为什么中国坐标正反运算计算器字符串转化为float时候提示输入的的字符串格式不正确？

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;
using System.IO;

namespace 坐标正反算
{
    public partial class Form1 : Form
    {
        public Form1()
        {
            InitializeComponent();
        }

        private struct rowDdta
        {
            public double x, y;    //A
            public double X, Y;    //B
            public double D, a, R;    //水平距离，坐标方位角；
        }

        List<rowDdta> Data = new List<rowDdta>();


        private void button1_Click(object sender, EventArgs e)
        {
            string str;
            string[] strs;
            rowDdta rD = new rowDdta();

            for (int i = 0; i < textBox1.Lines.Length; i++)
            {
                str = textBox1.Lines[i];
                strs = str.Split(',');

                rD.x = Convert.ToDouble(strs[0]);
                rD.y = Convert.ToDouble(strs[1]);
                rD.a = Convert.ToDouble(strs[2]);
                rD.D = Convert.ToDouble(strs[3]);

                rD.X = rD.x + rD.D * Math.Cos(rD.a);
                rD.Y = rD.y + rD.D * Math.Sin(rD.a);

                Data.Add(rD);

            }

            for (int i = 0; i < Data.Count; i++)
            {
                textBox2.Text = textBox2.Text + Data[i].X + "," + Data[i].Y + "\r\n";
            }

        }