Sitecore check access and roles programatically

The Sitecore security model have changed over time, but the general API to check security access and roles have been stable for many many years.

CHECK IF USER HAS ACCESS TO AN ITEM:

To check if a user have access to a Sitecore item:

// Get Sitecore item
var item = Sitecore.Context.Database.GetItem("/sitecore/content/home/item");

// Check if the current context user 
// have read access to the item
bool canRead = item.Access.CanRead();

// Check if a named user have read access
// to an item
using (new Sitecore.Security.Accounts.UserSwitcher(@"extranet\username", true))
{
  bool canRead = item.Access.CanRead();
}

CHECK IF USER HAVE A CERTAIN ROLE:

The Sitecore User object has a Roles property, but this property does only list the immediate roles, not the roles obtained from roles-in-roles. To check for all roles, you need to use the IsInRole property:

using Sitecore.Security.Accounts;

// Get the role to check
// Remember to prefix the role name with 
// the domain name
Role role = Role.FromName(@"extranet\rolename");

// Check if user have the role
var isInRole = Sitecore.Context.User.IsInRole(role);

MORE TO READ:

• Assign access rights to a security account by Sitecore
• Sitecore: Assigning Security to items from code by briancaos (Sitecore 6 post, but the code is also valid for Sitecore 7,8 and 9).