How to determine which Child Page is being displayed from Master Page?

It's better to let the ContentPage notify the MasterPage. That's why the ContentPage has a Master Property and MasterPage does not have Child property. Best pratice in this is to define a property or method on the MasterPage and use this through the Master property of the ContentPage.

If you use this technique it's best to explicitly specify the classname for the MasterPage. This makes to use the MasterPage in the ContentPage.

Example:

//Page_Load
MyMaster m = (MyMaster)this.Master;

m.TellMasterWhoIAm(this);

Hope this helps.

I use this:

string pageName = this.ContentPlaceHolder1.Page.GetType().FullName;

It retuns the class name in this format "ASP.default_aspx", but I find that easy to parse for most purposes.

Hope that helps!