LINQ where未知类型的表达

Question

I have a database where all tables have a primary key named ID .

I don't want to have to make multiple GetObjectForId for all my tables, i would like to make a generic method that takes IQueryable and a Type and then try to get an object from the IQueryable selection if possible.

I'm thinking something like this:

public IQueryable<Type> GetObjectForIdInCollection(IQueryable selection, Type t, int id)
{
    return selection.Cast<T>().Where(c => c.ID == id).FirstOrDefault()
} 

Obviously this code is totally wrong, but you get the idea what i'm trying to do.

Is this a good idea? How can i implement this method?

Answer 1

If you are using Code-First, then you can create BaseEntity class and derive all entities from this class.

public abstract class BaseEntity
{
    public int ID { get; set; }
}

Then,

public T GetObjectForIdInCollection<T>(IQueryable<T> selection, int id)
    where T : BaseEntity
{
    return selection.Where(c => c.ID == id).FirstOrDefault();

    // Or you can simply use:
    // return selection.FirstOrDefault(c => c.ID == id);
} 

Answer 2

you could create a base interface, which all your entities implement:

public interface IIdObject
{
    int Id { get; set; }    
}

// ...

public T GetObjectForIdInCollection<T>(IQueryable<T> selection, int id)
    where T : IIdObject
{
    return selection.FirstOrDefault(c => c.Id == id);
}  

but I don't think it is worth the effort.

Answer 3

Your classes should implement IEntity interface, also you can use extension method:

public interface IEntity
{
    int Id { get; set; }
}

public static class Extention
{
    public static Type GetObjectForIdInCollection<Type>(this IQueryable<Type> selection, int id)
         where Type : class, IEntity
    {
        return selection.FirstOrDefault(c => c.Id == id);
    }
}

Usage:

var item = source.GetObjectForIdInCollection(1);

Answer 4

I think you must inherit all your models from a base class (the base type BaseModel) that has ID as property, then :

public GetById<T>(int id, IRepository repo) where T : BaseModel
        {

                IQueryable<T> setQuery = repo.Get<T>();
                return setQuery.FirstOrDefault(q => q.Id== id);
        }