C# 检查 lambda 表达式中的属性是否为空
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Check if property is null in lambda expression
提问by mickyjtwin
I have a list of objects that I am trying to bind to a listview. I am sorting by two properties. The problem exists whereby some records may not have one of the properties. This is causing an error. I would like it to still bind the records that have the property.
我有一个我试图绑定到列表视图的对象列表。我按两个属性排序。存在的问题是某些记录可能不具有其中一项属性。这导致了错误。我希望它仍然绑定具有该属性的记录。
IEnumerable<ERec> list = retailerList.Cast<ERec>();
lvwRetailStores.DataSource = list.OrderByDescending(r => r.Properties["RS_Partner Type"].ToString())
.ThenBy(r => r.Properties["RS_Title"].ToString());
采纳答案by Adam Says - Reinstate Monica
list.Where(r => r.Properties["RS_Partner_Type"] != null && r.Properties["RS_Title"] != null)
.OrderByDescending(r => r.Properties["RS_Partner Type"].ToString())
.ThenBy(r => r.Properties["RS_Title"].ToString());
Or instead of != null, use whatever test the Properties collection has.
或者代替 != null，使用 Properties 集合的任何测试。
回答by Serguei
You can use a ternary expression in the lambda:
您可以在 lambda 中使用三元表达式：
list.OrderByDescending(r => r.Properties["RS_Partner_Type"] == null ? null : r.Properties["RS_Partner Type"].ToString())
.ThenBy(r => r.Properties["RS_Title"] == null ? null : r.Properties["RS_Title"].ToString());
回答by outis
Another common approach is to give the collection a suitable default value, and return that when the collection doesn't have a particular key. For instance, if Propertiesimplements IDictionary,
另一种常见的方法是为集合提供一个合适的默认值，并在集合没有特定键时返回该值。例如，如果Properties实现 IDictionary，
public static class IDictionaryExtension {
public static TValue GetValue<TKey, TValue>(this IDictionary<TKey, TValue> dict, TKey key, TValue default) {
TValue result;
return dict.TryGetValue(key, out result) ? result : dflt;
}
}
...
lvwRetailStores.DataSource = list.OrderByDescending(r => r.GetValue("RS_Partner Type", "").ToString())
.ThenBy(r => r.GetValue("RS_Title","").ToString());
回答by hiramknick
I've found that the ?? Operator works well. I use Parenthesis to evaluate for null,
我发现 ?? 操作员运作良好。我使用括号来评估空值，
For Example:
例如：
Datetime? Today = DateTimeValue
// Check for Null, if Null put Today's date
datetime GoodDate = Today ?? DateTime.Now
Datetime? Today = DateTimeValue
// Check for Null, if Null put Today's date
datetime GoodDate = Today ?? DateTime.Now
This same logic works in Lambda, just use parenthesis to ensure that the correct comparisons are used.
同样的逻辑在 Lambda 中也有效，只需使用括号来确保使用正确的比较。
