How to access the asp controls in code behind using vs2015?

See more:

ASP.NET5

Dear all,
Now i start the Visual studio 2015 i insert the Text Box in Designer page.like this

<asp:TextBox ID="txtDate" runat="server" CssClass="date" ></asp:TextBox>

and access the textbox in code behind i get the following Error:

"The name 'txtDate' does not exist in the current context" how to i solve the Error any one give me the Reference.

Note:
I am using Master page and working windows7 OS with SP1.

Thanks Advance!...

Regards,
Karthik M

What I have tried:

I tried to access the asp controls from code behind in ASP.net 2015.