作者:jack1881的天空_925 | 来源:互联网 | 2023-05-18 17:23
看一下.NET源代码Contains
中的HashSet
类代码,我找不到任何Contains
线程安全的原因?
我HashSet
提前加载了一个值,然后检查Contains
多线程.AsParallel()
环.
这有什么理由不安全吗?ConcurrentDictionary
当我实际上不需要存储值时,我不愿意使用.
1> xanatos..:
通常(通常)仅用于读取的集合是"非正式"线程安全的(.NET中没有我知道在读取期间自行修改的集合).有一些警告:
本身不能成为项目线程安全的(但有HashSet
这个问题应尽量减少,因为你不能从中提取项目.仍然是GetHashCode()
和Equals()
必须是线程安全的.如果,例如,他们访问所加载懒对象按需,它们可能不是线程安全的,或者它们可能缓存/记忆一些数据以加速后续操作)
您必须确保在最后一次写入之后有一个Thread.MemoryBarrier()
(在与写入相同的线程中完成)或等效的,否则在另一个线程上读取可能会读取不完整的数据
你必须确保在每个线程(不同于你写的那个)中,在第一次读取之前有一个Thread.MemoryBarrier()
.请注意,如果HashSet
在创建/启动其他线程之前已经"准备好"(最后使用Thread.MemoryBarrier()),那么Thread.MemoryBarrier()
就没有必要了,因为线程不能过时读取内存(因为他们不存在).各种操作导致隐含Thread.MemoryBarrier()
.例如,如果在HashSet
填充之前创建的线程,输入a Wait()
并且un-Waited
在HashSet
填充之后(加上它Thread.MemoryBarrier()
),则退出a Wait()
会导致隐式Thread.MemoryBarrier()
使用memoization/lazy loading /任何你想要调用它的类的简单示例可以打破线程的安全性.
public class MyClass
{
private long value2;
public int Value1 { get; set; }
// Value2 is lazily loaded in a very primitive
// way (note that Lazy *can* be used thread-safely!)
public long Value2
{
get
{
if (value2 == 0)
{
// value2 is a long. If the .NET is running at 32 bits,
// the assignment of a long (64 bits) isn't atomic :)
value2 = LoadFromServer();
// If thread1 checks and see value2 == 0 and loads it,
// and then begin writing value2 = (value), but after
// writing the first 32 bits of value2 we have that
// thread2 reads value2, then thread2 will read an
// "incomplete" data. If this "incomplete" data is == 0
// then a second LoadFromServer() will be done. If the
// operation was repeatable then there won't be any
// problem (other than time wasted). But if the
// operation isn't repeatable, or if the incomplete
// data that is read is != 0, then there will be a
// problem (for example an exception if the operation
// wasn't repeatable, or different data if the operation
// wasn't deterministic, or incomplete data if the read
// was != 0)
}
return value2;
}
}
private long LoadFromServer()
{
// This is a slow operation that justifies a lazy property
return 1;
}
public override int GetHashCode()
{
// The GetHashCode doesn't use Value2, because it
// wants to be fast
return Value1;
}
public override bool Equals(object obj)
{
MyClass obj2 = obj as MyClass;
if (obj2 == null)
{
return false;
}
// The equality operator uses Value2, because it
// wants to be correct.
// Note that probably the HashSet doesn't need to
// use the Equals method on Add, if there are no
// other objects with the same GetHashCode
// (and surely, if the HashSet is empty and you Add a
// single object, that object won't be compared with
// anything, because there isn't anything to compare
// it with! :-) )
// Clearly the Equals is used by the Contains method
// of the HashSet
return Value1 == obj2.Value1 && Value2 == obj2.Value2;
}
}