如何在實體框架中將字符串作為類型DbSet的類型參數傳遞？

Question

考慮以下示例代碼，將C＃泛型類型作為參數傳遞的最簡潔方法是什么？

public dynamic MyDbSet(string typeName)
{
    var typeofDbSet = typeof(DbSet<>);
    Type[] typeArgs = { Type.GetType(typeName) };
    var type = typeofDbSet.MakeGenericType(typeArgs);
    dynamic dbsetOfT = Activator.CreateInstance(type);

    return dbsetOfT;

    //Call it
    //var notificationTypes = MyDbSet("NotificationType");
    //var list = notificationTypes.ToList();
}

或類似這樣的東西：

public dynamic MyDbSet2(string typeName)
{
    var keyValuePairs = new Dictionary<string, dynamic>
    {
        {nameof(NotificationType), Set<NotificationType>().AsQueryable()}
    };

    return keyValuePairs[typeName];
    //Call it
    //var notificationTypes = MyDbSet2("NotificationType");
    //var list = notificationTypes.ToList();
}

Answer 1

Activator.CreateInstance(type)將失敗，因為DbSet沒有公共的無參數構造函數。
除非您確實需要將類型名稱作為字符串傳遞，否則執行此操作的最佳方法是創建泛型函數，獲取構造函數並調用它。 會是這樣的：

public DbSet<T> MyDbSet<T>() where T : class
{
    return (DbSet<T>)typeof(DbSet<T>).GetConstructor(BindingFlags.NonPublic |
       BindingFlags.Instance, null, Type.EmptyTypes, null).Invoke(null);
}

然后，您可以通過MyDbSet<NotificationType>()調用

更新：
由於需要傳遞名稱，因此您可以執行以下操作：

public dynamic MyDbSet(string typeName)
{
    return typeof(DbSet<>).MakeGenericType(Type.GetType(typeName)).
        GetConstructor(BindingFlags.NonPublic | BindingFlags.Instance,
        null, Type.EmptyTypes, null).Invoke(null);
}

然后，可以通過MyDbSet("<namespace>.NotificationType")調用。 指定名稱空間是必需的，因為否則Type.GetType將找不到類型並返回null

Answer 2

嘗試以下擴展代碼：

//Use var list = _context.MyDbSet("ConsoleAppEF.Student").ToList();
public static IQueryable MySet(this SchoolContext context, string typeName)
{
    var T = Type.GetType(typeName);
    // Get the generic type definition
    MethodInfo method = typeof(SchoolContext)
        .GetMethod("MySet", BindingFlags.Public | BindingFlags.Instance);

    // Build a method with the specific type argument you're interested in
    method = method.MakeGenericMethod(T);

    return method.Invoke(context, null) as IQueryable;
}

public static IQueryable<T> Set<T>(this SchoolContext context)
{
    // Get the generic type definition 
    MethodInfo method = typeof(SchoolContext)
        .GetMethod(nameof(SchoolContext.Set), BindingFlags.Public | BindingFlags.Instance);

    // Build a method with the specific type argument you're interested in 
    method = method.MakeGenericMethod(typeof(T));

    return method.Invoke(context, null) as IQueryable<T>;
}

public static IList ToList1(this IQueryable query)
{
    var genericToList = typeof(Enumerable).GetMethod("ToList")
        .MakeGenericMethod(new Type[] { query.ElementType });
    return (IList)genericToList.Invoke(null, new[] { query });
}

樣本DbContext：

public class SchoolContext : DbContext
{

    public SchoolContext() : base("SchoolContext")
    {

    }

    public DbSet<Student> Students { get; set; }

    public virtual new DbSet<TEntity> MySet<TEntity>() where TEntity : BaseEntity
    {
        return base.Set<TEntity>();
    }
}

樣本實體：

public class Student : BaseEntity
{
    public string LastName { get; set; }
    public string FirstMidName { get; set; }
}

public class BaseEntity
{
    public int Id { get; set; }
}

用它：

class Program
{
    static void Main(string[] args)
    {
        Seed();
        var _context = new SchoolContext();
        var list = _context.MySet("ConsoleAppEF.Student").ToList1();
    }

    private static void Seed()
    {
        var _context = new SchoolContext();
        var students = new List<Student>
        {
            new Student{FirstMidName="Carson",LastName="Alexander"},
        };

        students.ForEach(s => _context.Students.Add(s));
        _context.SaveChanges();
    }
}