How to remove a permission to a SPPrincipal on a Item (programmatically)

Question

I'm trying to work with privileges on elements of a list. I want the item to mantain the same privileges for all SPPrincipal I don't modify.

This is the code

    public static void AssignPermissionsToItem(SPListItem item, SPPrincipal obj, SPRoleType roleType) 
     { 
         if (!item.HasUniqueRoleAssignments) 
         { 
             item.BreakRoleInheritance(false, true); 
          } 

    SPRoleAssignment roleAssignment = new SPRoleAssignment(obj); 
    SPRoleDefinition roleDefinition = item.Web.RoleDefinitions.GetByType(roleType); 
    roleAssignment.RoleDefinitionBindings.Add(roleDefinition); 

    item.RoleAssignments.Add(roleAssignment); 
} 

This code (proposed to me by Alexander), destroys all the permissions and add the permission specified as parameter..

To mantain the past privileges I must set

item.BreakRoleInheritance(true, true); 

which mantains the old privileges.

I tried to make a RemoveAll on the roleAssignment before the add of the new one, but it mantains also the old privileges, like RemoveAll didn't to anything!

This is the code:

 public static void AddPermissionsToItem(SPListItem item, SPPrincipal obj, SPRoleType roleType)
    {
        if (!item.HasUniqueRoleAssignments)
        {
            item.BreakRoleInheritance(true, true);
        }

        SPRoleAssignment roleAssignment = new SPRoleAssignment(obj);
        roleAssignment.RoleDefinitionBindings.RemoveAll();
        roleAssignment.Update();

        SPRoleDefinition roleDefinition = item.Web.RoleDefinitions.GetByType(roleType);
        roleAssignment.RoleDefinitionBindings.Add(roleDefinition);

        item.RoleAssignments.Add(roleAssignment);
    }

My code (simplyfied):

  [WebMethod]
    public ResponseObject InsertInAllReplyers(string username)
    {
        ResponseObject SPResultStatus = new ResponseObject();
        try
        {

                SPUserToken myToken = TokenManager.GetToken(username);
                //ATTENZIONE!! SOLO PER LA MACCHINA LOCALE DIEGO VA MANTENUTO LO SPAZIO!!!
                string fullsite = SPContext.Current.Web.Url + "/sites/Reply Corp";
                //ATTENZIONE!! SOLO PER LA MACCHINA LOCALE DIEGO VA MANTENUTO LO SPAZIO!!!
                using (SPSite site = new SPSite(fullsite, myToken))
                {
                    site.AllowUnsafeUpdates = true;
                    using (SPWeb currentWeb = site.OpenWeb("All Replyers"))
                    {

                        currentWeb.AllowUnsafeUpdates = true;
                        SPList targetList = currentWeb.Lists["Contenuti"];
                        SPListItem listItem = targetList.AddItem();
                        listItem["Title"] = "Prova :)";
                        listItem["tt_contentid"] = "TT_ID252";
                        listItem["Tag"] = "Tag2";
                        listItem.Update();

                        var allreplyers = currentWeb.Groups["All Replyers"];
                        AssignPermissionsToItem(listItem, (SPPrincipal)allreplyers, SPRoleType.Reader);
                        AssignPermissionsToItem(listItem, (SPPrincipal)currentWeb.AllUsers[username], SPRoleType.Contributor);

                        SPResultStatus.SPResultStatus = ResponseObjectSPResultStatus.SUCCESS;
                        SPResultStatus.SPResultUrl = listItem.Url;
                        SPResultStatus.SPResultId = listItem.ID.ToString();
                        SPResultStatus.SPResultType = ResponseObjectSPResultType.LIST;

                        }

                }
        }
        catch (Exception exx)
        {
            return new ResponseObject() { SPResultStatus = ResponseObjectSPResultStatus.FAILURE, Message = exx.Message, Stacktrace = exx.StackTrace, ExceptionType = exx.GetType().ToString() };
        }
        return SPResultStatus;
    }

    public static void AssignPermissionsToItem(SPListItem item, SPPrincipal obj, SPRoleType roleType)
    {
        if (!item.HasUniqueRoleAssignments)
        {
            item.BreakRoleInheritance(false, true);
        }
        item.RoleAssignments.Remove(obj); 
        item.SystemUpdate(); 

        SPRoleAssignment roleAssignment = new SPRoleAssignment(obj);
        SPRoleDefinition roleDefinition = item.Web.RoleDefinitions.GetByType(roleType);

        roleAssignment.RoleDefinitionBindings.Add(roleDefinition);

        item.RoleAssignments.Add(roleAssignment);
    }

Answer 1

Make sure you do an update to your list item before adding the role again.

oListItem.RoleAssignments.Remove(principal);
oListItem.SystemUpdate();