摘要

缓存是一种非常常见的性能优化技术，在开发过程中经常会用到。.NET提供了内置的内存缓存类 MemoryCache，它可以很方便地存储数据并在后续的请求中快速读取，从而提高应用程序的响应速度。

正文

通过使用 Microsoft.Extensions.Caching.Memory，我们可以在 .NET Core 中轻松实现内存缓存功能，从而提高应用程序的性能和响应速度。在实际应用中，你可以根据具体需求来设置缓存的有效期和其他选项。

nuget 安装依赖 Microsoft.Extensions.Caching.Memory

一个简单例子

C#
public partial class Form1 : Form
{
    // 创建 MemoryCache 实例
    MemoryCache cache = new MemoryCache(new MemoryCacheOptions());

    System.Timers.Timer timer = new System.Timers.Timer();
    int idx = 0;
    public Form1()
    {
        InitializeComponent();
        timer.Interval = 1000;
        timer.Elapsed += (o, e) =>
        {
            this.Invoke(new Action(() =>
            {
                lblTime.Text = idx.ToString();
                idx++;
            }));
        };
    }

    private void btnCreateCache_Click(object sender, EventArgs e)
    {
        // 添加数据到缓存
        string key = "hi";
        string value = "Hello, World!";
        var cacheEntryOptions = new MemoryCacheEntryOptions
        {
            AbsoluteExpiration = DateTimeOffset.Now.AddMinutes(1) // 缓存有效期为 1 分钟
        };
        cache.Set(key, value, cacheEntryOptions);
        timer.Start();
    }

    private void btnGetCache_Click(object sender, EventArgs e)
    {
        // 从缓存中获取数据
        if (cache.TryGetValue("hi", out string cachedValue))
        {
            MessageBox.Show(cachedValue);
        }
        else
        {
            MessageBox.Show("没有找到cache");
        }
    }
}

C#
/// <summary>
/// 删除cache
/// </summary>
/// <param name="sender"></param>
/// <param name="e"></param>
private void btnDeleteCache_Click(object sender, EventArgs e)
{
    cache.Remove("hi");
}

缓存一个对象

C#
public class Person
{
    public string Name { get; set; }    

    public int Age { get; set; }

    public override string ToString()
    {
        return this.Name+" "+this.Age.ToString();
    }
}

C#
public partial class Form1 : Form
{
    // 创建 MemoryCache 实例
    MemoryCache cache = new MemoryCache(new MemoryCacheOptions());

    System.Timers.Timer timer = new System.Timers.Timer();
    int idx = 0;
    public Form1()
    {
        InitializeComponent();
        timer.Interval = 1000;
        timer.Elapsed += (o, e) =>
        {
            this.Invoke(new Action(() =>
            {
                lblTime.Text = idx.ToString();
                idx++;
            }));
        };
    }

    private void btnCreateCache_Click(object sender, EventArgs e)
    {
        Person person = new Person()
        {
            Name="Rick",
            Age=99
        };

        // 添加对像数据到缓存
        var cacheEntryOptions = new MemoryCacheEntryOptions
        {
            AbsoluteExpiration = DateTimeOffset.Now.AddMinutes(1) // 缓存有效期为 1 分钟
        };
        cache.Set<Person>("p1", person);
        timer.Start();
    }

    private void btnGetCache_Click(object sender, EventArgs e)
    {
        // 从缓存中获取数据
        if (cache.TryGetValue<Person>("p1", out Person cachedValue))
        {
            MessageBox.Show(cachedValue.ToString());
        }
        else
        {
            MessageBox.Show("没有找到cache");
        }
    }

    /// <summary>
    /// 删除cache
    /// </summary>
    /// <param name="sender"></param>
    /// <param name="e"></param>
    private void btnDeleteCache_Click(object sender, EventArgs e)
    {
        cache.Remove("p1");
    }
}

侦听几个事件，使用PostEvictionCallbacks这个回调

C#
private void btnCreateCache_Click(object sender, EventArgs e)
{
    Person person = new Person()
    {
        Name="Rick",
        Age=99
    };

    // 添加对像数据到缓存
    var cacheEntryOptions = new MemoryCacheEntryOptions
    {
        AbsoluteExpiration = DateTimeOffset.Now.AddMinutes(1), // 缓存有效期为 1 分钟
        PostEvictionCallbacks =
        {
            new PostEvictionCallbackRegistration
            {
                EvictionCallback=Cache_EntryRemoved,
                State =this
            }
        }
    };
    cache.Set<Person>("p1", person, cacheEntryOptions);
    timer.Start();
}

private static void Cache_EntryRemoved(object key, object value, EvictionReason reason, object state)
{
    // 在 PostEvictionCallback 中处理逻辑
    switch (reason.ToString())
    {
        case "Delete":
            MessageBox.Show("删除缓存了！");
            break;
        default:
            break;
    }
}

注意Reason，这里能知道是什么操作

C#
public enum EvictionReason
{
    None,

    /// <summary>
    /// Manually
    /// </summary>
    Removed,

    /// <summary>
    /// Overwritten
    /// </summary>
    Replaced,

    /// <summary>
    /// Timed out
    /// </summary>
    Expired,

    /// <summary>
    /// Event
    /// </summary>
    TokenExpired,

    /// <summary>
    /// Overflow
    /// </summary>
    Capacity,
}