如何在代码优先实体框架fluent API中指定多列唯一约束

using System.ComponentModel.DataAnnotations.Schema;
using System.Data.Entity.ModelConfiguration;

public class User
{
    public int Id { get; set; }
    public string Username { get; set; }
    public string Email { get; set; }
}

public class UserConfiguration : EntityTypeConfiguration<User>
{
    public UserConfiguration()
    {
        // 指定多列唯一约束
        HasKey(u => new { u.Username, u.Email });

        // 其他配置...
    }
}

using System.Data.Entity;

public class MyDbContext : DbContext
{
    public DbSet<User> Users { get; set; }

    protected override void OnModelCreating(DbModelBuilder modelBuilder)
    {
        modelBuilder.Configurations.Add(new UserConfiguration());

        base.OnModelCreating(modelBuilder);
    }
}