این کد رو توی همین سایت پیدا کردم!!
private void Attach_db(string db_Name, string Path_Mdf, string Path_Ldf)

{

try

{

SqlConnection con = new SqlConnection("server=.;trusted_connection=yes;");

SqlCommand cmd = new SqlCommand();

cmd.CommandText = "sp_attach_db @dbname = N'" + db_name + "', @filename1 = N'" + Path_Mdf + "', @filename2 = N'" + Path_Ldf + "'";

cmd.CommandType = CommandType.Text;

con.Open();

cmd.Connection = con;

cmd.ExecuteNonQuery();

con.Close();

}

catch (Exception e)

{

MessageBox.Show(e);

}

}