VB.NET | HttpWebRequest, How to see if the host is online? - vb.net

Question

I am coding a whitelist for my application in VB.NET.
I am using the HttpWebRequest Method and HttpWebResponse.
If the Whitelist Host is down, the whitelist is bypassed and the program is available to anyone which is a vulnerability.
Public Function GetWhitelist(ByVal PageURL As String) As String
Dim S As String = ""
Try
Dim Request As HttpWebRequest = WebRequest.Create("WHITELIST URL HERE")
Dim Response As HttpWebResponse = Request.GetResponse()
Using Reader As StreamReader = New StreamReader(Response.GetResponseStream())
S = Reader.ReadToEnd
End Using
Catch ex As Exception
Debug.WriteLine("Start Program Error. Handle:0")
End Try
Return S
End Function
I want to give the user an error if the website is down, any ideas?
Regards.

Answer 1

What is your error condition? This function alone doesn't prevent anything from happening in the event of an exception. The function still exits with returning a string. So any consuming code would have to look for error conditions and handle them accordingly.
If the request fails, you should meaningfully handle the exception. Two ideas off the top of my head would include:
1) Let the exception bubble up the stack:
Public Function GetWhitelist(ByVal PageURL As String) As String
Dim S As String = ""
Dim Request As HttpWebRequest = WebRequest.Create("WHITELIST URL HERE")
Dim Response As HttpWebResponse = Request.GetResponse()
Using Reader As StreamReader = New StreamReader(Response.GetResponseStream())
S = Reader.ReadToEnd
End Using
Return S
End Function
This would make the attempt to contact the host and, if that attempt failed, throw an exception instead of return a string.
2) Throw a custom exception:
Public Function GetWhitelist(ByVal PageURL As String) As String
Dim S As String = ""
Try
Dim Request As HttpWebRequest = WebRequest.Create("WHITELIST URL HERE")
Dim Response As HttpWebResponse = Request.GetResponse()
Using Reader As StreamReader = New StreamReader(Response.GetResponseStream())
S = Reader.ReadToEnd
End Using
Catch ex As Exception
Debug.WriteLine("Start Program Error. Handle:0")
Throw New SomeCustomException(String.Format("Unable to contact host: {0}", PageURL), ex)
End Try
Return S
End Function
This would provide a more targeted exception instead of whatever comes out of the response reader, would provide useful runtime information about the error for logging and analysis (namely the runtime value of the PageURL), and takes a step toward hiding the implementation details from code outside of this object (since that code doesn't really care about the HttpWebRequest and HttpWebResponse, it just wants to know if the URL is good or not).
Remember that throwing an exception is a perfectly acceptable exit path for a function. It doesn't always have to return a value. An exception is an appropriate way of indicating an error condition. Your current implementation, however, "swallows" the exception and provides no indication to the consuming code that anything went wrong. Instead it returns a "magic value" of String.Empty which consuming code may or may not ignore.

Answer 2

Change your exception handler. The correct way to do this is to just try the request and handle the exception if it fails.