Learn C# from Unity. Use NUnit for test framework.

Tests.cs

using System;
using NUnit.Framework;

namespace Syntax.Tests
{
    public struct Vector2 : IFormattable
    {
        public float x, y;

        public static Vector2 operator -(Vector2 a, Vector2 b)
            => new Vector2 {x = a.x - b.x, y = a.y - b.y};

        public string ToString(string format, IFormatProvider formatProvider)
        {
            if (string.IsNullOrEmpty(format))
            {
                format = "F1";
            }

            return $"({x.ToString(format, formatProvider)}, {y.ToString(format, formatProvider)})";
        }
    }

    [TestFixture]
    public class LearnStructTests
    {
        private void ModifyVector2(Vector2 vec)
        {
            vec.x = 2;
        }

        [Test]
        public void TestStruct()
        {
            var vec = new Vector2
            {
                x = 1,
                y = 20
            };
            ModifyVector2(vec);
            Assert.AreEqual(1, vec.x);
        }

        [Test]
        public void TestOperatorOverloading()
        {
            var target = new Vector2 {x = 2, y = 30};
            var player = new Vector2 {x = 1, y = 20};

            Assert.AreEqual(new Vector2 {x = 1, y = 10}, target - player);
        }

        [Test]
        public void TestFormatString()
        {
            var vec = new Vector2 {x = 1, y = 2};
            Assert.AreEqual("(1.0, 2.0)", $"{vec}");
            Assert.AreEqual("(1.00, 2.00)", $"{vec:F2}");
        }
    }
}