How can I set the Page.User Property in ASP.NET?

This is mostly a theory question, for example if I wanted to create my own alternative to 开发者_JAVA技巧forms authentication.

Page.User just returns Page.Context.User, which should be set in a handler for HttpApplication.AuthenticateRequest via module or global.asax. You can provide a custom implementation of IPrincipal, which could then return a custom implementation of IIdentity.

For example:

public class App : HttpApplication
{
    public App()
    {
        AuthenticateRequest += App_AuthenticateRequest;
    }

    void App_AuthenticateRequest(object sender, EventArgs e)
    {
        var cookie = Request.Cookies[FormsAuthentication.FormsCookieName];

        if (cookie == null) return;

        var userData = GetUserData(cookie.Value);
        var userIdentity = new MyIdentity(userData);

        Context.User = new MyPrincipal(userIdentity);
    }

    private string GetUserData(string value)
    {
        try
        {
            var ticket = FormsAuthentication.Decrypt(value);
            return ticket == null ? null : ticket.UserData;
        }
        catch (ArgumentException)
        {
            Response.Cookies.Remove(FormsAuthentication.FormsCookieName);
            return null;
        }
    }
}

You need to write your own authentication module - see this excellent article that explains how to write an ASP.NET authentication module supporting basic & digest authentication.

In case, you are thinking of only forms authentication scheme then you really don't have to write one. The in-build Forms Authenticate module is quite extensible (and offers a lot of flexibility).

After some googling, I found this blogpost. It describes quite well how to do it. Basically you need to create a custom HTTP module and handle the OnAuthenticateRequest event of HttpApplication object.