14/Form1.cs

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 Calc2
{
    public partial class Form1 : Form
    {
        public Form1()
        {
            InitializeComponent();
        }
        float a, b;
        int oper;
        bool znak = true;

        private void button15_Click(object sender, EventArgs e)
        {
            if (znak == true)
            {
                textBox1.Text = "-" + textBox1.Text;
                znak = false;
            }
            else if (znak == false)
            {
                textBox1.Text = textBox1.Text.Replace("-", "");
                znak = true;
            }
        }

        private void button11_Click(object sender, EventArgs e)
        {
            textBox1.Text = textBox1.Text + 0;
        }

        private void button12_Click(object sender, EventArgs e)
        {
            textBox1.Text = textBox1.Text + ",";
        }

        private void button16_Click(object sender, EventArgs e)
        {
            a = float.Parse(textBox1.Text);
            textBox1.Clear();
            oper = 1;
            label1.Text = a.ToString() + "+";
            znak = true;
        }

        
    }
}