What is the best way to return error message from function?

Question

If face a logic error error such (Expired user, invalid ID), then what is the best way to tell the parent method of this error from the following :

1- Throwing customized exception like the following :

 try
{
//if (ID doesn't match) then 
Throw new CustomException(-1,"ID doesn't match");
}
catch(CustomException ex)
{
throw ex
}
catch(Exception ex)
{
throw new CustomException(ex.ErrorCode,ex.message);
}

2- return error message and code like :

//if (ID doesn't match) then 
This.ErrorCode= -1;
This.Message= "ID doesn't match";

See [this question](http://stackoverflow.com/questions/3926086/handling-known-errors-and-error-messages-in-a-method?rq=1). — Tim M., Apr 08 '13 at 07:24
Also see [this question](http://stackoverflow.com/questions/409563/best-practices-for-exception-management-in-java-or-c-sharp) — Tim M., Apr 08 '13 at 07:29

Denys Denysenko · Accepted Answer · 2016-08-18T10:45:52.367

The better way is to throw custom exception. That's why they were introduced. If you need to provide specific info, like ErrorCode or something else, you could easily extend base Exception class to do so. Main reasons are:

You can ignore invalid error code returned from your funcion and this could lead you to the situation where your system state is corrupted whereas Exception is something you can't ignore.
If your funcion does something usable then it should return some data you interested in and not the error codes, this gives you more solid design.

score 4 · Answer 2 · answered Oct 31 '15 at 23:35

If face a logic error error such (Expired user, invalid ID), then what is the best way to tell the parent method of this error

Since you want the parent method to know the error, it seams that you are not sure that ID is valid and the User is not expired before you call the GetUser method. Right?
If you are not sure that parameters you pass into the function are valid, then using exceptions is not justified and you should return error information instead.
You can return error information more functional way similar to what Scala, Go and Rust languages suggest.

Create a generic class to return either an error or a value

public class Either(of ErrorType, ValueType)
    public readonly Success as boolean
    public readonly Error as ErrorType
    public readonly Value as ValueType

    public sub new(Error as ErrorType)
        me.Success = False
        me.Error = Error
    end sub

    public sub new(Value as ValueType)
        me.Success = True
        me.Value = Value
    end sub
end class

Create an enumeration of errors that your function can have

public enum UserError
    InvalidUserID
    UserExpired
end enum

Create a function that takes a User ID as an argument and returns either an error or a User

function GetUser(ID as integer) as Either(of UserError, User)

    if <business logic to find a user failed> then
        return new Either(of UserError, User)(UserError.InvalidUserID) 
    end if

    if <user expired> then
        return new Either(of UserError, User)(UserError.UserExpired)
    end if

    return new Either(of UserError, User)(User)
end function

In the caller (parent) method check for the error and apply business logic

dim UserID = 10
dim UserResult = GetUser(10)

if UserResult.Success then
    rem apply business logic to UserResult.Value
else
    rem apply business logic to UserResult.Error
end if

Note: if you rewrite this code using Exceptions you will get exactly the same amount of code.

You can use [language-ext](https://github.com/louthy/language-ext) nuget for functional programming extensions in c#. — Alexandra Petrova, Mar 16 '21 at 15:22

score 2 · Answer 3 · answered Apr 08 '13 at 07:27

In most cases you should throw an exception. Whether this is a standard or custom exception depends on the context, but is irrelevant to the process.

Throwing exceptions forces any implementation to handle bad data accordingly. If you rely on implementations to check property values for correct processing, then unexpected, harder to debug exceptions will be encountered sooner or later.

score 2 · Answer 4 · answered Apr 08 '13 at 07:27

I suppose you should use exceptions, because it is exact, what they are made for. You can put there the same information, as in you second case. But the difference between 1-st and 2-nd one is that you give your function consumers a possibility to be informed that something wrong, and it MUST be handled somehow. In second case you just say that function worked, but something wrong, and user MAY handle this information

fatihk · Answer 5 · 2013-04-08T07:41:04.963

0

It entirely depends of the workflow of your project. If it is an unexpected run timer error and it inhibits your program then exception is a good choice but for simple input checks and expectable outputs you should use enumerators

edited Apr 08 '13 at 07:41

answered Apr 08 '13 at 07:28

fatihk

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What is the best way to return error message from function?

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