In Entity layer we add entity in TITLE Class as
public int titleId { get; set; }
public string titleName { get; set; }
public Author authorObj { get; set; }
public Category categoryObj { get; set; }
In Author
public int authorId { get; set; }
public string authorName { get; set; }
In Category
public int categoryId { get; set; }
public string categoryName { get; set; }
public int price { get; set; }
In presentation layer
au.authorId = int.Parse(TextBox1.Text);
au.authorName = TextBox2.Text;
ca.categoryId = int.Parse(TextBox3.Text);
ca.categoryName = TextBox4.Text;
ca.price = int.Parse(TextBox5.Text);
ti.titleId = int.Parse(TextBox6.Text);
ti.titleName = TextBox7.Text;
ti.authorObj.authorId = int.Parse(TextBox1.Text);
ti.categoryObj.categoryId = int.Parse(TextBox3.Text);
// It showing Object reference not set to an instance error while executing,Please help me how to solve this issue.