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 Triangle
{
    public partial class form : Form
    {
        //Constructor
        public form()
        {
            InitializeComponent();
        }
        
        //Functions
        protected override void OnPaint(PaintEventArgs e)
        {
            base.OnPaint(e);

            Graphics g = e.Graphics;

            PointF point1 = new PointF((this.Size.Width / 2), (this.Size.Height / 2) - 150.0f);
            PointF point2 = new PointF((this.Size.Width / 2) + 150.0f, (this.Size.Height / 2) + 150.0f);
            PointF point3 = new PointF((this.Size.Width / 2) - 150.0f, (this.Size.Height / 2) + 150.0f);

            using (Pen outlinePen = new Pen(Color.Black, 5))
            {
                g.DrawLine(outlinePen, point1, point2);
                g.DrawLine(outlinePen, point2, point3);
                g.DrawLine(outlinePen, point3, point1);
            }
        }

        private void Generate(object sender, EventArgs e)
        {
            if (a.Value == 0 || b.Value == 0 || c.Value == 0)
            {
                MessageBox.Show("The sides cannot be negative.", "Generate ERROR", MessageBoxButtons.OK, MessageBoxIcon.Error);
                return;
            }

            if (a.Value + b.Value < c.Value || a.Value + c.Value < b.Value || b.Value + c.Value < a.Value)
            {
                MessageBox.Show("Sum of two sides is lower than the thirt side!", "Generate ERROR", MessageBoxButtons.OK, MessageBoxIcon.Error);
                return;
            }

            La.Text = a.Value.ToString();
            Lb.Text = b.Value.ToString();
            Lc.Text = c.Value.ToString();

            int perimetrN = 0;
            float areaN = 0.00f;

            perimetrN = (int)(a.Value + b.Value + c.Value);
            perimetr.Text = $"P = {perimetrN}";

            areaN = (float)(a.Value + b.Value + c.Value) / 2.00f;
            areaN = (float)Math.Sqrt(areaN * (areaN - (float)a.Value) * (areaN - (float)b.Value) * (areaN - (float)c.Value));
            
            area.Text = $"A = {areaN:F2}";

            if(c.Value == b.Value && b.Value == a.Value && c.Value == a.Value)
            {
                type.Text = "Equilateral ";
                MessageBox.Show("The triangle is equilateral.");
                return;
            }

            float aVal = (float)a.Value;
            float bVal = (float)b.Value;
            float cVal = (float)c.Value;

            bool isRight =
                Math.Abs(aVal * aVal + bVal * bVal - cVal * cVal) < 0.001f ||
                Math.Abs(aVal * aVal + cVal * cVal - bVal * bVal) < 0.001f ||
                Math.Abs(bVal * bVal + cVal * cVal - aVal * aVal) < 0.001f;

            string right = isRight ? " and right" : "";

            if (a.Value == b.Value || c.Value == b.Value || a.Value == c.Value)
            {
                type.Text = "Isosceles";
                MessageBox.Show($"The triangle is isosceles{right}.");
                return;
            }

            type.Text = $"Scalene{right}";
        }

        //Operations
        private void exitToolStripMenuItem_Click(object sender, EventArgs e)
        {
            Environment.Exit(0);
        }

        private void clearToolStripMenuItem_Click(object sender, EventArgs e)
        {
            a.Value = 0;
            b.Value = 0;
            c.Value = 0;
            La.Text = a.Value.ToString();
            Lb.Text = b.Value.ToString();
            Lc.Text = c.Value.ToString();
            perimetr.Text = "P = 0";
            area.Text = "A = 0";
            type.Text = "Type";
        }
    }
}