Error cs0019 operator cannot be applied to operands of type double and double

I have tried working on a vector addition code in C#, and part of it involves using math to determine X and Y values for a vector, and for some reason it gives me the error saying I cannot multiply an array value with a double precision floating point value. I have tried converting, but that has only created more errors. Here is my code:

using System;
using System.Windows.Forms;

    namespace VectorAddEdit
    {
        public partial class Form1 : Form
        {
            double[] mag = new double[5];
            double[] ang = new double[5];
            int cnt = 0;
            public Form1()
            {
                InitializeComponent();
            }

            private void textBox1_TextChanged(object sender, EventArgs e)
            {

            }

            private void textBox2_TextChanged(object sender, EventArgs e)
            {

            }

            private void textBox4_TextChanged(object sender, EventArgs e)
            {

            }

            private void textBox5_TextChanged(object sender, EventArgs e)
            {

            }

            private void button1_Click(object sender, EventArgs e)
            {
                while (cnt < 5)
                {
                    if (textBox1.Text == "" || textBox2.Text == "")
                    {
                        MessageBox.Show("You did not enter data into the correct boxes!");
                        System.Threading.Thread.Sleep(1000);
                        Application.Exit();
                    }

                    mag[cnt] = Convert.ToDouble(textBox1.Text);
                    ang[cnt] = Convert.ToDouble(textBox2.Text);

                    Console.ReadLine();

                    cnt++;
                }
            }

            private void button2_Click(object sender, EventArgs e)
            {
                double xsummag = 0;
                double ysummag = 0;
                double resultmag;
                double resultang;


                for (int i = 0; i < cnt; i++)
                {
                    xsummag = mag * Math.Cos(ang * Math.PI / 180) + xsummag;
                    ysummag = mag * Math.Sin(ang * Math.PI / 180) + ysummag;
                }

                resultmag = Math.Sqrt(Math.Pow(xsummag, 2) + Math.Pow(ysummag, 2));
                resultang = Math.Atan(ysummag / xsummag) * 180 / Math.PI;

                if (xsummag < 0 && ysummag > 0)
                    resultang = resultang + 180;

                else if (xsummag < 0 && ysummag < 0)
                    resultang = resultang + 180;

                else if (xsummag > 0 && ysummag < 0)
                    resultang = resultang + 360;

                textBox4.Text = resultmag.ToString();

                textBox5.Text = resultang.ToString();
            }

            private void button3_Click(object sender, EventArgs e)
            {
                System.Media.SoundPlayer player = new System.Media.SoundPlayer(@"C:WindowsMediaAlarm09.wav");

                player.Play();
                Application.DoEvents();
                System.Threading.Thread.Sleep(1000);

                Application.Exit();
            }
        }
    }

The error occurs on 2 lines where xsummag and ysummag are assigned.
If anyone can help me resolve this error, I would greatly appreciate it.

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private static object[] GetEnumLookupData<TEnum>() where TEnum: Enum
{
    var data = Enum.GetValues(typeof(TEnum))
        .OfType<TEnum>()
        .Where(e => e != default(TEnum))
        .Select(e => new {Id = e, Name = e.ToString()})
        .ToArray();

    return data;
}

This does not compile, but returns a error:

CS0019: Operator ‘!=’ cannot be applied to operands of type ‘TEnum’ and ‘TEnum’

What am I missing here? Isn’t it clear that TEnum is a value type?

The funniest thing is that if I omit (TEnum) in default(TEnum), so it is now e != default it compiles, but default generates porbably just a null because I see 0 value in created EF Core migration. WTF?!

I tried to use EqualityComparer<TEnum>.Default.Equals(...) and it kinda worked, however, EF Core 2.1 has a bug: migrations with seeding of data with emun as primary key are unstable. This bug is fixed only in EF Core 2.2 preview.
So I changed to int and …

private static object[] GetEnumLookupData<TEnum>() where TEnum: struct, Enum
{
    var comparer = EqualityComparer<TEnum>.Default;

    var data = Enum.GetValues(typeof(TEnum))
        .OfType<TEnum>()
        .Where(e => !comparer.Equals(e, default))
        .Select(e => new {Id = (int)e, Name = e.ToString()})
        .ToArray();

    return data;
}

CS0030 Cannot convert type ‘TEnum’ to ‘int’

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