Testing Generic EF Repo (https://stackoverflow.com/questions/76450849/trying-to-unit-test-generic-repository-based-on-net-ef-core-fails-when-dealing)

Test.cs

// this is a way to test Generic Repo that uses EF. Notice, I'm not advocating for generic repos.
using FluentAssertions;
using Microsoft.EntityFrameworkCore;

public static class MyDebug
{
    public static List<string> Sql {get; set; } = new();
    public static void Capture(string query)
    {
        if (query.Contains("Execute")) 
            Sql.Add(query);
    }
}
public class Customer
{
    public int CustomerId { get; set; }
    public string Name { get; set; } = "test data";
}

public class TestDbContext: DbContext
{
    public DbSet<Customer> Customers { get; set; } = default!;

    protected override void OnConfiguring(DbContextOptionsBuilder optionsBuilder)
    {
        optionsBuilder
            .UseSqlite("Data Source=file:f1?mode=memory&cache=shared")
            .LogTo(MyDebug.Capture);
    }
}

public class GenericRepository
{
    protected readonly DbContext DbContext;
    public GenericRepository(DbContext dbContext)
    {
        DbContext = dbContext;
    }
    public virtual void Delete<TEntity>(TEntity entity)
    where TEntity: class
    {
        if (DbContext.Entry(entity).State == EntityState.Detached)
        {
            DbContext.Attach(entity);
        }
        DbContext.Remove(entity);        
    }
}

public class CustomerRepositoryItegrationTest
{
    private readonly TestDbContext db;
    private readonly Customer customer;
    public CustomerRepositoryItegrationTest()
    {
        db = new TestDbContext();
        db.Database.EnsureCreated();
        customer = new Customer();
        db.Customers.Add(customer);
        var n = db.SaveChanges();
        Assert.Equal(1, n);
    }
    [Fact]
    public void Delete_Attached_Entity()
    {
        // Arrange
        var repo = new GenericRepository(db);

        // Act
        repo.Delete(customer);
        db.SaveChanges();

        // Assert
        MyDebug.Sql.Last().Should().Contain("DELETE")       ;
        db.Customers.Any(c => c.CustomerId == customer.CustomerId).Should().BeFalse();
    }

    [Fact]
    public void Delete_Dettached_Entity()
    {
        // Arrange
        var repo = new GenericRepository(db);
        db.Entry(customer).State = EntityState.Detached;

        // Act
        repo.Delete(customer);
        db.SaveChanges();

        // Assert
        MyDebug.Sql.Last().Should().Contain("DELETE")       ;
        db.Customers.Any(c => c.CustomerId == customer.CustomerId).Should().BeFalse();
    }
}

@ctrl-alt-d Thank you for your thorough answer and the time you spent to write it. You really helped me to confirm some concepts, and ideas.