带有类型作为键的字典中派生类型参数的函数

Question

我希望能够使用Key作为对象类型将函数存储在字典中。所有的函数都会接受来自同一个基类的对象。我希望能够（基于注册函数）将类转换为DbCommand。我在下面列出了我的代码，看起来是正确的，但语法错误，等等。我想我错过了一些非常简单的东西......任何帮助将不胜感激！

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

与你原来的解决方案的问题是，该方法的签名Func<T, DbCommand>不匹配预期值Func<TrackingRecord, DbCommand>。为了解决这个问题，因为你知道，T被约束到从TrackingRecord继承，你可以创建一个预期的签名（Func<TrackingRecord, DbCommand>）相匹配的包装功能部件和铸铁的参数T.

试试这个：

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; 
}