无法从 ViewComponent 中解析缓存服务答案

【问题标题】：Unable to resolve cache service from within ViewComponent无法从 ViewComponent 中解析缓存服务
【发布时间】：2020-02-10 23:30:08
【问题描述】：

我在 Startup.cs 中注册缓存服务如下：

    public void ConfigureServices(IServiceCollection services)
    {

        services.AddMemoryCache();            
        services.AddTransient<DataFacade, DataFacade>();
        ...
    }

在同一个项目中，我的 _Layout.cshtml 标头包含一个 ViewComponent

@await Component.InvokeAsync(nameof(ViewComponents.PageHeader))

这会调用 ModelBuilder 组件并具有如下构造函数：

using MyApp.Services;
using MyApp.Website.ModelBuilder;
using Microsoft.AspNetCore.Builder;
using Microsoft.AspNetCore.Mvc;

namespace MyApp.Website.ViewComponents
{
    [ViewComponent]
    public class Header : ViewComponent
    {
        private readonly HeaderModelBuilder _modelBuilder;

        public Header(HeaderModelBuilder modelBuilder)
        {
            _modelBuilder = modelBuilder;
        }

...并且 ModelBuilder 服务引用了 DataFacade 服务：

using MyApp.Data.ExternalCountryData;
using MyApp.Services;
using MyApp.Website.Constants;
using MyApp.Website.Models;

namespace MyApp.Website.ModelBuilder
{
    public class HeaderModelBuilder
    {
        private readonly DataFacade _dataFacade;

        public HeaderModelBuilder(DataFacade dataFacade)
        {
            _dataFacade = dataFacade;
        }

...它的构造函数如下：

using System;
using MyApp.Data;
using MyApp.Data.ExternalSiteData;
using MyApp.Domain.Services.Data;
using Microsoft.Extensions.Caching.Memory;

namespace MyApp.Services
{
    public class DataFacade
    {
        private MemoryCache _cache;
        public DataFacade(MemoryCache memoryCache)
        {
            _cache = memoryCache;
        }

但是，在运行代码时，Program.cs 会抛出以下内容：

InvalidOperationException：无法解析服务类型 'Microsoft.Extensions.Caching.Memory.MemoryCache' 尝试激活“MyApp.Services.DataFacade”。

如果我注释掉视图组件，应用程序的其他部分可以正常访问缓存。

为什么视图组件会出现问题？我是否错误地注册了其中一项服务？

【问题讨论】：

标签： c# dependency-injection .net-core razor-pages memorycache

【解决方案1】：

注入IMemoryCache 抽象而不是实现

public class DataFacade {
    private readonly IMemoryCache _cache;

    public DataFacade(IMemoryCache memoryCache) {
        _cache = memoryCache;
    }

    //...

参考Use IMemoryCache

添加内存缓存的源码在向服务集合注册时添加接口

/// <summary>
/// Adds a non distributed in memory implementation of <see cref="IMemoryCache"/> to the
/// <see cref="IServiceCollection" />.
/// </summary>
/// <param name="services">The <see cref="IServiceCollection" /> to add services to.</param>
/// <returns>The <see cref="IServiceCollection"/> so that additional calls can be chained.</returns>
public static IServiceCollection AddMemoryCache(this IServiceCollection services)
{
    if (services == null)
    {
        throw new ArgumentNullException(nameof(services));
    }

    services.AddOptions();
    services.TryAdd(ServiceDescriptor.Singleton<IMemoryCache, MemoryCache>());

    return services;
}

Source

【讨论】：