Add Entries in Web.Config File in SharePoint 2010

It may depend on core client requirment to modify the configuration files. I came across such requirment and below is the way to modify your configuration file. Remember one thing, before doing this activity first you take a backup of your configuration file.

First add SharePoint administration namespace using below segment:
using Microsoft.Sharepoint.Administration;

Now you can write below code segment on a button's click event.

SPWebService service = SPWebService.ContentService;

SPWebConfigModification myModification = new SPWebConfigModification();
myModification.Path = "configuration/SharePoint/SafeControls";
myModification.Name = "SafeControl[@Assembly='MyCustomAssembly'][@Namespace='MyCustomNamespace'][@TypeName='*'][@Safe='True']";
myModification.Sequence = 0;
myModification.Owner = "User Name";//In my case i used System account
myModification.Type = SPWebConfigModification.SPWebConfigModificationType.EnsureChildNode;
myModification.Value = "<SafeControl Assembly='MyCustomAssembly' Namespace='MyCustomNamespace' TypeName='*' Safe='True' />";
service.WebConfigModifications.Add(myModification);

service.Update();
service.ApplyWebConfigModifications();