using System;

using System.Collections.Generic;

using System.Linq;

​

public class Program

{

    public static void Main()

    {

        var settings = new List<DocumentNotification>

        {

            new DocumentNotification{Email = "abc.com;def.com", Identifier =null}, 

            new DocumentNotification{Email = "abc.com;def.com", Identifier = "1234567890"}

        };

        var receivers = new List<string>();

        receivers.AddRange(settings.FirstOrDefault(x => x.Identifier == null)?.Email?.Split(";") ?? Enumerable.Empty<string>());

        receivers.AddRange(settings.Where(x => x.Identifier == "1234567890" && !string.IsNullOrEmpty(x.Email)).Select(x => x.Email.Split(";")).SelectMany(x => x));

        Console.WriteLine(receivers.Count());

        foreach (var r in receivers)

            Console.WriteLine(r);

    }

}

​

public class DocumentNotification

{

    public int Id

    {

        get;

        set;

    }

​

    public Guid CustomerId

    {

        get;

        set;

    }

​

    public string Identifier

    {

        get;

        set;

    }

​

    public string Email

    {

        get;

        set;

    }

​

    public int AppType

    {

        get;

        set;

    }

​

    public DateTime CreatedDate

    {

        get;

        set;

    }

​

    public DateTime UpdatedDate

    {

        get;

        set;

    }

}