I am trying to make TransactionId increased using Is Identity in my code. But it shows this following error :
SqlException: Cannot insert explicit value for identity column in table 'TransactionHistories' when IDENTITY_INSERT is set to OFF.
I tried to put [DatabaseGeneratedAttribute(DatabaseGeneratedOption.Identity)] syntax. But it didn't work. 
public partial class TransactionHistories
    {
        [DatabaseGeneratedAttribute(DatabaseGeneratedOption.Identity)]
        public int TransactionId { get; set; }
        public DateTime? TransactionDate { get; set; }
        public long? CheckingAccountNumber { get; set; }
        public long? SavingAccountNumber { get; set; }
        [DisplayName("Transffered Amount")]
        public decimal? Amount { get; set; }
        public decimal? Transactionfee { get; set; }
        public long? TransactionTypeId { get; set; }
    }
            TransactionHistories = new TransactionHistories();
            TransactionTypes = new TransactionTypes();
            if (!ModelState.IsValid)
            {
                return Page();
            }
            SavingAccounts.Balance = SavingAccounts.Balance - Amount;
            CheckingAccounts.Balance = CheckingAccounts.Balance + Amount;
            //TransactionHistories.TransactionId = TransactionHistories.TransactionId + 1;
            TransactionHistories.TransactionDate = DateTime.Now;
            TransactionHistories.CheckingAccountNumber = CheckingAccounts.CheckingAccountNumber;
            TransactionHistories.SavingAccountNumber = SavingAccounts.SavingAccountNumber;
            TransactionHistories.Amount = Amount;
            TransactionHistories.Transactionfee = 0;
            TransactionHistories.TransactionType = "SavingToChecking";
Above is my model class and TransactionId is primary key which is the value I am trying to make it increase by 1 using Is Identity. How can I make this work? Please give me any advice and help.

 
    