Well, there is a C# function, but it's not actually a function. CType is VB's way of casting. There are two ways you can do what you want to do in C#:
RichTextBox rtb = TabControl1.SelectedTab.Controls[0] as RichTextBox;
Then you would access the text by using "rtb.Text". The other way you could do this is by direct casting:
((RichTextBox)TabControl1.SelectedTab.Controls[0]).Text
