Creating Bin Transfer in C# using Advanced Bin Management

I am trying to create a bin transfer script for some mass transfers that we are going through. I have nearly everything completed up to the actual transfer. However, I keep getting an error "Before you can create the Inventory Detail, please enter values for: Quantity". I am not sure where I am going wrong with the quantity. See my code below:

public class BinTransferOrder
{
    public string LocationInternalID { get; set; }
    public string ItemInternalID { get; set; }
    public string ItemQuantity { get; set; }
    public string BinFromID { get; set; }
    public string BinToID { get; set; }
}

public void CreateBinTransferOrder(List<BinTransferOrder> orders)
{
BinTransfer transfer = new BinTransfer();