how to check if the path is valid?

You cannot restrict users, but you can warn them they entered invalid path. Actually, that is wrong, you can restrict users by using Browser Dialog[^] without allowing writing into the Textbox

The way to check for invalid path (if you don't want to use BrowserDialog)

if (System.IO.Directory.Exists() ) {
    MsgBox("You entered invalid folder path!", MsgBoxStyle.Exclamation);

}

Put the check above in textbox validating event.

If this helps, please take time to accept the solution. Thank you.

File exists

if (!File.Exists(path))
   {
   // Report problem
   ...
   }

Folder exists

if (!Directory.Exists(path))
   {
   // Report problem
   ...
   }