C# 使用 LINQ 的 FirstOrDefault 检查 KeyValuePair 是否存在
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Check if KeyValuePair exists with LINQ's FirstOrDefault
提问by Steve
I have a dictionary of type
我有一个类型的字典
Dictionary<Guid,int>
I want to return the first instance where a condition is met using
我想返回满足条件的第一个实例
var available = m_AvailableDict.FirstOrDefault(p => p.Value == 0)
However, how do I check if I'm actually getting back a KeyValuePair? I can't seem to use != or == to check against default(KeyValuePair) without a compiler error. There is a similar thread herethat doesn't quite seem to have a solution. I'm actually able to solve my particular problem by getting the key and checking the default of Guid, but I'm curious if there's a good way of doing this with the keyvaluepair. Thanks
但是，如何检查我是否真的取回了 KeyValuePair？如果没有编译器错误，我似乎无使用 != 或 == 来检查 default(KeyValuePair) 。有一个类似的线程在这里并不十分似乎有一个解决方案。我实际上能够通过获取密钥并检查 Guid 的默认值来解决我的特定问题，但是我很好奇是否有使用键值对执行此操作的好方。谢谢
采纳答案by Marc Gravell
If you just care about existence, you could use ContainsValue(0)or Any(p => p.Value == 0)instead? Searching by valueis unusual for a Dictionary<,>; if you were searching by key, you could use TryGetValue.
如果你只关心存在，你可以用ContainsValue(0)orAny(p => p.Value == 0)代替？按值搜索对于 a 是不寻常的Dictionary<,>；如果您按关键字搜索，则可以使用TryGetValue.
One other approach:
另一种方：
var record = data.Where(p => p.Value == 1)
.Select(p => new { Key = p.Key, Value = p.Value })
.FirstOrDefault();
This returns a class- so will be nullif not found.
这将返回一个类-null如果找不到，也会返回一个类。
回答by pomarc
you could check if
你可以检查一下
available.Key==Guid.Empty
回答by Jon Skeet
I suggest you change it in this way:
我建议你以这种方式改变它：
var query = m_AvailableDict.Where(p => p.Value == 0).Take(1).ToList();
You can then see whether the list is empty or not, and take the first value if it's not, e.g.
然后您可以查看列表是否为空，如果不是，则取第一个值，例如
if (query.Count == 0)
{
// Take action accordingly
}
else
{
Guid key = query[0].Key;
// Use the key
}
Note that there's no real concept of a "first" entry in a dictionary - the order in which it's iterated is not well-defined. If you want to get the key/value pair which was firstentered with that value, you'll need an order-preserving dictionary of some kind.
请注意，字典中没有“第一个”条目的真正概念——它的迭代顺序没有明确定义。如果您想获得首先使用该值输入的键/值对，您将需要某种保序字典。
(This is assuming you actually want to know the key - if you're just after an existence check, Marc's solutionis the most appropriate.)
（这是假设您确实想知道密钥 - 如果您只是在进行存在检查之后，那么Marc 的解决方案是最合适的。）
回答by Samuel
What you want is an Anymethod that gives you the matching element as well. You can easily write this method yourself.
您想要的是一种Any同时为您提供匹配元素的方。您可以轻松地自己编写此方。
public static class IEnumerableExtensions
{
public static bool TryGetFirst<TSource>(this IEnumerable<TSource> source,
Func<TSource, bool> predicate,
out TSource first)
{
foreach (TSource item in source)
{
if (predicate(item))
{
first = item;
return true;
}
}
first = default(TSource);
return false;
}
}
回答by Florian Sandro V?lkl
Use the default() keyword.
使用 default() 关键字。
bool exists = !available.Equals(default(KeyValuePair<Guid, int>));
回答by kevinpo
A way to check against the default value of a struct such as KeyValuePair without specifying the type is to create a new instance using Activator:
在不指定类型的情况下检查结构（例如 KeyValuePair）的默认值的一种方是使用 Activator 创建一个新实例：
if (available.Equals(Activator.CreateInstance(available.GetType())))
{
Console.WriteLine("Not Found!");
}
