如何摆脱函数中的此错误消息？

Question

I have a function in a class but am receiving an error under "Withdraw", "not all code paths return a value". I thought adding a void would do the trick but can't seem to make it go away. Does know how I could revise my code? Here is the portion:

public virtual double Withdraw(double amount)
  {
     if (amount > balance)
     {
        MessageBox.Show("Debit amount exceeded account balance.", "Insufficient funds!", MessageBoxButtons.OK, MessageBoxIcon.Exclamation);
     }
     else
        return balance - amount;
  }

Answer 1

Since you've declared that your function returns a double , it needs to do that whichever branch of the if gets taken.

You need to return a value from the true side of the if , after MessageBox returns, eg.:

if (amount > balance)
{
    MessageBox.Show(...);
    return balance;
}
else ...

Answer 2

Not a direct answer but I think your Method is serving many purposes, the calculation and showing a message to the user, you should consider using two methods like this

public virtual double Withdraw(double amount)
{
    if (amount > balance)    
        throw new Exception("your message")        
    else
        return balance - amount;
}

code of the caller

try{
 Withraw(...)
}
catch{
 Your messageBox
}

Answer 3

your ode should always return some value, in any condition, so

public virtual double Withdraw(double amount)
  {
     if (amount > balance)
     {
        MessageBox.Show("Debit amount exceeded account balance.", "Insufficient funds!", MessageBoxButtons.OK, MessageBoxIcon.Exclamation);
        return SOME_NON_VALID_VALUE_FOR_YOUR_APP; //or raise an exception,
        // depends on architecture 
     }

     return balance - amount;       
  }

considering the logic of the code provided, if amount > balance it's not correct , otherwise return computation.

Answer 4

You line of code below does not return any value are main root cause:

if (amount > balance)
 {
    MessageBox.Show("Debit amount exceeded account balance.", "Insufficient funds!", MessageBoxButtons.OK, MessageBoxIcon.Exclamation);
 }

You should return a double value after MessageBox.Show .

Answer 5

You need to retrn something after MessageBox.Show idealy 0

  public virtual double Withdraw(double amount)
  {
     if (amount > balance)
     {
        MessageBox.Show("Debit amount exceeded account balance.", "Insufficient funds!", MessageBoxButtons.OK, MessageBoxIcon.Exclamation);
            return 0;
     }
     else
        return balance - amount;
  }

Answer 6

Your function is expected to returns a double but if amount>balance it will not.

public virtual double Withdraw(double amount)
{
    if (amount > balance)
    {
        //your messagebox code    
        return double.NaN; // or whatever you think is correct in this case.
    }
    else
        return balance - amount;
}

Answer 7

You are returning double value through your function.

But only mentioned in else part.

If , if(amount > balance) condition gets true, then also you will have to return the value.

See below code:

    public virtual double Withdraw(double amount)
      {
         if (amount > balance)
         {
            MessageBox.Show("Debit amount exceeded account balance.", "Insufficient funds!", MessageBoxButtons.OK, MessageBoxIcon.Exclamation);
         }
         else
            return balance - amount;

         return 0;
      }