How to find locked user accounts from active directory and unlock the user accounts

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C#

Could anyone please help me on to get the locked users from active directory user group and get it unlocked by using c#.

Note: Only users from AD account not other AD objects.

I have tried this:

string strUserName = o.SAMAccountName;

            using (var context = new PrincipalContext(ContextType.Domain))
            {
                using (var user1 = UserPrincipal.FindByIdentity(context, IdentityType.SamAccountName, strUserName))
                {
                    if (user1.IsAccountLockedOut())//Getting below error here//
                    {
                        /* Button visible to Unlock user */
                    }
                    else
                    {
                        e.Row.Cells[ColumnUnlock].Controls.Clear();
                    }
                }
            }

am getting error "Object reference not set to an instance of object".

I tried with different method and getting different error, please help

Thanks in Advance