IsReadable is True if you have permissions to read from the FolderItem. It is not a guarantee that the read will succeed. If you want to read from a file, you should simply attempt to do so. If the read operation can work, it will work.
Example
Var f As New FolderItem
f = FolderItem.ShowOpenFileDialog("text/plain")
If f.IsReadable Then
// conduct the read..
Else
MessageBox("The file cannot be read!")
End If
