From what I've seen, I think that this:
string connString = "Provider=Microsoft.Jet.OLEDB.4.0;"
+ "Data Source=\"" + dir + "\\\";"
+ "Extended Properties=\"text;HDR=yes;FMT=Delimited\";";
should be this:
string connString = "Provider=Microsoft.Jet.OLEDB.4.0;"
+ "Data Source=" + dir + "\\;"
+ "Extended Properties=\"text;HDR=Yes;FMT=Delimited(;)\"";