C#：如何将字符串转换为 class 以用于“是”操作？

Question

假设 predicate.Or 基于字符串列表动态生成 sql where 子句。 有没有办法重构它，这样我就不必对 case 语句进行硬编码？ 有没有办法摆脱 switch 语句并重构为更简单/优雅的 foreach？

//contentTypes will ALWAYS have members that are the string class name of some classes in our project
//ex List<string> contentTypes = new List<string>{"News", "Photos", "Video", "Product"}

foreach (string t in contentTypes)
            {
                switch (t)
                {
                    case "News":
                        predicate = predicate.Or(ci => ci is News);
                        break;
                    case "Photos":
                        predicate = predicate.Or(ci => ci is Photos);
                        break;
                    case "Video":
                        predicate = predicate.Or(ci => ci is Video);
                        break;
                    case "Product":
                        predicate = predicate.Or(ci => ci is Product);
                        break;
                }
            }

Answer 1

你绝对可以让它更干净：

foreach (string t in contentTypes)
{
    Expression<Func<ContentItem, bool>> typePredicate = t switch
    {
        nameof(News) => ci is News,
        nameof(Product) => ci is Photos,
        nameof(Video) => ci is Video,
        nameof(Product) => ci is Product,
        _ => throw new InvalidOperationException($"Unexpected content type: '{t}'")
    };
    predicate = predicate.Or(typePredicate);
}

另一种选择是创建一个Dictionary<string, Expression<Func<ContentItem, bool>>然后只使用：

foreach (string t in contentTypes)
{
    predicate = predicate.Or(typePredicates[t]);
}

（如果您想要比从索引器获得的KeyNotFoundException更清晰的异常，请使用TryGetValue 。）

如果你有很多类型要添加，你可以创建一个辅助方法来使字典创建非常无缝。 但我会坚持明确说明您确实想要支持哪些类型。