SharePoint Delegate control is a mechanism that we can inject a code or control to the SharePoint Master page with out touching any any code in the SharePoint Master page.
“ is a container type control which can holds child controls on it “
This is the simplest explanation for the SharePoint delegate controls but this also may confuse to you. So let me further simplify.
SharePoint Master Page design in a way that leaving place holders(Not a place holder really but for get an idea) to some controls (Delegate Controls) that has a machinist to inject.
Assume this is our SharePoint Master page.
The Red Rectangle (SmallSearchInputBox) is a delegate control. It means the textbox and other functionalities in that search box not written in the master page. Master page just have a place holder with id SmallSearchInputBox and which is going to dynamically replace by search box in the run time.
Master Page code is just look like this.
There are few delegate controls defined in the SharePoint Master page.
Why We need Delegate Controls
1. We can inject controls to master page with out changing any code. For an example you need a custom control to put in the place of small search box. So you can create a control and activate the control using a feature.Then control will display in the place of search box but good thing is you didn’t touch any master page code.
2.You can customize the master page delegate controls dynamically by user permissions,URLs , or any other custom rule
3. Recently we have a problem that we need to include some custom user validation for each and every SharePoint page. So we finally decided to put that code in AdditionalPageHead so it will get apply to all pages and we need to simply create a control to inject to the delegate control.
SharePoint 2010 and SharePoint 2013
AdditionalPageHead
GlobalNavigation
GlobalSiteLink3
GlobalDelegate2
GlobalDelegate0
PublishingConsole
TopNavigationDataSource
SmallSearchInputBox
QuickLaunchDataSource
TreeViewAndDataSource
New Delegate controls in SharePoint 2013
SuiteLinksDelegate
SuiteBarBrandingDelegate
PromotedActions