How to retrieve data from cookies ?

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C#

VB

ASP.NET

ASP.NET-Core

i have a small web application with a login page and default page with forms authentication set up exactly ASP.NET Forms Authentication .

Now on the default page i have a SQL Query to retrieve EmailID of logged user like

Select EmailID from users where username = "LoggedUser"

how can i pass logged user value from my cookie to loggeduser variable of my SQL Query ?

What I have tried:

Earlier i was using custom authentication like i will store

session("Loggeduser") = txtusername.text

then retrieve it into SQL query and everything works fine. later i found that method is not safe as client may be able to manipulate and login with someone else's username.