How to retrieve a list of Memory Cache keys in asp.net core?

As other answers point out, current implementation of Microsoft.Extensions.Caching.Memory.MemoryCache does not expose any members allowing to retrieve all cache keys, although there is a way around the problem if we use reflection.

This answer is partially based upon the one by MarkM, adds some speed to the solution by reducing reflection usage to a minimum, adds support for Microsoft.Extensions.Caching.Memory version 7 and packs everything into a single extension class.

using System;
using System.Collections;
using System.Collections.Generic;
using System.Linq;
using System.Reflection;
using System.Reflection.Emit;
using Microsoft.Extensions.Caching.Memory;

public static class MemoryCacheExtensions
{
#region Microsoft.Extensions.Caching.Memory_6_OR_OLDER

    private static readonly Lazy<Func<MemoryCache, object>> GetEntries6 =
        new Lazy<Func<MemoryCache, object>>(() => (Func<MemoryCache, object>)Delegate.CreateDelegate(
            typeof(Func<MemoryCache, object>),
            typeof(MemoryCache).GetProperty("EntriesCollection", BindingFlags.NonPublic | BindingFlags.Instance).GetGetMethod(true),
            throwOnBindFailure: true));

#endregion

#region Microsoft.Extensions.Caching.Memory_7_OR_NEWER

    private static readonly Lazy<Func<MemoryCache, object>> GetCoherentState =
        new Lazy<Func<MemoryCache, object>>(() =>
            CreateGetter<MemoryCache, object>(typeof(MemoryCache)
                .GetField("_coherentState", BindingFlags.NonPublic | BindingFlags.Instance)));

    private static readonly Lazy<Func<object, IDictionary>> GetEntries7 =
        new Lazy<Func<object, IDictionary>>(() =>
            CreateGetter<object, IDictionary>(typeof(MemoryCache)
                .GetNestedType("CoherentState", BindingFlags.NonPublic)
                .GetField("_entries", BindingFlags.NonPublic | BindingFlags.Instance)));

    private static Func<TParam, TReturn> CreateGetter<TParam, TReturn>(FieldInfo field)
    {
        var methodName = $"{field.ReflectedType.FullName}.get_{field.Name}";
        var method = new DynamicMethod(methodName, typeof(TReturn), new[] { typeof(TParam) }, typeof(TParam), true);
        var ilGen = method.GetILGenerator();
        ilGen.Emit(OpCodes.Ldarg_0);
        ilGen.Emit(OpCodes.Ldfld, field);
        ilGen.Emit(OpCodes.Ret);
        return (Func<TParam, TReturn>)method.CreateDelegate(typeof(Func<TParam, TReturn>));
    }

#endregion

    private static readonly Func<MemoryCache, IDictionary> GetEntries =
        Assembly.GetAssembly(typeof(MemoryCache)).GetName().Version.Major < 7
            ? (Func<MemoryCache, IDictionary>)(cache => (IDictionary)GetEntries6.Value(cache))
            : cache => GetEntries7.Value(GetCoherentState.Value(cache));

    public static ICollection GetKeys(this IMemoryCache memoryCache) =>
        GetEntries((MemoryCache)memoryCache).Keys;

    public static IEnumerable<T> GetKeys<T>(this IMemoryCache memoryCache) =>
        memoryCache.GetKeys().OfType<T>();
}

Usage:

var cache = new MemoryCache(new MemoryCacheOptions());
cache.GetOrCreate(1, ce => "one");
cache.GetOrCreate("two", ce => "two");

foreach (var key in cache.GetKeys())
    Console.WriteLine($"Key: '{key}', Key type: '{key.GetType()}'");

foreach (var key in cache.GetKeys<string>())
    Console.WriteLine($"Key: '{key}', Key type: '{key.GetType()}'");

Output:

Key: '1', Key type: 'System.Int32'
Key: 'two', Key type: 'System.String'
Key: 'two', Key type: 'System.String'

Notes:

• Reflection usage is reduced to a single call that builds the GetEntries delegate. When we’re working with the retrieved MemoryCache keys, reflection is not used. In contrast to a raw reflection approach, this decreases execution time and saves machine resources during traversal of long collections of MemoryCache keys.
• In the solution we are casting MemoryCache‘s internal dictionary to IDictionary instead of the native ConcurrentDictionary<object, CacheEntry> because CacheEntry type is internal.
• The code was verified using a matrix of console and webapi apps based on .NET Framework 4.8, .NET 6 and 7 with Microsoft.Extensions.Caching.Memory versions 6 and 7. LangVersion is 7.3.