如果为空，则将其设置为linq中的空字符串lambda表达式

本文关键字：字符串 表达式 linq lambda 如果 设置 | 更新日期: 2023-09-27 18:05:06

我有一个foreach循环，看起来像这样:

 foreach (var item in listOfFieldNames)
                {
    list.Where(s =>  s.GetType().GetProperty(item).GetValue(s, null).ToString().ToLower().Contains(searchString.ToLower()));
}

，它工作得很好，但每当"s"是null时，我得到nullreferenceexception

我想改变我的s = null为s = " "但我不知道如何做到这一点。你们能帮帮我吗?或者有一种方法可以跳过空记录继续循环而不会出现异常。这对我也有帮助。

试图找出这个相当长一段时间，无法找到答案:/尝试了一些。defaultifempty组合，但我不知道该在里面放什么

如果为空，则将其设置为linq中的空字符串lambda表达式

如果s有时为null，您可以使用:

list.Where(s => s != null && s.GetType().GetProperty(item).GetValue(s, null).ToString().ToLower().Contains(searchString.ToLower()));

后编辑

问题是GetValue返回null。在这种情况下，您可以使用空合并操作符??。我个人会将表达式展开，这样更容易阅读:

list.Where(s => 
{
    var property = s.GetType().GetProperty(item);
    var value = property.GetValue(s, null);
    if (value == null) return false;
    return value.ToString().Contains(searchString, StringComparison.OrdinalIgnoreCase);
});

在循环之前从列表中删除null引用:

list.RemoveAll(s => s == null);

// doing the .ToLower() on searchString before the query will improve performance.  
// The original form would execute each pass (as a note .ToUpper() is slightly faster for 
// conversions and comparisons since upper case letter come before lower case.
searchString = searchString.ToLower();
// assuming `list` is a List<T> this will allow the inner type of be used to get the 
// property list. It is possible to check that getters exist for the properties which 
// may be helpful if you have any setter only properties. If you work with any languages
// other than C# you may also want to exclude properties with indexers.
var properties = list.GetType()
                     .GetGenericArguments()[0].GetProperties()
                     .Where(p => listOfFieldNames.Contains(p.Name))
                     .ToList();
var query = from listItem in list
            // if you have any setter only properties p.GetValue() will cause an exception
            let values = properties.Select(p => p.GetValue(listItem))
                                   .Where(p => p != null)
            // you probably don't see the null coalesce but it is possible that .ToString()
            // was over ridden and could allow null. The empty string is simplier to inline
            // than to add another null filter after the .ToString()
                                   .Select(p => (p.ToString() ?? "").ToLower())
            // this will allow you to compare all properties that had values to your search
            where values.Contains(searchString)
            select listItem;
// this will create a new list that will contain all of the values from list that had at 
// least one property that matched your searc hstring
var matched = query.ToList();
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