我应该抛出什么异常并应该提供消息？

Question

I have a situation where a user operation changes the current status of a request.

Depending on the current status then only certain operations are possible ie

Pending can be Approved or Rejected or Cancelled Approved can be 'Cancel Requested' 'Cancel Requested' can be 'Cancellation Approved' or 'Cancellation Requested'

My question is what type of exception should I raise if the requested operation cannot be carried out? Should I use an inbuilt exception or should I create a custom exception such as InvalidCurrentStatusException or some such?

Looking at the documentation for InvalidOperationException seems a prime candidate as it is 'the exception that is thrown when a method call is invalid for the object's current state'.

If I go with the second option of a custom exception then I don't need to provide a message.

If I use the inbuilt InvalidOperationException should I provide a message and what should that message be?

UPDATE:

here's the code I have at the moment:

internal void CancelRequest(int requestID, int userID, string notes)
{

        DateTime editDate = DateTime.UtcNow;

        var request = this.FindByID(requestID, CancelRequestIncludes);

        if(request == null)
        {
            throw new ArgumentException(InvalidRequestMessage);
        }

        var currentStatus = request.LeaveRequestStatuses.Where(s => s.IsCurrent).FirstOrDefault();

        if (currentStatus.StatusID == (int)RequestStatuses.RequestPending)
        {
            SetNewRequestStatus(request, currentStatus, RequestStatuses.CancellationApproved, userID, notes, editDate);
        }
        else if (currentStatus.StatusID == (int)RequestStatuses.RequestApproved)
        {
            if (ValidApprover(request.UserID, userID))
            {
                SetNewRequestStatus(request, currentStatus, RequestStatuses.CancellationPending, userID, notes, editDate);
            }
            else
            {
                //throw an invalid approver exceptioon

            }
        }
        else
        {
            //throw exception as cant carry out cancellation
        }

        Context.SaveChanges();
}

Answer 1

You should never use an Exception (or any subclass) to control the state of the application, or indicate logical flow. Instead you should create a class that acts as a "result" in which you add a property that has the "status" -- as some others have already commented an enum is a prime example of a desirable use-case.

public enum RequestStatus
{
    Approved,
    Rejected,
    Cancelled,
    UnableToCarryOut
}

public class RequestResult
{
    public RequestStatus Status { get; set; }
    public string Message { get; set; }
}

Then simply pass around an instance of this object.

Answer 2

Custom exception would be the best option in this case. And you've guessed it right - there should be two types of those (as your comments suggest). The classes for those should be named to be self-describing: - InvalidApproverException - UnexpectedStatusException

Both can actually be derieved or even replaced by InvalidOperationException, but then you'll have to provide similar exception message every time. The good thing about custom exceptions is that you can add some more information about the context of the exception. Specifically, the InvalidApproverException - can expose the actual approver's id, who for the operation has failed. This could be useful later down the road. Also - it can internally set the message to some constant string: something similar to: "The caller is not the initiator of the request". As or the second exception (UnexpectedStatusException) - I'd expose the CurrentRequestStatus property on it as well and again have a constant error message. That just simplifies the handling down the road. Here is a sample code for the exception:

public class UnexpectedStatusException : Exception { private const string DefaultExceptionMessage = "The request is not in an expected state.";

public LeaveRequestStatus CurrentStatus
{
    get; private set;
}

public UnexpectedStatusException(LeaveRequestStatus currentStatus) : this(currentStatus, DefaultExceptionMessage)
{
}

public UnexpectedStatusException(LeaveRequestStatus currentStatus, string message) : base(message)
{
    this.CurrentStatus = currentStatus;
}

}

public enum LeaveRequestStatus { // Request statueses here }