空的列表或字典的内存使用情况? [英] Memory usage of an empty List or Dictionary?

问题描述

一个空的列表或字典使用多少内存?如:

How much memory is used by an empty List or Dictionary? Such as:

List<double> list = new List<double>();

在x86和x64 OS的64位上，指针本身至少占用32位，但是列表本身呢?有0条记录.

The pointer itself eats at least 32 bits on x86 and 64 of x64 OS, but what about the list itself? With 0 records.

问的原因是，可以通过将列表设置为null来节省一些字节吗?

The reason for asking is, can you save some bytes by setting lists to null?

(假设您有一个包含某些List<T>的类，在某些情况下正在使用该类，而在其他情况下则没有使用，在这种情况下，该类具有一个boolean，如IsEmpty和null而不是空列表可能会节省一些操作内存.特别是在您将在操作内存中拥有数千个此类的情况下，每一位都很重要.

(Imagine you have a class that contains some List<T> which in some cases is being used and in other case it is not, in that case having a boolean like IsEmpty and null instead of empty list might save some operating memory. Especially in case you would have thousands of such classes in operating memory, every bit counts.)

推荐答案

由dotPeek编译:

Decompiled by dotPeek :

public class List<T> : IList<T>, ICollection<T>, IList, ICollection, IReadOnlyList<T>, IReadOnlyCollection<T>, IEnumerable<T>, IEnumerable
{
    private T[] _items; //4 bytes for x86, 8 for x64
    private int _size; //4 bytes
    private int _version; //4 bytes
    [NonSerialized]
    private object _syncRoot; //4 bytes for x86, 8 for x64
    private static readonly T[] _emptyArray; //one per type
    private const int _defaultCapacity = 4; //one per type
    ...
}

在x86上总共有 20 个字节(List<T>成员有16个字节，元数据引用开销为4个字节)，在x64上有 32 个字节，包括对类型的引用对象，.net中的每个对象都有.进行该计算时，大致不计算出鳄鱼数.

you got total of 20 bytes on x86 (16 for List<T> members and 4 for metadata reference overhead) and 32 on x64, including reffernce to type of the object, which each object in .net have. This calculation is done roughly not counting alligment.

public class Dictionary<TKey, TValue> : ...
{
    private int[] buckets; //4 bytes for x86, 8 for x64
    private Dictionary<TKey, TValue>.Entry[] entries; //4 bytes for x86, 8 for x64
    private int count; //4 bytes
    private int version; //4 bytes
    private int freeList; //4 bytes
    private int freeCount; //4 bytes
    private IEqualityComparer<TKey> comparer; //4 bytes for x86, 8 for x64
    private Dictionary<TKey, TValue>.KeyCollection keys; //4 bytes for x86, 8 for x64
    private Dictionary<TKey, TValue>.ValueCollection values; //4 bytes for x86, 8 for x64
    private object _syncRoot; //4 bytes for x86, 8 for x64

    private const string VersionName = "Version"; //one per type
    private const string HashSizeName = "HashSize"; //one per type
    private const string KeyValuePairsName = "KeyValuePairs"; //one per type
    private const string ComparerName = "Comparer"; //one per type
}

44 (对于x86)和 72 (对于x64).再次进行粗略计算，因为需要不同对象的实例.

44 for x86 and 72 for x64. Again rough calculation, since instances of different objects are required.

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