Why are EF Core navigation properties null until explicitly loaded, and how can I consistently populate them?

EF Core lazy loading of navigation properties’ null value problem and solution

Problem Description

Consider the following model:

<code class="language-csharp">public class Mutant
{
    public long Id { get; set; }
    public long OriginalCodeId { get; set; }
    public virtual OriginalCode OriginalCode { get; set; }
}

public class OriginalCode
{
    public long Id { get; set; }
    public virtual List<Mutant> Mutants { get; set; }
}</code>

Define the relationship in the OnModelCreating method of DbContext:

<code class="language-csharp">modelBuilder.Entity<Mutant>()
    .HasOne(m => m.OriginalCode)
    .WithMany(oc => oc.Mutants)
    .HasForeignKey(m => m.OriginalCodeId)
    .OnDelete(DeleteBehavior.Restrict);</code>

When querying a Mutant, the OriginalCode property is initially empty. However, if you query OriginalCode before querying the Mutant, the OriginalCode attribute will be populated.

Cause Analysis

This behavior is documented in the "Loading related data" section of the EF Core documentation:

Entity Framework Core does not currently support lazy loading, so typically navigation properties will be empty until you load them via preloading or explicitly.

However, the "Preloading" section contains the following:

Tips Entity Framework Core will automatically fix the navigation properties of any other entities that were previously loaded into the context instance. Therefore, even if you do not explicitly include data for a navigation property, the property may still be populated if some or all of the related entities were previously loaded.

Solution

Solve the first problem (navigation attribute is empty):

Use available methods of loading relevant data, such as preloading:

<code class="language-csharp">var mutants = db.Mutants.Include(m => m.OriginalCode).ToList();</code>

Fix the second issue (preloading related entities causing navigation properties to be populated):

This behavior is "by design" and cannot be controlled. To avoid this problem:

• Use a new DbContext instance for each query.
• Use no tracking queries (AsNoTracking()).

EF Core v2.1 and later updates:

EF Core v2.1 supports lazy loading, but it is not enabled by default. To enable it:

• Mark all navigation properties with virtual.
• Install the Microsoft.EntityFrameworkCore.Proxies NuGet package.
• Use UseLazyLoadingProxies() to enable lazy loading.

With the above approach, you can effectively manage navigation property loading in EF Core, avoid null value issues and improve code predictability.

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