我有一些这种格式的JSON，其中包括一个集合。

{
  "Store": 12,
  "TransactionItems": {
    "$type": "System.Collections.ObjectModel.Collection`1[[BaseTransactionItem, Common]], mscorlib",
    "$values": [
      {
        "$type": "TransactionSaleItem, Common",
        "Description": "Grey Shirt",
        "ActualSalePrice": 40.0,
        "ActualSalePricePerItem": 40.0
      },
      {
        "$type": "TransactionReturnItem, Common",
        "Description": "Red Shirt",
        "ActualSalePrice": -30.0,
        "ActualSalePricePerItem": -30.0
      }
    ]
  },
  "CountryCode": "GB"
}

当使用Newtonsoft的nuget包实现这一点时，我可以访问TransactionItems，它有两个项目的集合，它们都是BaseTransactionItem类型。当循环遍历这些项时，我如何获得每个项的类型，即TransactionSaleItem和TransactionReturnItem？
简化的C#代码：

[DataContract(IsReference = true)]
public class Transaction : IEquatable<Transaction>
{
    [DataMember]
    public int Store { get; set; }

    [DataMember]
    public ICollection<BaseTransactionItem> TransactionItems { get; set; }

    [DataMember]
    public string CountryCode { get; set; }
}

[DataContract]
[KnownType(typeof(TransactionOrderItem))]
[KnownType(typeof(TransactionReturnItem))]
[KnownType(typeof(TransactionSaleItem))]
public class BaseTransactionItem : IEquatable<BaseTransactionItem>
{
}

[DataContract]
public class TransactionReturnItem : BaseTransactionItem, IEquatable<TransactionReturnItem>
{
}

[DataContract]
public class TransactionSaleItem : BaseTransactionItem, IEquatable<TransactionSaleItem>
{
}