public ActionResult EditSave(IEnumerable<HttpPostedFileBase> files, int referid)
       {
           // OASEntities db = new OASEntities();
           // The Name of the Upload component is "files"
           if (files != null)
           {
 
 
               int i =(int)db.letterdatas.Where(x => x.referletid == referid).Max(p => p.seq);
               foreach (var file in files)
               {
                   bool failtestsize = false;
                   if (file.ContentLength > 5096000) failtestsize = true;
                   if (failtestsize) return Content(new Exception("خطا : حجم فایل ارسالی بیش از حد مجاز می‌باشد").Message);
                 //  System.Threading.Thread.Sleep(2000);
                   var xi = db.letterdatas.Where(x => x.referletid == referid).Count();
                   if (xi > 4) return Content(new Exception("خطا : تعداد فایلهای ارسالی بیش از حد مجاز می‌باشد").Message);
 
 
                   i++;
                    
                  
                   var fileName = Path.GetFileName(file.FileName);
 
 
                 
                   byte[] filedata = new byte[file.ContentLength];
                   Stream st = file.InputStream;
                   st.Read(filedata, 0, file.ContentLength);
                   var letterdata = new letterdata
                   {
                       data = filedata,
                       extention = Path.GetExtension(file.FileName),
                       filename = fileName,
                       seq = i,
                       referletid = referid,
                       templetguid = null,
                       fileid = Guid.NewGuid(),
                       userid = db.users.Where(x => x.username == User.Identity.Name).Select(x => x.userid).FirstOrDefault()
 
 
                   };
                   db.letterdatas.Add(letterdata);
                   db.SaveChanges();
                  
               }
               
           }
            
           return Content("");
       }

int i =(int)db.letterdatas.Where(x => x.referletid == referid).Max(p => p.seq);
var xi = db.letterdatas.Where(x => x.referletid == referid).Count();

Url: C:\Users\Morteza\Downloads\ExplorerPCalV.1.1\Bin\ExplorerPCal.exe 
-> ExplorerPCal, Version=1.1.0.0, Culture=neutral, PublicKeyToken=a55a2614eb2aee77
Caller: ExplorerPCal.App -> appDispatcherUnhandledException
Exception classes:
   System.NullReferenceException
   System.Reflection.TargetInvocationException
   System.Reflection.TargetInvocationException

Exception messages:
   Object reference not set to an instance of an object.
   Exception has been thrown by the target of an invocation.

   Exception has been thrown by the target of an invocation.

Stack Traces:
   at ExplorerPCal.Utils.ConfigSetGet.GetConfigData(String key)

dotnet add package Microsoft.AspNetCore.DataProtection.EntityFrameworkCore

    public class MyKeysContext : DbContext, IDataProtectionKeyContext
    {
        // A recommended constructor overload when using EF Core 
        // with dependency injection.
        public MyKeysContext(DbContextOptions<MyKeysContext> options) 
            : base(options) { }

       // This maps to the table that stores keys.
        public DbSet<DataProtectionKey> DataProtectionKeys { get; set; }
    }

public void ConfigureServices(IServiceCollection services)
{
   // using Microsoft.AspNetCore.DataProtection;
    services.AddDataProtection()
        .PersistKeysToDbContext<MyKeysContext>();
}

protected override void OnModelCreating(ModelBuilder builder)
{
    base.OnModelCreating(builder);

    if (Database.ProviderName == "Microsoft.EntityFrameworkCore.Sqlite")
    {
        // SQLite does not have proper support for DateTimeOffset via Entity Framework Core, see the limitations
        // here: https://docs.microsoft.com/en-us/ef/core/providers/sqlite/limitations#query-limitations
        // To work around this, when the Sqlite database provider is used, all model properties of type DateTimeOffset
        // use the DateTimeOffsetToBinaryConverter
        // Based on: https://github.com/aspnet/EntityFrameworkCore/issues/10784#issuecomment-415769754
        // This only supports millisecond precision, but should be sufficient for most use cases.
        foreach (var entityType in builder.Model.GetEntityTypes())
        {
            var properties = entityType.ClrType.GetProperties().Where(p => p.PropertyType == typeof(DateTimeOffset));
            foreach (var property in properties)
            {
                builder
                    .Entity(entityType.Name)
                    .Property(property.Name)
                    .HasConversion(new DateTimeOffsetToBinaryConverter());
            }
        }
    }
}