EF Code First Lazy loading Not Working

问题:

I am using code first with EF6 but cannot seem to get lazy loading to work. Eager loading is working fine. I have the following classes:

public class Merchant : User
{
    ...

    public virtual ICollection<MerchantLocation> MerchantLocations { get; set; }
}

public class MerchantLocation : BaseEntity
{
    ...

    public int MerchantId { get; set; }
    public virtual Merchant Merchant { get; set; }       
}

public class User : BaseEntity
{
    ...
}

public class BaseEntity
{
    ...

    public int Id { get; set; }
}

I test my lazy loading of the locations via the following code (which fails):

public void Test_Lazy_Loading() {
    using (var context = new MyDbContext()) {
        var merchant = context.Users.OfType<Merchant>.First();
        merchant.MerchantLocations.ShouldNotBeNull(); // fails
    }
}

However eager loading works fine:

public void Test_Eager_Loading() {
    using (var context = new MyDbContext()) {
        var merchant = context.Users.OfType<Merchant>.Include("MerchantLocations").First();
        merchant.MerchantLocations.ShouldNotBeNull(); // passes
    }
}

MerchantLocations is marked as public virtual so I'm not sure what the problem is. I have also added the following in my DbContext constructor:

Configuration.LazyLoadingEnabled = true;
Configuration.ProxyCreationEnabled = true;

edit: I have also noticed that the merchant object being returned in the above tests is not an EF proxy. It is a plain Merchant. I suspect that this is causing the problem.

回答1:

I realized that the problem was that the Merchant class did not meet requirements for proxy generation. Specifically, I needed to add a protected parameterless constructor. I only had a private one.

回答2:

Another thing that can cause lazy loading to fail is navigation properties that are not virtual. That was not the case for OP, but this question is a top Google result so it may help some.

And yet another possible cause is a mapped database column that doesn't exist. I was surprised to see that break lazy loading rather than throw a database exception.