c# - Struct 不缓存，但 Class 缓存

Question

我有一个用于图片上传的洪水限制器:

public class FloodLimiter
{
    public static void OnImageUploaded(string ipAddress, int sizeBytes)
    {
        var dict = GetImageUploadFloodLimitDict();
        if (!dict.ContainsKey(ipAddress))
        {
            dict.Add(ipAddress, new ImageUploadLimits());
        }
        dict[ipAddress].OnFileUploaded(sizeBytes);
    }

    public static bool CanUploadImages(string ipAddress)
    {
        var dict = GetImageUploadFloodLimitDict();
        if (!dict.ContainsKey(ipAddress)) return true;
        return dict[ipAddress].BelowLimits;
    }

    private static readonly object FloodLimitLock = new object();
    /// <summary>
    /// Dictionary of IP's, and total images uploaded + total image size uploaded
    /// </summary>
    private static Dictionary<string, ImageUploadLimits> GetImageUploadFloodLimitDict()
    {
        const string cacheIndex = Settings.CachePrefix + "ImageUploadFloodLimiter";
        var context = HttpContext.Current;
        if (context.Cache[cacheIndex] == null)
        {
            lock (FloodLimitLock)
            {
                if (context.Cache[cacheIndex] == null)
                {
                    var dict = new Dictionary<string, ImageUploadLimits>();
                    context.Cache.Add(cacheIndex, dict, null, DateTime.Now.AddMinutes(Settings.ImageUpload.FloodLimitRecycleMinutes), Cache.NoSlidingExpiration, CacheItemPriority.AboveNormal, null);
                }
            }
        }
        return (Dictionary<string, ImageUploadLimits>)context.Cache[cacheIndex];
    }
}

如果 ImageUploadLimits 定义为:

private struct ImageUploadLimits
{
    private int _totalImagesUploaded;
    private int _totalBytesUploaded;
    public bool BelowLimits { get {
        return _totalImagesUploaded < Settings.ImageUpload.MaximumImagesCanUploadInPeriod && _totalBytesUploaded < (Settings.ImageUpload.MaximumImageUploadMbInPeriod * 1000000);
    }}

    public void OnFileUploaded(int sizeBytes)
    {
        _totalImagesUploaded ++;
        _totalBytesUploaded += sizeBytes;
    }
}

缓存永远不会更新(它总是返回 1 张上传的图片)。

如果我将 ImageUploadLimits 定义为:

private class ImageUploadLimits
{
    private int _totalImagesUploaded;
    private int _totalBytesUploaded;
    public bool BelowLimits { get {
        return _totalImagesUploaded < Settings.ImageUpload.MaximumImagesCanUploadInPeriod && _totalBytesUploaded < (Settings.ImageUpload.MaximumImageUploadMbInPeriod * 1000000);
    }}

    public void OnFileUploaded(int sizeBytes)
    {
        _totalImagesUploaded++;
        _totalBytesUploaded += sizeBytes;
    }
}

它工作得很好!

为什么 struct 版本没有缓存？这与 Structs 被 Val 传递和 Classes 被 Ref 传递有关吗？

Answer 1

当您从字典中读取结构时，您会得到该结构的副本。 OnFileUploaded 方法对值所做的更改只会影响副本，不会影响字典中的原始内容。

如果使用结构体，则需要将副本写回字典:

ImageUploadLimits copy = dict[ipAddress];
copy.OnFileUploaded(sizeBytes);
dict[ipAddress] = copy;

当您使用一个类时，您会从字典中获得引用的副本。仍然只有一个对象，但有两个对它的引用。

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注意:术语通过引用 和通过值 指的是参数的传递方式。参数始终按值传递，无论它是结构还是类，除非您使用 ref 或 out 关键字。按值传递的是结构的副本或对象引用的副本。