Campo único en la clase modelo .net core

campo único en la clase modelo .net core

//single index
[Index("IX_First", IsUnique = true)]
public int FirstColumn { get; set; }

// composite index
[Index("IX_FirstAndSecond", 1, IsUnique = true)]
public int FirstColumn { get; set; }

[Index("IX_FirstAndSecond", 2, IsUnique = true)]
public int SecondColumn { get; set; }

//using bulder

protected override void OnModelCreating(ModelBuilder builder)
{
    builder.Entity<User>()
        .HasIndex(u => u.Email)
        .IsUnique();
}

public class User
{
   public int UserId{get;set;}
   [StringLength(450)]
   [Index(IsUnique=true)]
   public string UserName{get;set;}
}