(windows.forms) Why doesn't it calculate correctly?

Question

C++ / C#

When I enter the number 6, it is squared, it turns out 36, it is divided by 10, it turns out 3.6 and is written to the variable k, in the second variable the number is rounded down, as a result, it turns out 3, then 3 is subtracted from 3.6, it should it will be 0.6, but in the output I see 0.59999.

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Windows.Forms;

namespace WindowsFormsApp5
{
    public partial class Form1 : Form
    {
        public Form1()
        {
            InitializeComponent();
        }
        double n, b, c, d = 0, k;
        int a = 0;
        private void textBox1_KeyPress(object sender, KeyPressEventArgs e)
        {
            char number = e.KeyChar;
            if (!Char.IsDigit(number) && number != 8 && number != 44)
            {
                e.Handled = true;
            }
        }

        private void button1_Click(object sender, EventArgs e)
        {
            if (textBox1.Text == "")
            {
                label2.Text = "Введите число";
            }
            else
            {
                label2.Visible = false;
                n = int.Parse(textBox1.Text);
                for (int i = 1; i <= n; i++)
                {
                    b = i;
                    c = i * i;
                    d = 0;

                    while (b >= 1)
                    {
                        b = b / 10;
                        d += 1;
                    }
                    for (int j = 1; j <= d; j++)
                    {
                        c = c / 10;
                    }
                    k = c;//здесь получается 3,6
                    c = Math.Floor(c);//здесь 3
                    k = k - c;//здесь 0,59999
                    for (int l = 1; l <= d; l++)
                    {
                        k = k * 10;
                    }
                    if (k == i)
                    {
                        a += 1;
                    }

                }
                label1.Text = a.ToString();
                a = 0;
            }
        }
    }
}

Answer 1

Welcome to float.
Because there are no integer calculations. 4/2 == 1.9999. Or 2.00001. Read about the representation of floating point numbers in memory

Answer 2

it turns out 3, then 3 is subtracted from 3.6, it should turn out 0.6

In general, no, it shouldn't.
But if you round correctly
Math.Round(double_num,2); , then everything is guaranteed with the specified accuracy.