I have a custom list named “Employees" which has the following columns.

In this blog you will see how to set the calculated field formula using SharePoint Object Model.

Formula for FullName

=[LastName]&","&[FirstName]

Description

Combines the last name and first name separated by comma (Ramalingam, Vijai).

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using Microsoft.SharePoint;

namespace CalculatedField
{
 class Program
 {
 static void Main(string[] args)
 {
 using (SPSite site = new SPSite("http://shashankpc:2012/sites/test"))
 {
 using (SPWeb web = site.OpenWeb())
 {
 SPList list = web.Lists.TryGetList("Employees");
 if (list != null)
 {
 SPFieldCalculated calculatedField = list.Fields.GetFieldByInternalName("FullName") as SPFieldCalculated;
 calculatedField.Formula="=[LastName]&\",\"&[FirstName]";
 calculatedField.Update();
 }
 }
 }
 }
 }
}