如何合并C＃谓词

Question

嗨，我有两个谓词。 我想知道是否有可能将它们以某种方式结合在一起？ 第一：

Predicate<object> viewFilter = f =>
{
    ExternalUser u = f as ExternalUser;
    return u.AgentName.ToLower().StartsWith(agentName.ToLower());
};

第二：

Predicate<object> viewFilter = f =>
{
    ExternalUser u = f as ExternalUser;
    return u.ExternalLogin.ToLower().StartsWith(login.ToLower());
};

这就是连接谓词的外观。

Predicate<object> viewFilter = f =>
{
    ExternalUser u = f as ExternalUser;
    return u.ExternalLogin.ToLower().StartsWith(login.ToLower()) && u.AgentName.ToLower().StartsWith(agentName.ToLower());
};

对于WPF数据网格过滤器（网格源为ICollectionView），我需要它。 因此，在创建谓词后，将其设置为如下所示的过滤器：

private void FilterByLogin(ICollectionView view, string login)
{
    Predicate<object> viewFilter = f =>
    {
        ExternalUser u = f as ExternalUser;
        return u.ExternalLogin.ToLower().StartsWith(login.ToLower());
    };
    view.Filter = viewFilter;
}

Answer 1

我将创建两个新方法：

public static Prediate<object> Cast<T>(Predicate<T> inner)
{
    return new Predicate<object>(o => inner((T)o));
}

public static Predicate<T> And<T>(Predicate<T> p1, Predicate<T> p2)
{
    return x => p1(x) && p2(x);
}

那么您可以从谓词中删除类型转换并将其移至顶层：

Predicate<ExternalUser> first = u => u.AgentName.ToLower().StartsWith(agentName.ToLower());
Predicate<ExternalUser> second = u => u.ExternalLogin.ToLower().StartsWith(login.ToLower());

view.Filter = Cast(And(first, second));

Answer 2

确实应该像这样简单：

Predicate<object> viewFilter1 = f =>
{
    ExternalUser u = f as ExternalUser;
    return u.AgentName.ToLower().StartsWith(agentName.ToLower());
};

Predicate<object> viewFilter2 = f =>
{
    ExternalUser u = f as ExternalUser;
    return u.ExternalLogin.ToLower().StartsWith(login.ToLower());
};

Predicate<object> viewFilter = f => viewFilter1(f) && viewFilter2(f);