fileNameValidity using try and catch.

    Public Function isFileNameValid(ByVal fileName As String)
        Dim isValid As Boolean = False

        Try
            IO.File.WriteAllText(fileName, "")
            IO.File.Delete(fileName)
            isValid = True
        Catch ex As Exception
            isValid = False
        End Try

        Return isValid
    End Function

'-------------------------

sub writeFileWhenThisHappens()
    dim content as string = "this text goes into the file"

    if isFileNameValid(textFieldWhereUserWritesTheFilename.text) then
        IO.File.WriteAllText(textFieldWhereUserWritesTheFilename.text, content)
    end if

end sub