Which event called first? Master Page Page_Load or Content Page Page_Load
The content page load event will fire before the master page load event.
See here for full order (MSDN: Events in ASP.NET Master and Content Pages)
Copying incase link goes dead:
The following is the sequence in which events occur when a master page is merged with a content page:
- Content page PreInit event.
- Master page controls Init event.
- Content controls Init event.
- Master page Init event.
- Content page Init event.
- Content page Load event.
- Master page Load event.
- Master page controls Load event.
- Content page controls Load event.
- Content page PreRender event.
- Master page PreRender event.
- Master page controls PreRender event.
- Content page controls PreRender event.
- Master page controls Unload event.
- Content page controls Unload event.
- Master page Unload event.
- Content page Unload event.
As far as MasterPage is indeed a user control all rules applied to it as for user controls:
Master pages run Page_Load() from the deepest level of nesting outwards. So your nested webpage will first run the page_load event.