SharePoint @ Big Scholar

To check if a user is in a SharePoint Group (which also checks if the user is in an AD Group within that SharePoint group), use the following code:

 using System.DirectoryServices.AccountManagement;

        public bool IsUserInSharePointGroup(string webUrl, string groupName, string username)
        {
            bool userIsInGroup = false;

            SPSecurity.RunWithElevatedPrivileges(delegate
            {
                try
                {
                    SPWeb web = SPContext.Current.Web;

                    // Find the group
                    SPGroup group = web.SiteGroups[groupName];
                    string upperCaseUserName = username.ToUpper();

                    foreach (SPUser user in group.Users)
                    {
                        // Check if this is an AD Group
                        if (!user.IsDomainGroup)
                        {
                            // Verify if the user name matches the user name in group
                            if (user.LoginName.ToUpper().Equals(upperCaseUserName))
                            {
                                userIsInGroup = true;
                                return;
                            }
                        }
                        else
                        {
                            // this is an AD group
                            var pc = new PrincipalContext(ContextType.Domain);
                            var myuser = UserPrincipal.FindByIdentity(pc, IdentityType.SamAccountName, username);
                            var mygroup = GroupPrincipal.FindByIdentity(pc, user.LoginName);
                            if (myuser.IsMemberOf(mygroup))
                            {
                                userIsInGroup = true;
                                return;
                            }
                        }
                    }
                }
                catch (Exception ex)
                {
                    //Trace error
                }
            });
            return userIsInGroup;
        }