Unit Test case example showing that it's the contract of a method and not the implementation that should dictate the number of tests.

StringReverser.cs

using System;
using System.Collections.Generic;
using System.Linq;
using Xunit;

public class StringReverser
{
    // Method 1: Concise Implementation (2 lines)
    // Contract: Reverse the input string and return the result.
    public static string ReverseStringConcise(string input)
    {
        if (string.IsNullOrEmpty(input)) return input;
        return string.Concat(input.Reverse());
    }

    // Method 2: Verbose Implementation (15 lines)
    // Contract: Reverse the input string and return the result.
    public static string ReverseStringVerbose(string input)
    {
        if (string.IsNullOrEmpty(input))
        {
            return input;
        }

        char[] charArray = new char[input.Length];
        for (int i = 0; i < input.Length; i++)
        {
            charArray[i] = input[input.Length - 1 - i];
        }

        string reversed = "";
        foreach (var c in charArray)
        {
            reversed += c;
        }

        return reversed;
    }
}

public class StringReverserTests
{
    // Shared test cases using MemberData
    public static IEnumerable<object[]> TestCases =>
        new List<object[]>
        {
            new object[] { "Hello", "olleH" },               // Normal case
            new object[] { "", "" },                         // Empty string
            new object[] { "A", "A" },                       // Single character
            new object[] { null, null },                     // Null input
            new object[] { "12345", "54321" },               // Numeric string
            new object[] { "!@#$", "$#@!" },                 // Special characters
            new object[] { "AbCdE", "EdCbA" },               // Mixed case
            new object[] { "a b c", "c b a" },               // String with spaces
            new object[] { "😊🎉", "🎉😊" },                 // Unicode characters
            new object[] { "aaaa", "aaaa" },                 // Repeated characters
            new object[] { "a\nb\nc", "c\nb\na" },           // String with newlines
            new object[] { "LongString12345", "54321gnirtSgnoL" } // Longer string
        };

    [Theory]
    [MemberData(nameof(TestCases))]
    public void ReverseStringConcise_ShouldReturnCorrectlyReversedString(string input, string expected)
    {
        // Act
        var result = StringReverser.ReverseStringConcise(input);

        // Assert
        Assert.Equal(expected, result);
    }

    [Theory]
    [MemberData(nameof(TestCases))]
    public void ReverseStringVerbose_ShouldReturnCorrectlyReversedString(string input, string expected)
    {
        // Act
        var result = StringReverser.ReverseStringVerbose(input);

        // Assert
        Assert.Equal(expected, result);
    }
}