Working with
Errors in SOAP
While working with Web Services, you eventually will need to handle errors
in a more constructive way. The question is, how do you want to handle the errors that pop
up? The following error message is an example of what you would see if you did not have
any mechanism for handling errors:
System.IndexOutOfRangeException: Exception of type System.IndexOutOfRangeException was
This error message is typical of
what you would see if your Web Service ran into an error. To keep this from happening,
Listings 13.11 and 13.12 show a simple example to catch the error and return it inside the
contents of the SOAP message.
Listing 13.11. WebMethod Error (C#)
[WebMethod]
public object ReturnErrorString()
{
// Catch the error then return it inside a SOAP message.
string[] sret = new string[3];
try
{
sret[0] = "one";
sret[1] = "two";
sret[3] = "three";//Error
}
catch(Exception e)
{
sret[0] = e.Message;
}
return sret;
} |
Listing 13.12. WebMethod Error (Visual Basic .NET)
<WebMethod()> Public Function ErrorStringCatch() As String()
' Catch the error then return it inside a SOAP message.Dim sret(3) As String
Try
sret(0) = "Hello"
sret(1) = "Bye"
sret(13) = "Testing" 'This will generate an error
Catch e As Exception
sret(0) = e.Message
End Try
ErrorStringCatch = sret
End Function |
If you look closely at the code in Listings 13.11 and 13.12, you will
notice that we intentionally added a runtime error while adding the third item to the
array.
Now when you run into an error, it will be returned inside the SOAP message. This is
one way to approach handling errors and sending them back to the client, but this is not
the preferred method for doing so. Instead, one of two things should be done. First, you
cannot catch the exception that will send it back to the client to handle. Second, you can
identify what the problem is inside the catch and try to resolve the problem or throw
another exception that will be sent back to the client. This gives you the capability to
identify the error inside your Web Service client and deal with it there. Otherwise, if
you just returned the error message inside the array, you would need to check the first
item of the array and try to identify whether it is an error message. If you did this, the
return message might look something like Listing 13.13.
Listing 13.13. SOAP Return Values
<?xml version="1.0" encoding="utf-8" ?> <Object n1:type="ArrayOfString"Listing 13.12 xmlns:n1="http://www.w3.org/2001 |
What if you are working only with numbers? Then how do you go about sending back
error messages? If you look at Listings 13.14 and 13.15, you will see that you can use the
try-catch method and then rethrow the error.
Listing 13.14. Throw Exception (C#)
[WebMethod]
public object ReturnErrorString()
{
// Catch the error then return it inside a SOAP message.
string[] sret = new string[3];
try
{
sret[0] = "one";
sret[1] = "two";
sret[3] = "three";//Error
}
catch(Exception e)
{
Throw(e);//Return the exception to the client
}
return sret;
} |
Listing 13.15. Throw Exception (Visual Basic .NET)
<WebMethod()> Public Function ErrorStringCatch() As String()
' Catch the error then return it inside a SOAP message.Dim sret(3) As String
Try
sret(0) = "Hello"
sret(1) = "Bye"
sret(13) = "Testing" 'This will generate an error
Catch e As Exception
Throw(e) 'Return the exception to the client
End Try
ErrorStringCatch = sret
End Function |
If you look at the Catch statement, the Throw method is being called to
pass the error on to the client. By adding this line of code, you change the behavior of
the Web Service dramatically. A simple change in one line of code can make a world of
difference when it comes to debugging. Don’t forget that you need to use the try-catch
statement on the client side as well, to be able to handle the exception being passed back
by the server. To show how the client should be implemented, take a look at the code in
Listings 13.16 and 13.17.
Listing 13.16. The try-catch Statement on the Client Side (C#)
private void Button2_Click(object sender, System.EventArgs e)
{
try
{
localhost.Service1 s = new localhost.Service1();
s. ErrorStringCatch ();
}
catch(Exception ex)
{
TextBox1. Text = ex.Message;
}
} |
Listing 13.17. The try-catch Statement on the Client Side (Visual Basic .NET)
Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) |
In Listings 13.16 and 13.17, the user clicks the button to invoke the method
of the Web Service. If there is an error on the server side, the Web Service passes back
the error information, and it can be received in the catch statement.When an error does
get caught, it is displayed in the text box that is on the page.