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如何在实体框架中传递字符串作为泛型dbset的类型参数?

reflection entity-framework c#

Elham Azadfar · 技术社区 · 6 年前

考虑下面的示例代码,将c泛型类型作为参数传递的最简洁方法是什么?

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();
}

或者像这样的:

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();
}

0 回复 | 直到 6 年前

Sohaib Jundi 6 年前

Activator.CreateInstance(type) 会失败的 DbSet 没有公共的无参数构造函数。
除非您真的需要将类型名作为字符串传递,否则最好的方法是创建泛型函数,获取构造函数并调用它。可能是这样的:

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);
}

那么,你可以顺路叫它 MyDbSet<NotificationType>()

更新:
由于需要传递名称,因此可以执行以下操作:

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

那么,你可以顺路叫它 MyDbSet("<namespace>.NotificationType") 是的。指定命名空间是必需的,否则 Type.GetType 找不到类型并返回 null

Sohaib Jundi 6 年前

请尝试此扩展代码:

//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 });
}

示例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>();
    }
}

示例实体:

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

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

使用它:

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();
    }
}