create table table_abc (id nvarchar(20) not null);

drop table table_abc 

string query = " alter table table_abc add column username nvarchar(100); "

string sql="alter table table_abc drop column username";

string sql=" alter table table_abc alter column displayname nvarchar(200) "

string sql=" EXEC sp_rename 'table_abc.username', 'displayname', 'COLUMN' ";

DataSet ColumnList=new DataSet(); 
string sql="select 1 * from table_abc";//exe this sql to dataset

string columnname="";
foreach(DataColumn dc in ColumnList.Table[0].Columns)
{
//GET ALL COLUMN NAMES
columnname=dc.ColumnName;
}