当我有一个实体时，动态地在 DbContext 中查找指定的通用 DbSet

Question

I have following classes and DbContext :

public class Order : BaseEntity
{
    public Number {get; set;}
}
public class Product : BaseEntity;
{
    public Name {get; set;} 
}

public class Context : DbContext
{
    ....
    public DbSet<Order> Orders { set; get; }
    public DbSet<Product> Products { set; get; }
    ....
}   

I have a list of objects that want to add to my context, too, but I don't know how can I find appropriate generic DbSet according each entity type dynamically.

IList<BaseEntity> list = new List<BaseEntity>();
Order o1 = new Order();
o1.Numner = "Ord1";
list.Add(o1);

Product p1 = new Product();
p1.Name = "Pencil";
list.Add(p1);

Context cntx = new Context();  
foreach (BaseEntity entity in list)
{
      cntx.Set<?>().Add(entity);         
}

How can I do that?

Answer 1

DbContext has a method called Set , that you can use to get a non-generic DbSet , such as:

var someDbSet = this.Set(typeof(SomeEntity));

So in your case:

foreach (BaseEntity entity in list)
{
      cntx.Set(entity.GetType()).Add(entity);         
}

Answer 2

The question does not specify EF version and the proposed answer does not work anymore for Entity Framework Core (in EF Core, DbContext does not have a non-generic Set method, at least at the date of this answer).

Yet you can still have a working extension method using Jon Skeet's answer to this question . My code is added below for convenience.

Update: Added the generic function call as well returning IQueryable<T> thanks to the comment from Shaddix.

public static IQueryable Set(this DbContext context, Type T)
{
    // Get the generic type definition
    MethodInfo method = typeof(DbContext).GetMethod(nameof(DbContext.Set), 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 DbContext context)
{
    // Get the generic type definition 
    MethodInfo method = typeof(DbContext).GetMethod(nameof(DbContext.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>;
} 

Answer 3

Unfortunately, the below proposed version does not work since .NET Core 3.0. You still can get an IQueryable back, but you cannot cast it to DbSet anymore.

IQueryable<TEntity> as DbSet<TEntity> => null

What's worse is that starting with EF Core 3.0, the new FromSqlRaw and FromSqlInterpolated methods (which replace FromSql ) can only be specified on query roots, ie directly on the DbSet<> and not on IQueryable . Attempting to specify them anywhere else will result in a compilation error.

See https://github.com/dotnet/efcore/issues/15704#issuecomment-493230352

Answer 4

To avoid error "System.Reflection.AmbiguousMatchException: 'Ambiguous match found.'" I used version below:

    public static IQueryable Set(this DbContext context, Type T)
    {
        MethodInfo method = typeof(DbContext).GetMethods()
            .Where(p => p.Name == "Set" && p.ContainsGenericParameters).FirstOrDefault();

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

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

Answer 5

In EF Core we only have Set<TEntity>() method(s) which have a constraint that TEntity must be a type of class , so this prevents you accidentally passing a interface type that EF is impossible to find out any existing DbSet.

So if it's possible, you can always restrict your generic type parameter as the same as Set<TEntity>() requiring, for example this will not work:

public void GetDbSet<T>(DbContext db) {
    db.Set<T>()
//     ~~~~~~~~
// Error CS0452 The type 'T' must be a reference type in order to use it as parameter 'TEntity' in the generic type or method 'DbContext.Set<TEntity>()'
}

but this will:

public void GetDbSet<T>(DbContext db) where T : class {
    db.Set<T>()
}

Answer 6

In addition to Pablo's answer, you can add a class to your project:

namespace System.Data.Entity
{
    public static class EntityFrameworkExtensions
    {
        public static IEnumerable<object> AsEnumerable(this DbSet set)
        {
            foreach (var entity in set)
            {
                yield return entity;
            }
        }
    }
}

This class adds an extention method AsEnumerable to DbSet instance.

When you want to use it, for example to Count or filter a row:

var someDbSet = this.Set(typeof(SomeEntity));
var count = (from a in someDbSet.AsEnumerable() select a).Count();