Unable to initialize list property of ASP MVC3 ViewModel

Question

Below are two ViewModels we use in an ASP MVC3 site we're building. In the code below that, I am trying to populate the TradingPartners IList property of the AgentIdDetail property of the AgentWithTraining variable named bigAgent.

To better illustrate:

bigAgent is a AgentWithTraining

AgentWithTraining has an AgentIdDetail ICollection list object as a property AgentIdDetail has an IList list object by the name of TradingPartner

public class AgentWithTraining
{
    public Monet.Models.Agent Agent { get; set; }
    public ICollection<AgentProdTrainDetail> AgentProdTrainDetails { get; set; }
    public ICollection<AgentIdDetail> AgentIdDetails { get; set; }
}

public class AgentIdDetail
{
    public string AgentId { get; set; }
    public string CompanyCode { get; set; }
    public IList<string> TradingPartners { get; set; }
}

The problem is the following line of code:

bigAgent.AgentIdDetails = new AgentIdDetail();

This gives me the error:

Cannot implicitly convert type 'Monet.ViewModel.AgentIdDetail' to 'System.Collections.Generic.ICollection<Monet.ViewModel.AgentIdDetail>'. An explicit conversion exists (are you missing a cast?)

Can someone explain how I need to go about initializing bigAgent.AgentIdDetails? Below is the full section of code I am using, just in case that is helpful.

        //Grab Trading partner information and add it to 'trainlist' object
        List<AgentIdToTradingPartner>tradingParter = new List<AgentIdToTradingPartner>();

        var symNumToAgId = from s in db.SymetraNumberToAgentId
                           where s.SymetraNumber == id
                           select s;

        //*******************************************
        //This line is causing the problem
        //*******************************************
        bigAgent.AgentIdDetails = new AgentIdDetail();

        foreach (var s in symNumToAgId)
        {
            AgentIdDetail item = new AgentIdDetail();

            item.AgentId = s.AgentId;
            item.CompanyCode = s.CompanyCode;

            tradingParter = db.AgentIdToTradingPartner
                              .Where(r => r.AgentId == s.AgentId).ToList();

            item.TradingPartners = new List<string>();

            foreach (var t in tradingParter)
            {
                item.TradingPartners.Add(t.TradingPartner.ToString());
            }

            bigAgent.AgentIdDetails.Add(item);
        }

Answer 1

You cannot initilize this way because the AgentIdDetails is an ICollection<AgentIdDetail> and not a single instance of AgentIdDetail.

You have to initilize the collection and then add a new item on this collection. For sample:

// initilize as a List<AgentIdDetail>
bigAgent.AgentIdDetails = new List<AgentIdDetail>();

foreach (var s in symNumToAgId)
{
    AgentIdDetail item = new AgentIdDetail();

    item.AgentId = s.AgentId;
    item.CompanyCode = s.CompanyCode;

    tradingParter = db.AgentIdToTradingPartner
                      .Where(r => r.AgentId == s.AgentId).ToList();

    item.TradingPartners = new List<string>();

    foreach (var t in tradingParter)
    {
        item.TradingPartners.Add(t.TradingPartner.ToString());
    }

    bigAgent.AgentIdDetails.Add(item);
}

As a good pratice, I like to initilize my Collection properties on ViewModels on the constructor, so I do not need to worry about initilize when I fill it, unless I need a new one, for sample:

public class AgentWithTraining
{
    public Monet.Models.Agent Agent { get; set; }
    public ICollection<AgentProdTrainDetail> AgentProdTrainDetails { get; set; }
    public ICollection<AgentIdDetail> AgentIdDetails { get; set; }

    public AgentWithTraining()
    {
        this.AgentProdTrainDetails = new List<AgentProdTrainDetail>();
        this.AgentIdDetails = new List<AgentIdDetail>();
    }
}