GetCustomAttribute(MemberInfo element, Type attributeType, Boolean inherit)
at System.Reflection.CustomAttributeExtensions.GetCustomAttribute[T](MemberInfo element)
at Microsoft.EntityFrameworkCore.Design.Internal.DbContextOperations.<>c.b__12_5(TypeInfo t)
at System.Linq.Enumerable.WhereSelectListIterator2.MoveNext()
at System.Linq.Enumerable.WhereEnumerableIterator1.MoveNext()
at System.Linq.Enumerable.ConcatIterator1.MoveNext()
at System.Linq.Enumerable.DistinctIterator1.MoveNext()
at System.Linq.Enumerable.WhereEnumerableIterator1.MoveNext()
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.AddMigration.<>c__DisplayClass0_1.<.ctor>b__0()
at Microsoft.EntityFrameworkCore.Design.OperationExecutor.OperationBase.<>c__DisplayClass3_01.b__0()
at Microsoft.EntityFrameworkCore.Design.OperationExecutor.OperationBase.Execute(Action action)
Could not load type 'EF.EFCore.ApplicationDbContext' from assembly 'EF, Version=1.0.0.0, Culture=neutral, PublicKeyToken=null'.

I am sure the problem is related to separation of the project, into component form

services.AddDbContext<ApplicationDbContext>(options =>
 options.UseSqlServer(conString,builder => builder.MigrationsAssembly(typeof(Startup).Assembly.FullName)));

the above code is the code I am using for my ApplicationDBContext class that derived form IdentityContext, either add or remove MigrationAssembly this does not make a different. Also, adding IDbContextFactory does not make a big deal
I changed the ApplicationDbContext, that is derived from IdentityContext to the following

protected override void OnConfiguring(DbContextOptionsBuilder optionsBuilder)

 // Entity
 string conString = "Server=(local); Database=OnlineStoreDB;User Id=sa;Password=123@asu;";

 optionsBuilder.UseSqlServer(conString, x => x.MigrationsAssembly("EF"));

 base.OnConfiguring(optionsBuilder);
 // base.OnConfiguring(optionsBuilder);
 //if (!optionsBuilder.IsConfigured)
 //
 // optionsBuilder.UseSqlServer(conString);
 //
 //else
 //
 // optionsBuilder.UseSqlServer(conString);
 //

Our approach was to create a small helper-classlibrary with the following content:

public class ApiDbContextMigrationsFactory : IDesignTimeDbContextFactory<ApiDbContext>

 public ApiDbContext CreateDbContext(string args)
 
 var optionsBuilder = new DbContextOptionsBuilder<ApiDbContext>();
 optionsBuilder.UseSqlServer(@"Server=(localdb)mssqllocaldb;Database=Api.Local.Migration;Trusted_Connection=True;ConnectRetryCount=0",
 b => b.MigrationsAssembly(typeof(ApiDbContext).Assembly.FullName));

 return new ApiDbContext(optionsBuilder.Options);

 

And then we create the migration from inside the HelperForMigrations-Library-Folder with the following command:

dotnet ef migrations add nameofMigration --startup-project ./ --project ../ApiDbContextProject --context ApiDbContext