[*.cs]
dotnet_diagnostic.LC055.severity = error

LC055: OnModelCreating override skips the base configuration

In Plain Terms

You inherited a house with the wiring already done, then rebuilt the kitchen from the floor plan without looking at the wiring diagram. The lights you never touched stop working.

Goal

Detect OnModelCreating overrides in a DbContext whose base context configures the model, when nothing in the context ever calls base.OnModelCreating(...).

The Problem

Contexts such as ASP.NET Core Identity’s IdentityDbContext set up keys, indexes, and relationships for their own entities in OnModelCreating. Shared base contexts in your own code often add global query filters, value conversions, or owned types there. An override that forgets base.OnModelCreating(builder) throws all of that away.

With Identity the app usually fails at startup with “The entity type ‘IdentityUserLogin<string>’ requires a primary key to be defined”. With a project base context the model is silently wrong: a soft-delete or tenant filter is missing, and queries return rows they should hide.

public class AppDbContext : IdentityDbContext<AppUser>
{
    // Violation: Identity's model configuration never runs.
    protected override void OnModelCreating(ModelBuilder builder)
    {
        builder.Entity<Order>().HasKey(o => o.Id);
    }
}

The Fix

Call the base implementation first, then add or refine your own configuration:

protected override void OnModelCreating(ModelBuilder builder)
{
    base.OnModelCreating(builder);
    builder.Entity<Order>().HasKey(o => o.Id);
}

Analyzer Logic

ID: LC055

Category: Correctness

Severity: Warning

1. Find an OnModelCreating(ModelBuilder) override with a body in a type that derives from DbContext.
2. Look at the method it overrides. Stay quiet when that is EF Core’s own DbContext.OnModelCreating, which is empty, or a source override whose body is empty or only forwards to its own empty base. A base from another assembly (Identity, a shared package) is assumed to configure the model.
3. Report on the method name when no base.OnModelCreating(...) call appears anywhere in the context, including other methods, lambdas, and other partial parts.

When it stays quiet (non-goals)

• Contexts that derive from DbContext directly.
• Base overrides in source that are empty or only call base.OnModelCreating(...) down to DbContext.
• A base OnModelCreating declared abstract, which cannot be called.
• Any base.OnModelCreating(...) call in the type, even conditional or in a helper. The rule does not check that every path calls it.
• Types that are not DbContexts, even when they have an OnModelCreating(ModelBuilder) method.

A compiled base context whose OnModelCreating happens to be empty is still reported, because its body cannot be inspected. Adding the base call is harmless there.

Code Fix

Inserts base.OnModelCreating(<parameter>); as the first statement, so your configuration still runs after the base configuration and can override it. An expression-bodied override (=> builder.Entity<Order>()...) becomes a block with the base call followed by the original expression.

Test Cases

Violations

public class AppDbContext : IdentityDbContext
{
    protected override void OnModelCreating(ModelBuilder builder)
    {
        builder.Entity<Order>().HasKey(o => o.Id);
    }
}

Valid

public class AppDbContext : IdentityDbContext
{
    protected override void OnModelCreating(ModelBuilder builder)
    {
        base.OnModelCreating(builder);
        builder.Entity<Order>().HasKey(o => o.Id);
    }
}

public class ShopContext : DbContext
{
    protected override void OnModelCreating(ModelBuilder builder)
    {
        builder.Entity<Order>().HasKey(o => o.Id);
    }
}