string connString = \"Provider=Microsoft.ACE.OLEDB.12.0;Data Source=D:\\\\data\\\\[Proj_Resource Details 20110118.xlsx];Extended Properties=Excel 12.0\";
Have you tried it as just
string connString = "Provider=Microsoft.ACE.OLEDB.12.0;Data Source=D:\\data\\Proj_Resource Details 20110118.xlsx;Extended Properties=Excel 12.0";
without the []s?
By the way, if you are not escaping anything, just use @
string connString = @"Provider=Microsoft.ACE.OLEDB.12.0;Data Source=D:\data\Proj_Resource Details 20110118.xlsx;Extended Properties=Excel 12.0";