如果找不到匹配的内容，则FirstOrDefault返回NullReferenceException

问题描述

这是我的代码:

string displayName = Dictionary.FirstOrDefault(x => x.Value.ID == long.Parse(options.ID)).Value.DisplayName;

如果x.Value.ID与options.ID匹配，则代码工作正常.但是，如果没有，我会得到一个NullReferenceException.

The code works fine if x.Value.ID matches options.ID. However, I get a NullReferenceException if it doesn't.

推荐答案

FirstOrDefault返回类型的默认值.对于引用类型为null.多数民众赞成在例外的原因.

FirstOrDefault returns the default value of a type if no item matches the predicate. For reference types that is null. Thats the reason for the exception.

所以您只需要先检查null:

string displayName = null;
var keyValue = Dictionary
    .FirstOrDefault(x => x.Value.ID == long.Parse(options.ID));
if(keyValue  != null)
{
    displayName = keyValue.Value.DisplayName;
}

但是，如果要搜索值，那么词典的关键字是什么? Dictionary<tKey,TValue>用于通过键查找值.也许您应该重构它.

But what is the key of the dictionary if you are searching in the values? A Dictionary<tKey,TValue> is used to find a value by the key. Maybe you should refactor it.

另一种选择是提供DefaultIfEmpty的默认值:

Another option is to provide a default value with DefaultIfEmpty:

string displayName = Dictionary
    .Where(kv => kv.Value.ID == long.Parse(options.ID))
    .Select(kv => kv.Value.DisplayName)   // not a problem even if no item matches
    .DefaultIfEmpty("--Option unknown--") // or no argument -> null
    .First();                             // cannot cause an exception

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