Entity Framework Core 在没有 T 的情况下使用 Dbset

Question

I want to be able use a type instead of T with Dbset

For example Dbset(myType) instead of DbSet

EF core 3.1 requires you to use DbSet and all the functionality is encapsulated in Extension methods.

Answer 1

Here's a solution I went with for EF 3.1

Hoping that since I've already been through the pain, I can share my solution for the next poor soul who has to do this.

Usage

var dbSet = new DbSetFacade(myType);
dynamic results = await dbSet.ToListAsync();

I'll just share ToList,Count for now to keep it brief!

public class DbSetFacade
{
    private static ReadOnlyDictionary<EfQueryExtensionType, MethodInfo> _extensionMethodInfos = GetExtensionMethodInfos();

    private readonly object _dbSet;
    private readonly ReadOnlyDictionary<EfQueryExtensionType, MethodInfo> _genericInstanceMethodInfos;

    public DbSetFacade(DbContext context, Type entityType)
    {
        if (context == null) throw new ArgumentNullException(nameof(context));
        if (entityType == null) throw new ArgumentNullException(nameof(entityType));

        _dbSet = GetDbSetInstance(context, entityType);

        _genericInstanceMethodInfos = new ReadOnlyDictionary<EfQueryExtensionType, MethodInfo>(new Dictionary<EfQueryExtensionType, MethodInfo>
        {
            [EfQueryExtensionType.AsNoTracking] = _extensionMethodInfos[EfQueryExtensionType.AsNoTracking].MakeGenericMethod(entityType),
            [EfQueryExtensionType.ToListAsync] = _extensionMethodInfos[EfQueryExtensionType.ToListAsync].MakeGenericMethod(entityType),
            [EfQueryExtensionType.CountAsyncWithoutPredicate] = _extensionMethodInfos[EfQueryExtensionType.CountAsyncWithoutPredicate].MakeGenericMethod(entityType),
        });
    }

    enum EfQueryExtensionType
    {
        AsNoTracking,
        ToListAsync,
        //FirstOrDefaultAsyncWithPredicate,
        //FirstOrDefaultAsyncWithoutPredicate,
        //ContainsAsync,
        //CountAsyncWithPredicate,
        CountAsyncWithoutPredicate
    }

    public Task<int> CountAsync(CancellationToken token = default) => _genericInstanceMethodInfos[EfQueryExtensionType.CountAsyncWithoutPredicate].Invoke(null, new object[] { _dbSet, token }) as Task<int>;


    public async Task<dynamic> ToListAsync(CancellationToken token = default)
    {
        var noTrackingSet = _genericInstanceMethodInfos[EfQueryExtensionType.AsNoTracking].Invoke(null, new object[] { _dbSet });
        var toList = _genericInstanceMethodInfos[EfQueryExtensionType.ToListAsync].Invoke(null, new object[] { noTrackingSet, token }) as Task;

        await toList.ConfigureAwait(false);

        return (object)((dynamic)toList).Result;
    }

    private static object GetDbSetInstance(DbContext context, Type type)
    {
        return typeof(DbContext).GetMethods()
            .FirstOrDefault(p => p.Name == "Set" && p.ContainsGenericParameters)
            .MakeGenericMethod(type)
            .Invoke(context, null);
    }

    static ReadOnlyDictionary<EfQueryExtensionType, MethodInfo> GetExtensionMethodInfos()
    {
        var extensionMethods = typeof(EntityFrameworkQueryableExtensions)
            .GetTypeInfo()
            .GetMethods()
            .Where(x => x.ContainsGenericParameters && x.GetParameters()[0].ParameterType.IsGenericType);

        return new ReadOnlyDictionary<EfQueryExtensionType, MethodInfo>(new Dictionary<EfQueryExtensionType, MethodInfo>
        {
            [EfQueryExtensionType.AsNoTracking] = extensionMethods.FirstOrDefault(x => x.Name == "AsNoTracking"),
            [EfQueryExtensionType.ToListAsync] = extensionMethods.FirstOrDefault(x => x.Name == "ToListAsync" && x.GetParameters().Count() == 2),
            [EfQueryExtensionType.CountAsyncWithoutPredicate] = extensionMethods.FirstOrDefault(x => x.Name == "CountAsync" && x.GetParameters().Count() == 2)
        });
    }
}