Improve the verification of CK3 Mod Directory (#2739) #patch

Added logic to verify that CK3ModsPath points to the standard Crusader
Kings III mods directory, including handling trailing separators and
platform-specific path comparisons. Introduced unit tests to ensure
correct validation and error handling for both valid and invalid paths.

Sentry event ID: 2bfcd84eccba4d66ba420aba627f2d05

Author: IhateTrains

ImperatorToCK3.UnitTests/ConfigurationTests.cs

@@ -1,5 +1,8 @@
 using commonItems.Mods;
 using ImperatorToCK3.Exceptions;
+using System;
+using System.IO;
+using System.Reflection;
 using Xunit;
 
 namespace ImperatorToCK3.UnitTests;
@@ -32,4 +35,40 @@ public void DetectSpecificCK3ModsThrowsExceptionForUnsupportedModCombinations()
 		Assert.Equal("The converter doesn't support combining The Fallen Eagle with Asia Expansion Project!",
 			ex.Message);
 	}
+
+	[Fact]
+	public void VerifyCK3ModsPathThrowsWhenNotPointingToStandardModsDirectory() {
+		var tempRoot = Path.Combine(Path.GetTempPath(), Guid.NewGuid().ToString());
+		Directory.CreateDirectory(tempRoot);
+		try {
+			var config = new Configuration {
+				CK3ModsPath = tempRoot
+			};
+			var verifyMethod = typeof(Configuration).GetMethod("VerifyCK3ModsPath", BindingFlags.Instance | BindingFlags.NonPublic);
+			var exception = Assert.Throws<TargetInvocationException>(() => verifyMethod!.Invoke(config, null));
+			var userError = Assert.IsType<UserErrorException>(exception.InnerException);
+			var expectedSuffix = Path.Combine("Paradox Interactive", "Crusader Kings III", "mod");
+			Assert.Contains(expectedSuffix, userError.Message);
+		}
+		finally {
+			Directory.Delete(tempRoot, recursive: true);
+		}
+	}
+
+	[Fact]
+	public void VerifyCK3ModsPathAcceptsStandardDirectoryWithTrailingSeparator() {
+		var tempRoot = Path.Combine(Path.GetTempPath(), Guid.NewGuid().ToString());
+		var modsPath = Path.Combine(tempRoot, "Paradox Interactive", "Crusader Kings III", "mod");
+		Directory.CreateDirectory(modsPath);
+		try {
+			var config = new Configuration {
+				CK3ModsPath = modsPath + Path.DirectorySeparatorChar
+			};
+			var verifyMethod = typeof(Configuration).GetMethod("VerifyCK3ModsPath", BindingFlags.Instance | BindingFlags.NonPublic);
+			verifyMethod!.Invoke(config, null);
+		}
+		finally {
+			Directory.Delete(tempRoot, recursive: true);
+		}
+	}
 }

ImperatorToCK3/Configuration.cs

@@ -256,7 +256,18 @@ private void VerifyCK3ModsPath() {
 		if (!Directory.Exists(CK3ModsPath)) {
 			throw new UserErrorException($"{CK3ModsPath} does not exist!");
 		}
-		
+
+		var normalizedCK3ModsPath = Path.TrimEndingDirectorySeparator(
+			CK3ModsPath.Replace(Path.AltDirectorySeparatorChar, Path.DirectorySeparatorChar)
+		);
+		var expectedSuffix = Path.Combine("Paradox Interactive", "Crusader Kings III", "mod");
+		var comparison = OperatingSystem.IsWindows() || OperatingSystem.IsMacOS()
+			? StringComparison.OrdinalIgnoreCase
+			: StringComparison.Ordinal;
+		if (!normalizedCK3ModsPath.EndsWith(expectedSuffix, comparison)) {
+			throw new UserErrorException($"{CK3ModsPath} is not a valid CK3 mods directory! It should end with {expectedSuffix}.");
+		}
+
 		// If the mods folder contains any files, at least one on them should have a .mod extension.
 		var filesInFolder = Directory.GetFiles(CK3ModsPath);
 		if (filesInFolder.Length > 0) {