SharePoint Feature Receiver : System.InvalidCastException: Unable to cast object of type 'Microsoft.SharePoint.SPSite' to type 'Microsoft.SharePoint.SPWeb

If you trying to activate masterpage in the site collection level and if you are using below code this will thrown error

this code is referring only website not site collection

  public override void FeatureActivated(SPFeatureReceiverProperties properties)

        {

           SPWeb currentWeb = (SPWeb)properties.Feature.Parent;

            currentWeb.MasterUrl = "/_catalogs/masterpage/CustomMaster.master";

            currentWeb.CustomMasterUrl = "/_catalogs/masterpage/CustomMaster.master";

            currentWeb.Update();

        }

Error: 

Feature receiver assembly 'TestMasterPages, Version=1.0.0.0, Culture=neutral, PublicKeyToken=9c3395ff06cc524a', class 'TestMasterPages.Features.CustomMaster.CustomMasterEventReceiver', method 'FeatureActivated' for feature '0ab67b2b-2360-45f2-b26e-e33a49ceea3c' threw an exception: System.InvalidCastException: Unable to cast object of type 'Microsoft.SharePoint.SPSite' to type 'Microsoft.SharePoint.SPWeb'.   

 at TestMasterPages.Features.CustomMaster.CustomMasterEventReceiver.FeatureActivated(SPFeatureReceiverProperties properties)   

 at Microsoft.SharePoint.SPFeature.DoActivationCallout(Boolean fActivate, Boolean fForce)

Solution:

  public override void FeatureActivated(SPFeatureReceiverProperties properties)

        {

          SPSite currentSite = (SPSite)properties.Feature.Parent;

            SPWeb currentWeb = currentSite.RootWeb;

            currentWeb.MasterUrl = "/_catalogs/masterpage/CustomMaster.master";

            currentWeb.CustomMasterUrl = "/_catalogs/masterpage/CustomMaster.master";

            currentWeb.Update();

        }