Change Masterpage on SPWeb

Code:

 using (SPSite site = new SPSite("http://siteUrl")) 
 { 
     using (SPWeb web = site.OpenWeb()) 
     { 
         string newMasterPage = "newMasterPage.master";
         string webAppRelativePath = site.ServerRelativeUrl.TrimEnd(new char[] { '/' }); 

         //Site master page 
         web.CustomMasterUrl = string.Format("{0}/_catalogs/masterpage/{1}", webAppRelativePath, newMasterPage); 

         //System master page 
         web.MasterUrl = string.Format("{0}/_catalogs/masterpage/{1}", webAppRelativePath, newMasterPage); 

         web.Update();    
     }
 }

Here we apply the new MasterPage to both site pages and system pages.