帶有類型作爲鍵的字典中派生類型參數的函數

Question

我希望能夠使用Key作爲對象類型將函數存儲在字典中。所有的函數都會接受來自同一個基類的對象。我希望能夠（基於註冊函數）將類轉換爲DbCommand。我在下面列出了我的代碼，看起來是正確的，但語法錯誤，等等。我想我錯過了一些非常簡單的東西......任何幫助將不勝感激！

private Dictionary<object, Func<TrackingRecord, DbCommand>> conversionFunctions = new Dictionary<object, Func<TrackingRecord, DbCommand>>(); 


public void RegisterConversionFunction<T>(Func<T, DbCommand> conversionFunction) where T : TrackingRecord 
    { 
     conversionFunctions.Add(typeof(T), conversionFunction); 
    } 

public DbCommand GetConverstion<T>(T trackingRecord) where T : TrackingRecord 
    { 
     DbCommand command = null; 

     if (conversionFunctions.ContainsKey(trackingRecord.GetType())) 
     { 
      Func<T, DbCommand> conversionFunction; 
      if (conversionFunctions.TryGetValue(trackingRecord.GetType(), out conversionFunction)) 
      { 
       command = conversionFunction.Invoke(trackingRecord); 
      } 
     } 
     else 
     { 
      command = DefaultConversion(trackingRecord); 
     } 
     return command; 
    } 

Answer 1

與你原來的解決方案的問題是，該方法的簽名Func<T, DbCommand>不匹配預期值Func<TrackingRecord, DbCommand>。爲了解決這個問題，因爲你知道，T被約束到從TrackingRecord繼承，你可以創建一個預期的簽名（Func<TrackingRecord, DbCommand>）相匹配的包裝功能部件和鑄鐵的參數T.

試試這個：

private Dictionary<object, Func<TrackingRecord, DbCommand>> conversionFunctions = new Dictionary<object, Func<TrackingRecord, DbCommand>>(); 

public void RegisterConversionFunction<T>(Func<T, DbCommand> conversionFunction) where T : TrackingRecord 
{ 
    conversionFunctions.Add(typeof(T), tr => conversionFunction((T)tr)); 
} 

public DbCommand GetConverstion<T>(T trackingRecord) where T : TrackingRecord 
{ 
    DbCommand command = null; 
    Func<TrackingRecord, DbCommand> conversionFunction; 

    if (conversionFunctions.TryGetValue(typeof(T), out conversionFunction)) 
     command = conversionFunction.Invoke(trackingRecord); 
    else 
     command = DefaultConversion(trackingRecord); 

    return command; 
}