Getting into C# testing

Here are the tests I am running for a school registration system : using System.Collections.Generic;
using System.ComponentModel.DataAnnotations;
using SportsWeb.Models;
using Xunit;

namespace SportsWeb.Tests.Models
{
public class RegistrationViewModelValidationTests
{
private static IList Validate(object model)
{
var ctx = new ValidationContext(model, serviceProvider: null, items: null);
var results = new List();
Validator.TryValidateObject(model, ctx, results, validateAllProperties: true);
return results;
}

    [Fact]
    public void Missing_Season_IsInvalid()
    {
        // ARRANGE: Simulating a missing drop-down selection
        var vm = new RegistrationViewModel
        {
            AthleteName = "Bram",
            Season = null, 
            Sport = "Basketball"
        };

        // ACT
        var results = Validate(vm);

        // ASSERT
        Assert.Contains(results, r => r.MemberNames.Contains(nameof(RegistrationViewModel.Season)));
    }

    [Fact]
    public void Missing_Sport_IsInvalid()
    {
        // ARRANGE: Simulating no radio button being selected
        var vm = new RegistrationViewModel
        {
            AthleteName = "Bram",
            Season = "Fall",
            Sport = null 
        };

        var results = Validate(vm);

        Assert.Contains(results, r => r.MemberNames.Contains(nameof(RegistrationViewModel.Sport)));
    }

    [Fact]
    public void Valid_RegistrationModel_Passes()
    {
        // ARRANGE
        var vm = new RegistrationViewModel
        {
            AthleteName = "Bram",
            Season = "Winter",
            Sport = "Basketball"
        };

        // ACT
        var results = Validate(vm);

        // ASSERT
        Assert.Empty(results);
    }
}

}

using System;
using SportsWeb.Models;
using SportsWeb.Services;
using Xunit;

namespace SportsWeb.Tests.Services
{
public class RegistrationServiceTests
{
private readonly IRegistrationService _service;

    public RegistrationServiceTests()
    {
        // Testing the concrete service implementation
        _service = new RegistrationService(); 
    }

    [Fact]
    public void CalculateFee_ReturnsStandardRate_ForWinterBasketball()
    {
        // Arrange
        var vm = new RegistrationViewModel { Season = "Winter", Sport = "Basketball" };

        // Act
        decimal fee = _service.CalculateFee(vm);

        // Assert
        Assert.Equal(150m, fee);
    }

    [Theory]
    [InlineData("Summer", "Soccer", 100)]
    [InlineData("Spring", "Tennis", 120)]
    public void CalculateFee_ReturnsCorrectRates_ForAllSeasons(string season, string sport, decimal expectedFee)
    {
        // Arrange
        var vm = new RegistrationViewModel { Season = season, Sport = sport };

        // Act
        decimal fee = _service.CalculateFee(vm);

        // Assert
        Assert.Equal(expectedFee, fee);
    }
}

}

here are the controller tests : using Microsoft.AspNetCore.Mvc;
using Moq;
using SportsWeb.Controllers;
using SportsWeb.Models;
using SportsWeb.Services;
using Xunit;

namespace SportsWeb.Tests.Controllers
{
public class RegistrationControllerTests
{
private readonly Mock _mockService;
private readonly RegistrationController _controller;

    public RegistrationControllerTests()
    {
        _mockService = new Mock<IRegistrationService>();
        _controller = new RegistrationController(_mockService.Object);
    }

    [Fact]
    public void CreatePost_InvalidModel_ReturnsSameView()
    {
        // ARRANGE
        // Simulate ASP.NET Core catching a missing radio button selection
        _controller.ModelState.AddModelError("Sport", "Please select a sport.");

        var vm = new RegistrationViewModel { AthleteName = "Bram", Season = "Fall", Sport = null };

        // ACT
        var result = _controller.Create(vm) as ViewResult;

        // ASSERT
        Assert.NotNull(result);
        Assert.False(_controller.ModelState.IsValid);
        Assert.Same(vm, result.Model); // Ensure the form is sent back to the user with their inputs intact

        // Verify the database/service was NOT called since the model was invalid
        _mockService.VerifyNoOtherCalls(); 
    }

    [Fact]
    public void CreatePost_ValidModel_CallsServiceAndRedirects()
    {
        // ARRANGE
        var vm = new RegistrationViewModel { AthleteName = "Bram", Season = "Winter", Sport = "Basketball" };

        // Set up the mock to return true when saving
        _mockService.Setup(s => s.ProcessRegistration(vm)).Returns(true);

        // ACT
        var result = _controller.Create(vm) as RedirectToActionResult;

        // ASSERT
        Assert.NotNull(result);
        Assert.Equal("Success", result.ActionName); // Ensure it redirects to the Success page

        // Verify the service was called exactly once with the provided model
        _mockService.Verify(s => s.ProcessRegistration(vm), Times.Once);
    }
}

}