1. public static class OperationsUtlity  
2.      {  
3.         public static DataTable createDataTable()  
4.         {  
5.             DataTable table = new DataTable();     
6.                 //columns  
7.             table.Columns.Add("ID", typeof(int));     
8.             table.Columns.Add("NAME", typeof(string));     
9.             table.Columns.Add("CITY", typeof(string));    
10.   
11.                 //data  
12.             table.Rows.Add(111, "Devesh", "Ghaziabad");     
13.             table.Rows.Add(222, "ROLI", "KANPUR");     
14.             table.Rows.Add(102, "ROLI", "MAINPURI");     
15.             table.Rows.Add(212, "DEVESH", "KANPUR");  
16.             table.Rows.Add(102, "NIKHIL", "GZB");  
17.             table.Rows.Add(212, "HIMANSHU", "NOIDa");  
18.             table.Rows.Add(102, "AVINASH", "NOIDa");  
19.             table.Rows.Add(212, "BHUPPI", "GZB");  
20.               
21.              return table;  
22.           }  
23.          
24.     } 

Create a static class

1. public static class  CSVUtlity  { }

Add Extention Method 

1. public static void ToCSV(this DataTable dtDataTable, string strFilePath){ 

1. public static void ToCSV(this DataTable dtDataTable, string strFilePath) {  
2.     StreamWriter sw = new StreamWriter(strFilePath, false);  
3.     //headers    
4.     for (int i = 0; i < dtDataTable.Columns.Count; i++) {  
5.         sw.Write(dtDataTable.Columns[i]);  
6.         if (i < dtDataTable.Columns.Count - 1) {  
7.             sw.Write(",");  
8.         }  
9.     }  

apkact.com

Convert To csv

1.     sw.Write(sw.NewLine);  
2.     foreach(DataRow dr in dtDataTable.Rows) {  
3.         for (int i = 0; i < dtDataTable.Columns.Count; i++) {  
4.             if (!Convert.IsDBNull(dr[i])) {  
5.                 string value = dr[i].ToString();  
6.                 if (value.Contains(',')) {  
7.                     value = String.Format("\"{0}\"", value);  
8.                     sw.Write(value);  
9.                 } else {  
10.                     sw.Write(dr[i].ToString());  
11.                 }  
12.             }  
13.             if (i < dtDataTable.Columns.Count - 1) {  
14.                 sw.Write(",");  
15.             }  
16.         }  
17.         sw.Write(sw.NewLine);  
18.     }  
19.     sw.Close();  
20. } 

Export to CSV on button click

1. private void btnCSV_Click(object sender, EventArgs e) {  
2.     DataTable dt = OperationsUtlity.createDataTable();  
3.     string filename = OpenSavefileDialog();  
4.     dt.ToCSV(filename);  
5. }