在c#中使用lock陳述句

我需要使用鎖結構，并編輯以下方法以并行執行：

    public void Withdraw(int amountToWithdraw)
            {
                if (amountToWithdraw <= 0)
                {
                    throw new ArgumentException("The amount should be greater than 0.");
                }
    
                if (amountToWithdraw > MaxAmountPerTransaction)
                {
                    throw new ArgumentException($"The value {amountToWithdraw} exceeds transaction limit: {MaxAmountPerTransaction}.");
                }
    
                if (amountToWithdraw > Amount)
                {
                    throw new ArgumentException("Insufficient funds.");
                }
                
                WithdrawAndEmulateTransactionDelay(amountToWithdraw);
    
            }

這是結果

private readonly object balanceLock = new object();
public void Withdraw(int amountToWithdraw)
        {
            if (amountToWithdraw <= 0)
            {
                throw new ArgumentException("The amount should be greater than 0.");
            }

            if (amountToWithdraw > MaxAmountPerTransaction)
            {
                throw new ArgumentException($"The value {amountToWithdraw} exceeds transaction limit: {MaxAmountPerTransaction}.");
            }

            if (amountToWithdraw > Amount)
            {
                throw new ArgumentException("Insufficient funds.");
            }

            lock (balanceLock)
            {
                WithdrawAndEmulateTransactionDelay(amountToWithdraw);
            }

        }

這是對不應更改的方法 WithdrawAndEmulateTransactionDelay 的描述

private void WithdrawAndEmulateTransactionDelay(int amountToWithdraw)
        {
            Thread.Sleep(1000);
            Amount -= amountToWithdraw;
        }

但是，單元測驗失敗了。我的代碼中的錯誤在哪里？

uj5u.com熱心網友回復：

看來，您應該將最后一個驗證放在鎖中：在您當前的實作中，有可能

1. 執行緒 #1 嘗試撤銷cash1，這是有效cash1 < Account的 ()，驗證通過
2. 執行緒 #2 嘗試撤銷cash2，這是有效cash2 < Account的 ()，驗證通過
3. 然而cash1 cash2 > Account
4. 執行緒 #1WithdrawAndEmulateTransactionDelay現在呼叫Amount == Amount - cash1 < cash2
5. 執行緒 #2 呼叫WithdrawAndEmulateTransactionDelay; 因為Amount - cash1 < cash2你的測驗失敗了

private readonly object balanceLock = new object();

public void Withdraw(int amountToWithdraw) {
  // These validations are not depended on Amount, they don't want lock
  if (amountToWithdraw <= 0)
    throw new ArgumentOutOfRangeException(nameof(amountToWithdraw), 
      "The amount should be greater than 0.");

  if (amountToWithdraw > MaxAmountPerTransaction)
    throw new ArgumentOutOfRangeException(nameof(amountToWithdraw), 
      $"The value {amountToWithdraw} exceeds transaction limit: {MaxAmountPerTransaction}.");

  // from now on we start using Amount, so we need the lock:
  lock (balanceLock) {
    if (amountToWithdraw > Amount)
      throw new ArgumentException("Insufficient funds.", nameof(amountToWithdraw));

    WithdrawAndEmulateTransactionDelay(amountToWithdraw);
  } 
}

uj5u.com熱心網友回復：

我也會避免所有這些例外。錯誤的輸入是這個代碼通常是意料之中的，所以它并不例外。

試試這個代碼：

public TransactionStatus Withdraw(int amountToWithdraw)
{
    bool successful = false;
    string message = "OK";
    int balanceBefore = Amount;
    int balanceAfter = Amount;
    if (amountToWithdraw <= 0)
    {
        message = "The amount should be greater than 0.";
    }
    else if (amountToWithdraw > MaxAmountPerTransaction)
    {
        message = $"The value {amountToWithdraw} exceeds transaction limit: {MaxAmountPerTransaction}.";
    }
    else
    {
        lock (balanceLock)
        {
            if (amountToWithdraw > Amount)
            {
                message = "Insufficient funds.";
            }
            else
            {
                Thread.Sleep(1000);
                Amount -= amountToWithdraw;
                successful = true;
                balanceAfter = Amount;
            }
        }
    }
    return new TransactionStatus()
    {
        Successful = successful, Message = message, BalanceBefore = balanceBefore, BalanceAfter = balanceAfter
    };
}

public struct TransactionStatus
{
    public bool Successful;
    public string Message;
    public int BalanceBefore;
    public int BalanceAfter;
}

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