How to get an id of a saved entity in Entity Framework?

EF 6 and EF Core both, automatically populates the DB-generated Id properties (primary key or foreign key) when you call the SaveChanges() method. For example, consider the following Student entity.

public class Student
{
    public int StudentID { get; set; }
    public string StudentName { get; set; }
    public DateTime? DateOfBirth { get; set; }
    public decimal Height { get; set; }
    public float Weight { get; set; }
}

Now, when you add a new Student and call SaveChanges() method, EF will assign a newly generated id to the StudentID property.

using(var context = new SchoolContext())
{             
     var std = new Student(){ StudentName = "Steve" };
     context.Add(std);
     context.SaveChanges();
               
     Console.Write(std.StudentID); // 1
 }

EF execute each INSERT command followed by SELECT scope_identity() statement. SCOPE_IDENTITY returns the last identity value inserted into an identity column in the same scope. The above example will execute the following SQL in the database.

The same way, EF also populates the foreign key property. For example, consider the following Student entity that includes a Grade property.

public class Student
{
    public int StudentID { get; set; }
    public string StudentName { get; set; }
    public DateTime? DateOfBirth { get; set; }
    public decimal Height { get; set; }
    public float Weight { get; set; }

    public Grade Grade { get; set; }
}

public class Grade
{
    public int GradeId { get; set; }
    public string GradeName { get; set; }
    public string Section { get; set; }
}

Now, when you add a new Student with a new Grade then EF will populate the Student.Grade.GradeId also.

using (var context = new SchoolContext())
{
    var std = new Student() { StudentName = "Steve", 
                              Grade = new Grade(){  GradeName="1st Grade", Section="C" } };
    context.Students.Add(std);
    context.SaveChanges();

    Console.WriteLine(std.Grade.GradeId);// 1
}

EF will first execute the insert command for a new Grade and then use this new GradeId in the insert command for a Student.