如何告訴泛型類實例有關其泛型擁有的集合？

Question

我有一個抽象類“RiskBaseCollection”，它使用泛型本質上是字典的容器。 存儲在此字典中的項目類型必須在構造函數中傳遞，並且它們必須從名為“RiskBase”的基類繼承：

我在字典容器類中有一個方法，它調用數據庫查詢來填充字典的內容（數據庫調用取決於另一個字段中設置的內容，但這對我來說並不重要）。

在填充字典時，對於每個數據庫記錄，我想查看是否已經創建並存儲了類的適當實例：如果已經創建和存儲，我將檢索它並向其添加其他值。 如果沒有，我會創建一個 RiskBase 類的新實例。 我已經用泛型做到了這一點，這樣實際上創建了正確類型的 RiskBase。

我正在努力解決的是，我想告訴“RiskBase”類的每個實例“擁有”它的集合。 我嘗試了各種方法，但無法讓它發揮作用。

這是 Collection 類和整個 RiskBase 類的摘錄：

public abstract class RiskBaseCollection<T> where T : RiskBase, new() 
{
    public Dictionary<int, T> Items = new Dictionary<int, T>();

    #region Non-critical stuff
      //Removed - not important to my issue.         
    #endregion

    public void ReadData()
    {
        if (ValidConnectionInfo == false)
        {
            return;
        }

        //Create the connection:
        using (OleDbConnection conn = new OleDbConnection(ReaderConnectionString))
        {
            if (conn != null) conn.Open();
            if (conn.State == System.Data.ConnectionState.Open)
            {
                using (OleDbCommand command = new OleDbCommand(SQL, conn))
                {
                    if (command != null)
                    {
                        using (OleDbDataReader reader = command.ExecuteReader())
                        {
                            if (reader != null && reader.HasRows)
                            {
                                while (reader.Read())
                                {
                                    int RiskID = reader.GetValue<int>("ID");
                                    string Title = reader.GetValue<string>("TITLE");
                                    string Status = reader.GetValue<string>("STATUS");
                                    DateTime RaisedDate = reader.GetValue<DateTime>("RAISED_DATE");
                                    DateTime ExpiryDate = reader.GetValue<DateTime>("EXPIRY_DATE");

                                    //Get the FLOC string for the current record
                                    string FunctionalLocation = reader.GetValue<string>("FUNCTIONAL_LOCATION");

                                    T thisRisk = (Items.ContainsKey(RiskID)) ? (T)Items[RiskID] : new T();
                                    thisRisk.RiskID = RiskID;
                                    thisRisk.Title = Title;
                                    thisRisk.Status = Status;
                                    thisRisk.RaisedDate = RaisedDate;
                                    thisRisk.ExpiryDate = ExpiryDate;
                                    thisRisk.AddFunctionalLocation(FunctionalLocation);
                                    thisRisk.Owner = this;      //<=Error: Cannot implicitly convert type 'CRT.Risks.RiskBaseCollection<T>' to 'CRT.Risks.RiskBaseCollection<CRT.Risks.RiskBase>'  
                                    Items[RiskID] = thisRisk;  
                                }
                            }
                        }
                    }
                }
            }
        }
    }
}





public class RiskBase
{
    public RiskBaseCollection<RiskBase> Owner { get; set; }
    public virtual RiskType RiskType { get; set; }

    public RiskBase() { }
    public RiskBase(RiskBaseCollection<RiskBase> riskBaseCollection, RiskType riskType)
    {
        Owner = riskBaseCollection;
        RiskType = riskType;
    }

    public virtual int RiskID { get; set; }
    public virtual string Title { get; set; }
    public virtual string Status { get; set; }
    public virtual DateTime RaisedDate { get; set; }
    public virtual DateTime ExpiryDate { get; set; }

    public virtual List<string> FunctionalLocations { get; set; } = new List<string>();
    public virtual void AddFunctionalLocation(string functionalLocation)
    {
        if (!FunctionalLocations.Contains(functionalLocation)) FunctionalLocations.Add(functionalLocation);
    }
}

本質上，我正在尋找一種為 RiskBase 類設置“所有者”屬性的方法。

Answer 1

使RiskBaseCollection<T>實現協變通用接口，並將RiskBase.Owner屬性的類型更改為此接口：

// new interface
interface IRiskBaseCollection<out T>
{
    // interface members
}

public abstract class RiskBaseCollection<T> : IRiskBaseCollection<T> 
    where T : RiskBase, new() 
{
    // ...
    thisRisk.Owner = this;  // this now compiles!
}

public class RiskBase
{
    public IRiskBaseCollection<RiskBase> Owner { get; set; }    // interface reference
    // ...
}