C＃将两个列表与对象进行比较

Question

i'm making simple program, and i'm stuck on simple question. How can i compare two lists with objects , find that object and print all class elements that belongs to that object?

 List<Skaitytojas> MyObjectList = new List<Skaitytojas>();
 List<Knyga> KnyguList = new List<Knyga>();

 private void button2_Click(object sender, EventArgs e)
    {
        Reader reader = new Reader();

        reader._surname = Convert.ToString(textBox1.Text);
        reader._id = Convert.ToInt32(textBox2.Text);

        MyObjectList.Add(reader);
        MessageBox.Show("you created new user");
    }

private void button4_Click(object sender, EventArgs e)
    {
        Book new_book = new Book();

       new_book.book_id = Convert.ToInt32(textBox3.Text);
        new_book.date1 = Convert.ToString(textBox4.Text);
        new_book.date2 = Convert.ToString(textBox5.Text);
        new_book.date3 = Convert.ToString(textBox6.Text);
       new_book._id2 = Convert.ToInt32(textBox7.Text);

        BookList.Add(new_book);
        MessageBox.Show("you added new book!");
    }

I want to compare reader._id with new_book.id2, its gonna be same number , and if i find it i want to print all information about it: book id,date1,date2 etc. But i don't know how to do it, i tried it this way:

public void print_book(int x, ListBox f)
    {
        List<Book> BookList = new List<Book>();
        foreach (var k in BookList)
        {
            if (x == k._id2)
            {
                f.Items.Add(x);
            }
        }
    }

Call function with button

private void button3_Click(object sender, EventArgs e)
    {

        int x = Convert.ToInt32(textBox2.Text);
        Metodai m = new Metodai();
        m.print_book(x, this.listBox1);
    }

When i try to do this function the list is still empty, whats wrong with it? Maybe there is another way to do it? Can you help me?

Answer 1

The line

foreach (var k in BookList)

You've just instantiated BookList in the line before so it's looping through nothing.

Answer 2

You created a function variable 'BookList' with the same name as that of instance variable.

There are two ways to fix it.

// Fix #1
public void print_book(int x, ListBox f)
{
    //List<Book> BookList = new List<Book>(); // Comment this line
    foreach (var k in BookList)
    {
        if (x == k._id2)
        {
            f.Items.Add(x);
        }
    }
}

// Fix #2
public void print_book(int x, ListBox f)
{
    List<Book> BookList = new List<Book>(); // 'BookList' has never been used
    foreach (var k in this.BookList) // Add 'this'
    {
        if (x == k._id2)
        {
            f.Items.Add(x);
        }
    }
}