URL Exists

Checks whether a given URL is accessible by performing a HEAD request, optionally falling back to GET.

Uses a static shared HttpClient to optimize performance across calls.
If the server returns MethodNotAllowed or NotImplemented for HEAD, a fallback GET is performed if enabled.
Returns False in case of timeout, DNS failure, invalid URL, or any exception.
This function is compatible with both .NET Framework and .NET 5+ via conditional compilation.

    Public Function URLExists(ByVal url As String,
                              Optional timeoutMs As Integer = 5000,
                              Optional fallbackToGet As Boolean = True) As Boolean

        If String.IsNullOrWhiteSpace(url) Then Return False

        ' Reuse a single HttpClient across calls
        Static httpClient As HttpClient =
        New HttpClient(New HttpClientHandler With {.AllowAutoRedirect = True})

        Using cts As New CancellationTokenSource(timeoutMs)
            Try

#If NET5_0_OR_GREATER Then
                ' ---- .NET 5+ : use synchronous Send ----
                Using headReq As New HttpRequestMessage(HttpMethod.Head, url)
                    Using headResp As HttpResponseMessage =
                    httpClient.Send(headReq, HttpCompletionOption.ResponseHeadersRead, cts.Token)

                        If headResp.StatusCode = HttpStatusCode.OK Then Return True

                        If fallbackToGet AndAlso
                       (headResp.StatusCode = HttpStatusCode.MethodNotAllowed OrElse
                        headResp.StatusCode = HttpStatusCode.NotImplemented) Then

                            Using getReq As New HttpRequestMessage(HttpMethod.Get, url)
                                Using getResp As HttpResponseMessage =
                                httpClient.Send(getReq, HttpCompletionOption.ResponseHeadersRead, cts.Token)
                                    Return getResp.StatusCode = HttpStatusCode.OK
                                End Using
                            End Using
                        End If

                        Return False
                    End Using
                End Using
#Else
            ' ---- .NET Framework 4.x : block on SendAsync safely ----
            Using headReq As New HttpRequestMessage(HttpMethod.Head, url)
                Dim headResp As HttpResponseMessage =
                    httpClient.SendAsync(headReq, HttpCompletionOption.ResponseHeadersRead, cts.Token) _
                              .ConfigureAwait(False).GetAwaiter().GetResult()
                Try
                    If headResp.StatusCode = HttpStatusCode.OK Then Return True

                    If fallbackToGet AndAlso
                       (headResp.StatusCode = HttpStatusCode.MethodNotAllowed OrElse
                        headResp.StatusCode = HttpStatusCode.NotImplemented) Then

                        headResp.Dispose()
                        Using getReq As New HttpRequestMessage(HttpMethod.Get, url)
                            Dim getResp As HttpResponseMessage =
                                httpClient.SendAsync(getReq, HttpCompletionOption.ResponseHeadersRead, cts.Token) _
                                          .ConfigureAwait(False).GetAwaiter().GetResult()
                            Try
                                Return getResp.StatusCode = HttpStatusCode.OK
                            Finally
                                getResp.Dispose()
                            End Try
                        End Using
                    End If

                    Return False
                Finally
                    headResp.Dispose()
                End Try
            End Using
#End If

            Catch ex As OperationCanceledException
                ' Timeout
                Return False
            Catch
                ' DNS/connection/invalid URL, etc.
                Return False
            End Try
        End Using
    End Function

    Public Function URLExists(ByVal url As String,
                              Optional timeoutMs As Integer = 5000,
                              Optional fallbackToGet As Boolean = True) As Boolean

        If String.IsNullOrWhiteSpace(url) Then Return False

        ' Reuse a single HttpClient across calls
        Static httpClient As HttpClient =
        New HttpClient(New HttpClientHandler With {.AllowAutoRedirect = True})

        Using cts As New CancellationTokenSource(timeoutMs)
            Try

#If NET5_0_OR_GREATER Then
                ' ---- .NET 5+ : use synchronous Send ----
                Using headReq As New HttpRequestMessage(HttpMethod.Head, url)
                    Using headResp As HttpResponseMessage =
                    httpClient.Send(headReq, HttpCompletionOption.ResponseHeadersRead, cts.Token)

                        If headResp.StatusCode = HttpStatusCode.OK Then Return True

                        If fallbackToGet AndAlso
                       (headResp.StatusCode = HttpStatusCode.MethodNotAllowed OrElse
                        headResp.StatusCode = HttpStatusCode.NotImplemented) Then

                            Using getReq As New HttpRequestMessage(HttpMethod.Get, url)
                                Using getResp As HttpResponseMessage =
                                httpClient.Send(getReq, HttpCompletionOption.ResponseHeadersRead, cts.Token)
                                    Return getResp.StatusCode = HttpStatusCode.OK
                                End Using
                            End Using
                        End If

                        Return False
                    End Using
                End Using
#Else
            ' ---- .NET Framework 4.x : block on SendAsync safely ----
            Using headReq As New HttpRequestMessage(HttpMethod.Head, url)
                Dim headResp As HttpResponseMessage =
                    httpClient.SendAsync(headReq, HttpCompletionOption.ResponseHeadersRead, cts.Token) _
                              .ConfigureAwait(False).GetAwaiter().GetResult()
                Try
                    If headResp.StatusCode = HttpStatusCode.OK Then Return True

                    If fallbackToGet AndAlso
                       (headResp.StatusCode = HttpStatusCode.MethodNotAllowed OrElse
                        headResp.StatusCode = HttpStatusCode.NotImplemented) Then

                        headResp.Dispose()
                        Using getReq As New HttpRequestMessage(HttpMethod.Get, url)
                            Dim getResp As HttpResponseMessage =
                                httpClient.SendAsync(getReq, HttpCompletionOption.ResponseHeadersRead, cts.Token) _
                                          .ConfigureAwait(False).GetAwaiter().GetResult()
                            Try
                                Return getResp.StatusCode = HttpStatusCode.OK
                            Finally
                                getResp.Dispose()
                            End Try
                        End Using
                    End If

                    Return False
                Finally
                    headResp.Dispose()
                End Try
            End Using
#End If

            Catch ex As OperationCanceledException
                ' Timeout
                Return False
            Catch
                ' DNS/connection/invalid URL, etc.
                Return False
            End Try
        End Using
    End Function

Posted by ipercube