在Linq Where表达式中对Null检查String.ToLower [英] Null check String.ToLower in Linq Where expression

问题描述

我有这种方法

private IList<Order> FilterOrders(string filterText)
{
    string filterTextLowerCase = filterText.ToLower();
    var filtered = _orders.Where(order =>
        order.OrderIdFullOrderNumber.ToLower().Contains(filterTextLowerCase) ||
        order.Name.ToLower().Contains(filterTextLowerCase) ||
        order.Status.ToLower().Contains(filterTextLowerCase) ||
        order.TimeRemaining.ToLower().Contains(filterTextLowerCase) ||
        order.Address.ToLower().Contains(filterTextLowerCase) ||
        order.City.ToLower().Contains(filterTextLowerCase) ||
        order.State.Abbrev.ToLower().Contains(filterTextLowerCase) ||
        order.PostalCode.ToLower().Contains(filterTextLowerCase)).ToList();
    return filtered;
}

如您所见，属性中可能存在null引用异常，那么在此Where表达式中检查null的最佳方法是什么?

As you can see there's a possible nullreference exception in properties, what would be the best way to check for null in this Where expression?

推荐答案

很遗憾，目前我无法使用C#6.0功能.

Unfortunately I couldn't use C# 6.0 features at this point.

我最终创建了一个扩展方法来过滤列表，然后与另一个扩展方法Contains进行比较，该方法接受@ robert-mckee的建议并接受@ jeppe-stig-nielsen的演示，接受一个StringComparison参数.

I ended up creating an extension method to filter the list, comparing with another extension method, Contains, that accepts a StringComparison parameter as suggested from @robert-mckee and demonstrated from @jeppe-stig-nielsen.

public static IList<Order> FilterOrders(this IList<Order> orders, string filterText)
        {
            var filtered = orders.Where(order =>
                order.OrderIdFullOrderNumber.Contains(filterText, StringComparison.OrdinalIgnoreCase) ||
                order.Name.Contains(filterText, StringComparison.OrdinalIgnoreCase) ||
                order.Status.Contains(filterText, StringComparison.OrdinalIgnoreCase) ||
                order.TimeRemaining.Contains(filterText, StringComparison.OrdinalIgnoreCase) ||
                order.Address.Contains(filterText, StringComparison.OrdinalIgnoreCase) ||
                order.City.Contains(filterText, StringComparison.OrdinalIgnoreCase) ||
                order.State.Abbrev.Contains(filterText, StringComparison.OrdinalIgnoreCase) ||
                order.PostalCode.Contains(filterText, StringComparison.OrdinalIgnoreCase)).ToList();
            return filtered;
        }

public static bool Contains(this string source, string value, StringComparison comparison)
        {
            if (string.IsNullOrWhiteSpace(source))
            {
                return false;
            }

            return source.IndexOf(value, comparison) >= 0;
        }

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