在此程式码中：

public static void Seed(IConfiguration configuration,
        ForumDbContext dbContext,
        RoleManager<IdentityRole> roleManager,
        UserManager<User> userManager)
    {
        if (dbContext.Users.Count()==0) 
CreateUsers(configuration, dbContext, roleManager, userManager)
                    .GetAwaiter().GetResult();

        
    }

我在下面的行中得到了一个运行时异常：if (dbContext.Users.Count()==0)即：
系统类型初始化异常："" Microsoft. EntityFrameworkCore. EnumerableMethods "的类型初始值设定项引发了异常。"无效操作异常：序列包含多个匹配元素
我的DBContext代码如下：

public class ForumDbContext : IdentityDbContext<User>
{
    public ForumDbContext(DbContextOptions options) : base(options)
    {

    }
    protected override void OnModelCreating(ModelBuilder modelBuilder)
    {
        base.OnModelCreating(modelBuilder);
        modelBuilder.Entity<Topic>().ToTable("Topics");
        modelBuilder.Entity<Attachment>().ToTable("Attachments");
        modelBuilder.Entity<Subscription>().ToTable("Subscriptions");
    }

    public DbSet<Topic> Topics { get; set; }
    public DbSet<Subscription> Subscriptions {get;set;}
    public DbSet<Attachment> Attachments { get; set; }
}

My User类定义如下：

public class User:IdentityUser
    {
        public string Discriminator { get; set; } = "User";
        public string Country { get; set; } = "";
        public string Signature { get; set; } = "";
        public bool IncludeSignature { get; set; } = true;

        public DateTime CreateDate { get; set; }

        [NotMapped]
        public bool SignatureFormatted = false;
        

        [NotMapped]
        public int NrPosts { get; set; }
    }