Programatically change Site and System Master Page in SharePoint 2010

Here is the code to change both Site and System Master Page in SharePoint 2010.

This code has been written in Event Receiver. When the feature is deactivated, automatically, v4.master is applied

 public override void FeatureActivated(SPFeatureReceiverProperties properties)
        {
            SPSite oSite = (SPSite)properties.Feature.Parent;

            using (SPWeb oWeb = oSite.OpenWeb())
            {
                oWeb.MasterUrl = oWeb.ServerRelativeUrl + "/_catalogs/masterpage/CustomMaster.master";
                oWeb.CustomMasterUrl = oWeb.ServerRelativeUrl + "/_catalogs/masterpage/CustomMaster.master";
                oWeb.Update();
            }
        }


        // Uncomment the method below to handle the event raised before a feature is deactivated.

        public override void FeatureDeactivating(SPFeatureReceiverProperties properties)
        {
            SPSite oSite = (SPSite)properties.Feature.Parent;

            using (SPWeb oWeb = oSite.OpenWeb())
            {
                oWeb.MasterUrl = oWeb.ServerRelativeUrl + "/_catalogs/masterpage/v4.master";
                oWeb.CustomMasterUrl = oWeb.ServerRelativeUrl + "/_catalogs/masterpage/v4.master";
                oWeb.Update();
            }
        }

Here, oWeb.MasterUrl refers to System Master Page and oWeb.CustomMasterUrl refers to Site Master Page.

Cheers !