c# - Why use this construct - PropertyChangedEventHandler handler = this.PropertyChanged? -

the article http://msdn.microsoft.com/en-us/magazine/dd419663.aspx has following code sample:

public event propertychangedeventhandler propertychanged;  protected virtual void onpropertychanged(string propertyname) {            propertychangedeventhandler handler = this.propertychanged;     if (handler != null)     {         var e = new propertychangedeventargs(propertyname);         handler(this, e);     } } 

my question gained introducing variable 'handler' - following code seems work fine:

public event propertychangedeventhandler propertychanged;  protected virtual void onpropertychanged(string propertyname) {        if (propertychanged!= null)     {         var e = new propertychangedeventargs(propertyname);         propertychanged(this, e);     } } 

the reasoning behind local variable in multi-threaded environment, event devoid of subscribers (ie, become null) in gap between checking null , firing event.

by taking local variable, avoiding potential issue - checking event in thread-safe way. raise issue event might thrown item had unhooked.