实体框架中的通用查询方法

Question

In my DB I have tables who have an attribute int DeleteState . I want a generic method to query those tables. In other words a method who does this: Context.Table.Where(x => x.DeleteState == 0) .

I thought I could do this:

public static class Extensions
{
  public static IQueryable<T> Exists<T>(this IQueryable<T> qry) where T : IDeletable
  {
    return qry.Where(x => x.DeleteState == 0);
  }
}

Where IDeletable is this:

public interface IDeletable
{
    int DeleteState { get; set; }
}

Now I only have to add the IDeletable in the EF model:

public partial class Table : EntityObject, IDeletable { ... }

I did this with the templating mechanism.

Unfortunately, it doesn't work :( It compiles fine, but throws at runtime:

Unable to cast the type 'Table' to type 'IDeletable'. LINQ to Entities only supports casting Entity Data Model primitive types

if I call it like that:

Context.Table.Exists();

How can I solve this problem? Could you think of a fix or a different method to achieve similar results? Thx

Answer 1

The problem you have is that the Entity Framework can only work with an Expression Tree. Your function executes a query directly instead of building an Expression Tree.

A simpler solution would be to add a Model Defined Function .

A model defined function can be called directly on an instance of your context.

Answer 2

Have you tried converting your objects to IDeletable before you actually query? eg

public static IQueryable<T> Exists<T>(this IQueryable<T> qry)
{     
    return qry.Select<T, IDeletable>(x => x).Where(x => x.DeleteState == 0).Cast<T>();
}

I haven't tested this code, however, the error rings a bell and I remember I had to do something similar.

Answer 3

Maybe:

public static IQueryable<T> Exists<T>(this IQueryable<T> qry)
{
    return qry.Where(x => (!typeof(IDeletable).IsAssignableFrom(x.GetType()) || typeof(IDeletable).IsAssignableFrom(x.GetType()) && ((IDeletable)x).DeleteState == 0));
}

Answer 4

Tsss, this is the answer: Linq Entity Framework generic filter method

I forgot about the class here:

... where T : , IDeletable