Hi,
you declare the SqlVariable in Varchar data type.so u must give the single quote in front and rear of the value.
SqlCommand cmd = new SqlCommand("select * from tbadd where Dop between (@param1) and (@param2)", con);
cmd.Parameters.Add("@param1", SqlDbType.VarChar, 50).Value = '"+dateTimePicker1.Value+"';
cmd.Parameters.Add("@param2", SqlDbType.VarChar, 50).Value ='"+dateTimePicker2.Value+"';