为什么这种隐式演员是不可能的？

Question

给定下面的类和接口,我想知道为什么隐式转换:

ISomeModelAbstract<IBasicModel> x = new ConcreteClass();

是不可能的.我试过了

public interface ISomeModelAbstract<out T> where T: IBasicModel

但后来我无法使用GetById和GetAll方法.我感谢任何帮助或提示.谢谢.

public interface IBasicModel {
    string _id { get; set; }
}

public class SomeModel: IBasicModel {
    public string _id { get; set; }

    /* some other properties! */
}

public interface ISomeModelAbstract<T> where T: IBasicModel
{
    bool Save(T model);
    T GetById(string id);
    IEnumerable<T> GetAll();
    bool Update(string id, T model);
    bool Delete(string id);
}

public abstract class SomeModelAbstract<T> : ISomeModelAbstract<T> where T : IBasicModel
{
    public bool Save(T model)
    {
        throw new System.NotImplementedException();
    }

    public T GetById(string id)
    {
        throw new System.NotImplementedException();
    }

    public IEnumerable<T> GetAll()
    {
        throw new System.NotImplementedException();
    }

    public bool Update(string id, T model)
    {
        throw new System.NotImplementedException();
    }

    public bool Delete(string id)
    {
        throw new System.NotImplementedException();
    }
}

public interface IConcreteClass: ISomeModelAbstract<SomeModel> { }

public class ConcreteClass: SomeModelAbstract<SomeModel>, IConcreteClass { }