C# – Authorization Asp.net web.config

using System.Configuration;
using System.Web.Configuration;

Configuration config = WebConfigurationManager.OpenWebConfiguration(Request.ApplicationPath);
AuthorizationSection section = (AuthorizationSection)config.GetSection("system.web/authorization");

AuthorizationRule rule = new AuthorizationRule(AuthorizationRuleAction.Allow);
rule.Roles.Add("admins");
section.Rules.Add(rule);

config.Save();

Imports System.Configuration
Imports System.Web.Configuration

Dim config As Configuration = WebConfigurationManager.OpenWebConfiguration(Request.ApplicationPath)
Dim section As AuthorizationSection = CType(config.GetSection("system.web/authorization"), AuthorizationSection)

Dim rule As New AuthorizationRule(AuthorizationRuleAction.Allow)
rule.Roles.Add("admins")
section.Rules.Add(rule)

config.Save()

ASP.NET needs web.config write permission for this to work, so be careful.

Updated NOTE on this solution: Checking with FileAccess.ReadWrite will fail for Read-Only files so the solution has been modified to check with FileAccess.Read.

ORIGINAL: I've used this code for the past several years, and I haven't had any issues with it.

Understand your hesitation about using exceptions, but you can't avoid them all of the time:

protected virtual bool IsFileLocked(FileInfo file)
{
    try
    {
        using(FileStream stream = file.Open(FileMode.Open, FileAccess.Read, FileShare.None))
        {
            stream.Close();
        }
    }
    catch (IOException)
    {
        //the file is unavailable because it is:
        //still being written to
        //or being processed by another thread
        //or does not exist (has already been processed)
        return true;
    }

    //file is not locked
    return false;
}