各種派生類型集合上的通用LINQ to SQL擴展方法

Question

我正在嘗試為IQueryable<T>創建通用擴展方法。 T是一個抽象Player ，IQueryable可以包含具體類型的Goalkeeper和Striker 。

public abstract class Player
{
    public string Name { get; set; }
    public string Fouls { get; set; }
}

public class Goalkeeper : Player
{
    public int Saves { get; set; }

}

public class Striker : Player
{
    public int Goals { get; set; }
}

有效的擴展方法（簡單的方法）如下所示：

public static IQueryable<Goalkeeper> NotPerforming(this IQueryable<Goalkeeper> goalkeepers)
{
    return goalkeepers.Where(g => g.Saves < goalkeepers.Average(x => x.Saves));
}

public static IQueryable<Striker> NotPerforming(this IQueryable<Striker> strikers)
{
    return strikers.Where(g => g.Goals < strikers.Average(x => x.Goals));
}

我可以這樣使用：

var badGoalies = players.OfType<Goalkeeper>().NotPerforming();
var badStrikers = players.OfType<Striker>().NotPerforming();

所以現在我想查詢所有表現不佳的球員 。

var badPlayers = players.NotPerforming();

我似乎無法正常工作。

public static IQueryable<T> NotPerforming<T>(this IQueryable<T> players)
    where T : Player
{
    // what to do here?
}

我嘗試過類似...

return players.OfType<Striker>().NotPerforming()
          .Union(
              players.OfType<Goalkeeper>().NotPerforming()
          );

這不起作用。

這樣做的最佳實踐方法是什么？並且不離開LINQ-to-SQL，因為我想保持鏈接擴展方法的性能，並牢記良好的性能？

Answer 1

您可以創建另一個擴展方法，並在其中添加邏輯：

public static IQueryable<Player> NotPerforming(this IQueryable<Player> players)
{
    var notPerformingGoalKeepers = players.NotPerformingGoalkeepers();
    var notPerformingStrikers = players.NotPerformingStrikers();

    return notPerformingGoalKeepers.Cast<Player>()
        .Concat(notPerformingStrikers);
}

public static IQueryable<Goalkeeper> NotPerformingGoalkeepers(this IQueryable<Player> players)
{
    var goalkeepers = players.OfType<Goalkeeper>();
    return goalkeepers.Where(g => g.Saves < goalkeepers.Average(x => x.Saves));
}

public static IQueryable<Striker> NotPerformingStrikers(this IQueryable<Player> players)
{
    var strikers = players.OfType<Striker>();
    return strikers.Where(g => g.Goals < strikers.Average(x => x.Goals));
}

並使用：

var badPlayers = players.NotPerforming();

或另一種方法：

public static IQueryable<Player> NotPerforming<T>(this IQueryable<Player> players) where T : Player
{
    if (typeof(T) == typeof(Goalkeeper))
    {
        return players.OfType<Goalkeeper>().NotPerforming();
    }

    if (typeof(T) == typeof(Striker))
    {
        return players.OfType<Striker>().NotPerforming();
    }

    return null;
}

private static IQueryable<Goalkeeper> NotPerforming(this IQueryable<Goalkeeper> goalkeepers)
{
    return goalkeepers.Where(g => g.Saves < goalkeepers.Average(x => x.Saves));
}

private static IQueryable<Striker> NotPerforming(this IQueryable<Striker> strikers)
{
    return strikers.Where(g => g.Goals < strikers.Average(x => x.Goals));
}

並使用：

var badStrikers = players.NotPerforming<Striker>();
var badGoalkeepers = players.NotPerforming<Goalkeeper>();

var badPlayers = players.NotPerforming<Striker>()
    .Concat(players.NotPerforming<Goalkeeper>());