Submitting data from PeopleEditor control in Web Part gives error

Question

I have a web part that submits data to a list and it uses a PeopleEditor control for selecting users.

Once user is selected, when I submit the form it gives following error

Invalid data has been used to update the list item. The field you are trying to update may be read only.

I have included tag prefix as

<%@ Register TagPrefix="SharePoint" Namespace="Microsoft.SharePoint.WebControls" Assembly="Microsoft.SharePoint, Version=15.0.0.0, Culture=neutral, PublicKeyToken=71e9bce111e9429c" %>

Control is inserted as

<SharePoint:PeopleEditor ID="a" runat="server"  PrincipalSource="All" SharePointGroup="Everyone" MultiSelect="false" />

And on click of a submit button-

SPWeb web = SPContext.Current.Web;
SPListCollection vlist = web.Lists;

SPList list = web.Lists[new Guid("{xxxxxxxxx-xxxx-xxxx-xxxx-xxxxxxxxxxxx}")];
SPListItem listitem = list.Items.Add();
listitem["FormCompletedBy"] = a.ResolvedEntities;
listitem.Update();

Where

FormCompletedBy

is a field in SharePoint List where the data is to be submitted.

Answer 1

You can save User data as below

string[] userarray = a.CommaSeparatedAccounts.ToString().Split(',');;
SPFieldUserValueCollection usercollection = new SPFieldUserValueCollection();

for (int i = 0; i < userarray.Length; i++)
{
     SPFieldUserValue usertoadd = ConvertLoginAccount(userarray[i]);
     usercollection.Add(usertoadd);
}
listItem["FormCompletedBy"] = usercollection;
listItem.Update();

public SPFieldUserValue ConvertLoginAccount(string userid)
{
    SPFieldUserValue uservalue;
    using (SPSite site = new SPSite(SPContext.Current.Site.Url))
    {
        using (SPWeb web = site.OpenWeb())
       {
          SPUser requireduser = web.EnsureUser(userid);
          uservalue = new SPFieldUserValue(web, requireduser.ID, requireduser.LoginName);
       }
    }
    return uservalue;
}