In Entity Framework Core 1.0.1 Many-To-Many without creating relational entity is not supported and the easiest way to implement is creating the relational entity.

I will show that in a small example:

public class Course
{
    public int Id { get; set; }       
    public string Title { get; set; }

    public List<StudentAndCourse> StudentAndCourse { get; set; }
}
public class Student
{
    public int Id { get; set; }       
    public string FirstName { get; set; }
    public string LastName { get; set; }

    public List<StudentAndCourse> StudentAndCourse { get; set; }
}

 

Relational Entity:

public class StudentAndCourse
{
    public int StudentId { get; set; }
    public Student Student{ get; set; }

    public int CourseId { get; set; }
    public Course Course { get; set; }
        
}

 

The Fluent API in DbContext:

protected override void OnModelCreating(ModelBuilder modelBuilder)
{            
    modelBuilder.Entity<StudentAndCourse>()
       .HasKey(sc => new { sc.StudentId, sc.CourseId });

    modelBuilder.Entity<StudentAndCourse>()
        .HasOne(sc => sc.Student)
        .WithMany(s => s.StudentAndCourse)
        .HasForeignKey(sc => sc.StudentId);

    modelBuilder.Entity<StudentAndCourse>()
        .HasOne(sc => sc.Course)
        .WithMany(c => c.StudentAndCourse)
        .HasForeignKey(sc => sc.CourseId);
}