Is EF Core DbContext thread-safe? Parallel queries with IDbContextFactory

// ======================================================================
// Q: Is DbContext thread-safe? Can I run queries in parallel?
//
// No. One DbContext = one unit of work, one operation at a time. A second
// concurrent operation throws InvalidOperationException: A second operation was
// started on this context instance.... Common causes: Task.WhenAll on the same
// context, a forgotten await, async void, a singleton holding a scoped DbContext
// (captive dependency, Q5).
// In a BackgroundService: create a new scope
// (IServiceScopeFactory.CreateAsyncScope()) per message or iteration, so each
// unit of work has its own context. Parallel queries only make sense if they are
// independent and slow: each one takes a connection from the pool.
// ======================================================================

// ✗ same DbContext in parallel tasks
var orders = db.Orders.CountAsync(ct);
var products = db.Products.CountAsync(ct);
await Task.WhenAll(orders, products);

// ✓ separate context per parallel operation
public class DashboardQuery(IDbContextFactory<AppDbContext> factory)
{
    public async Task<(int Orders, int Products)> GetAsync(CancellationToken ct)
    {
        await using var db1 = await factory.CreateDbContextAsync(ct);
        await using var db2 = await factory.CreateDbContextAsync(ct);
        var orders = db1.Orders.CountAsync(ct);
        var products = db2.Products.CountAsync(ct);
        return (await orders, await products);
    }
}
builder.Services.AddDbContextFactory<AppDbContext>(o => o.UseNpgsql(cs));