如何将列表转换为可枚举 [英] How to cast list to enumerable

问题描述

以下代码有问题:

public IEnumerable<ISession> GetSessions()
{
    // ...

    using (ProvaDbEntities DBEntities = new ProvaDbEntities(Utilities.ToEntitiesConnectionString()))
    {
        ObjectQuery<session> sessions = DBEntities.session;
        IEnumerable<session> q1 = from session in sessions
                                  where session.site == this.Name
                                  select session;

        List<Session> sessionList = new List<Session>();
        foreach (var s in q1)
        {
            sessionList.Add(new Session(s.id.ToString(),s.username, s.site, new DateTime()));
        }

        IEnumerable<Session> res = sessionList;

        return sessionList;
    }
}

例外是:

无法将对象类型'System.Collections.Generic.List`1 [prova3.Session]'强制转换为类型'System.Collections.Generic.IEnumerable'1 [TAP2009.AuctionSite.Interfaces.ISession]'. /p>

Is not possible to cast object type 'System.Collections.Generic.List`1[prova3.Session]' to type 'System.Collections.Generic.IEnumerable`1[TAP2009.AuctionSite.Interfaces.ISession]'.

查看此SO问题似乎是正确的.我错了吗?

Looking at this SO question it seems to be correct. Am I wrong?

推荐答案

应该没问题，只要Session实现ISession- if ，而您使用的是C#4和.NET 4.如果不是，那就不会.

It should be fine, so long as Session implements ISession - if you're using C# 4 and .NET 4. If you're not, it won't be.

请注意，您提到的问题在两种情况下均使用相同的"T"，而您遇到的例外情况是有关将List<Session>转换为IEnumerable<ISession>的问题.您尚未说明要从何处获得异常，这使得很难确切地了解发生了什么.您确定这确实是实际上的失败代码吗?您确定要获取异常而不是编译时失败吗?

Note that the question you referred to use the same "T" in both cases - whereas the exception you've got is about converting a List<Session> to an IEnumerable<ISession>. You haven't stated where you're getting the exception, which makes it a bit harder to see exactly what's going on... Are you sure this is actually the code which is failing? Are you sure you're getting an exception rather than a compile-time failure?

如果您不使用.NET 4和C#4，则协方差的解决方法在此非常简单-使用Cast<T>() LINQ运算符:

If you're not using .NET 4 and C# 4, the workaround for covariance is reasonably simple here - use the Cast<T>() LINQ operator:

return sessionList.Cast<ISession>();

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