Update Person or Group Field of SharePoint List Programmatically

We can Update Sharepoint Person or group field using server object model and client object model. Please check below example:

Using Server Object model:

using Microsoft.SharePoint;
SPSite site = new SPSite("SiteName");
SPWeb web = site.OpenWeb();
SPList list = web.Lists["ListName"];
string testName = "Manish Loke"; //use display name instead of login name
if (testName != "") {
    SPUser userTest = web.EnsureUser(testName);
    testName = userTest.ID.ToString() + ";#" + userTest.LoginName.ToString();
    SPItem item;
    item["ColumnName"] = testName;
    item.Update();
}

Using Client object model:

using Microsoft.SharePoint;
using Microsoft.SharePoint.Client;
ClientContext context = new ClientContext("SiteName");
List list = context.Web.Lists.GetByTitle("ListName");
ListItem item;
context.Load(list);
context.ExecuteQuery();
string testName = "Manish Loke"; //use display name instead of login name
if (testName != "") {
    User userTest = context.Web.EnsureUser(testName);
    context.Load(userTest);
    context.ExecuteQuery();
    testName = userTest.Id.ToString() + ";#" + userTest.LoginName.ToString();
    ListItem item;
    item["ColumnName"] = testName;
    item.Update();
    context.ExecuteQuery();
}