c# - 错误: The cast to value type 'System.Int32' failed because the materialized value is null

Question

我遇到了问题

The cast to value type 'System.Int32' failed because the materialized value is null. Either the result type's generic parameter or the query must use a nullable type.

我已经查找了可以解决我遇到的问题的答案，但找不到类似的案例。

我试图从 account id 作为输入找出 tblTransactions 中的 max 交易 ID。如果该账户没有任何交易，就会导致错误。这两个表之间的关系为 tblTransaction。 accountId = tblAccount.Id

我的 Controller :

//GET: Balance

    [Authorize]
    public ActionResult Index()
    {
        string useracc = (string)Session["AccountNumber"];

        var accountInstance = db.Accounts.FirstOrDefault(w => w.AccountNumber.ToString() == useracc);


        List<Transaction> AccountTransactions = db.Transactions.Where(w => w.AccountId == accountInstance.Id&&w.IsCancelled ==false).Select(w => w).OrderByDescending(w => w.Date).ToList();

            var accountStatement = AccountTransactions.Where(w => w.TransactionTypeId == 2 && w.Date.Year >= 2015).OrderByDescending(w => w.Date).ToList();

            var lastTransactionId = db.Transactions.Where(w => w.AccountId == accountInstance.Id && w.IsCancelled == false && w.TransactionTypeId == 2 && w.Date.Year >= 2015).Max(t => t.Id);

            var needDueDate = db.SystemInvoices.Where(s => s.Id == lastTransactionId).Select(s => s.DueDate).FirstOrDefault();



            List<customBalanceInfoItem> currCustomBalance = new List<customBalanceInfoItem>();

            customBalanceInfoItem displayItem = new customBalanceInfoItem();

            displayItem.AccountNumber = accountInstance.AccountNumber;
            displayItem.DueDate = needDueDate;

            currCustomBalance.Add(displayItem);
            return View(currCustomBalance);
        }

错误发生在变量lastTransactionId上。

Answer 1

使用DefaultIfEmpty()修改它，检查here

修改后的查询

var lastTransactionId = db.Transactions.Where(w => w.AccountId == accountInstance.Id && w.IsCancelled == false && w.TransactionTypeId == 2 && w.Date.Year >= 2015)
                                       .Select(t=>t.Id)
                                       .DefaultIfEmpty(-1)
                                       .Max()

你可以定义需要返回的值，如果集合为空，就像我将其设置为-1，否则它将是默认值