Error does not get caught - exception

Question

I wrote a script that upload a xml file, look at the following code snippet:
open System.Xml
open System.IO
open System
type XmlErrorLoad =
| Success of XmlDocument
| FileNotFound of FileNotFoundException
| OtherException of Exception
let doc (filename:string) =
try
let file = XmlDocument()
file.Load(filename)
Success file
with
| :? FileNotFoundException as ex -> FileNotFound ex
| :? Exception as ex -> OtherException ex
let fileNotExists = doc("C:\Temp\ip2.xml")
match fileNotExists with
| Success s -> ()
| FileNotFound ex -> printfn "File not found: %s" ex.Message
| OtherException ex -> printfn "Exception: %s" ex.Message
In this case, ip2.xml file does not exist and it should throw an error FileNotFound and print on the screen.
"File not found: %s" ex.Message
But I've got the message
val fileNotExists : XmlErrorLoad =
FileNotFound
System.IO.FileNotFoundException: Could not find file 'C:\Temp\ip2.xml'.
File name: 'C:\Temp\ip2.xml'
at System.IO.__Error.WinIOError(Int32 errorCode, String maybeFullPath)
at System.IO.FileStream.Init(String path, FileMode mode, FileAccess access, Int32 rights, Boolean useRights, FileShare share, Int32 bufferSize, FileOptions options, SECURITY_ATTRIBUTES secAttrs, String msgPath, Boolean bFromProxy, Boolean useLongPath, Boolean checkHost)
at System.IO.FileStream..ctor(String path, FileMode mode, FileAccess access, FileShare share, Int32 bufferSize)
at System.Xml.XmlDownloadManager.GetStream(Uri uri, ICredentials credentials, IWebProxy proxy, RequestCachePolicy cachePolicy)
at System.Xml.XmlUrlResolver.GetEntity(Uri absoluteUri, String role, Type ofObjectToReturn)
at System.Xml.XmlTextReaderImpl.OpenUrlDelegate(Object xmlResolver)
at System.Threading.CompressedStack.runTryCode(Object userData)
at System.Runtime.CompilerServices.RuntimeHelpers.ExecuteCodeWithGuaranteedCleanup(TryCode code, CleanupCode backoutCode, Object userData)
at System.Threading.CompressedStack.Run(CompressedStack compressedStack, ContextCallback callback, Object state)
at System.Xml.XmlTextReaderImpl.OpenUrl()
at System.Xml.XmlTextReaderImpl.Read()
at System.Xml.XmlLoader.Load(XmlDocument doc, XmlReader reader, Boolean preserveWhitespace)
at System.Xml.XmlDocument.Load(XmlReader reader)
at System.Xml.XmlDocument.Load(String filename)
at FSI_0004.doc(String filename) in D:\f#\samples\ip.fsx:line 20

Answer 1

I've tried your code as input on F# Interactive and got the desired message
File not found: Could not find file 'C:\Temp\ip2.xml'.
Besides the output F# Interactive lists all the values defined in the code, so I also got following printed:
type XmlErrorLoad =
| Success of XmlDocument
| FileNotFound of FileNotFoundException
| OtherException of Exception
val doc : filename:string -> XmlErrorLoad
val fileNotExists : XmlErrorLoad =
FileNotFound
System.IO.FileNotFoundException: Could not find file 'C:\Temp\ip2.xml'.
File name: 'C:\Temp\ip2.xml'
at System.IO.__Error.WinIOError(Int32 errorCode, String maybeFullPath)
at System.IO.FileStream.Init(String path, FileMode mode, FileAccess access, Int32 rights, Boolean useRights, FileShare share, Int32 bufferSize, FileOptions options, SECURITY_ATTRIBUTES secAttrs, String msgPath, Boolean bFromProxy, Boolean useLongPath, Boolean checkHost)
at System.IO.FileStream..ctor(String path, FileMode mode, FileAccess access, FileShare share, Int32 bufferSize)
at System.Xml.XmlDownloadManager.GetStream(Uri uri, ICredentials credentials, IWebProxy proxy, RequestCachePolicy cachePolicy)
at System.Xml.XmlUrlResolver.GetEntity(Uri absoluteUri, String role, Type ofObjectToReturn)
at System.Xml.XmlTextReaderImpl.OpenUrlDelegate(Object xmlResolver)
at System.Threading.CompressedStack.runTryCode(Object userData)
at System.Runtime.CompilerServices.RuntimeHelpers.ExecuteCodeWithGuaranteedCleanup(TryCode code, CleanupCode backoutCode, Object userData)
at System.Threading.CompressedStack.Run(CompressedStack compressedStack, ContextCallback callback, Object state)
at System.Xml.XmlTextReaderImpl.OpenUrl()
at System.Xml.XmlTextReaderImpl.Read()
at System.Xml.XmlLoader.Load(XmlDocument doc, XmlReader reader, Boolean preserveWhitespace)
at System.Xml.XmlDocument.Load(XmlReader reader)
at System.Xml.XmlDocument.Load(String filename)
at FSI_0003.doc(String filename)
val it : unit = ()
You get this output only in F# Interactive as part of REPL feedback. In a normal program this output wouldn't be produced.