Fix #1849: Language version lost when switch language

ILSpy/DecompilationOptions.cs

@@ -78,7 +78,7 @@ namespace ICSharpCode.ILSpy
 
 		public DecompilationOptions(LanguageVersion version, Decompiler.DecompilerSettings settings, Options.DisplaySettings displaySettings)
 		{
-			if (!Enum.TryParse(version.Version, out Decompiler.CSharp.LanguageVersion languageVersion)) 
+			if (!Enum.TryParse(version?.Version, out Decompiler.CSharp.LanguageVersion languageVersion)) 
 				languageVersion = Decompiler.CSharp.LanguageVersion.Latest;
 			var newSettings = this.DecompilerSettings = settings.Clone();
 			newSettings.SetLanguageVersion(languageVersion);

ILSpy/Docking/DockWorkspace.cs

@@ -175,8 +175,12 @@ namespace ICSharpCode.ILSpy.Docking
 
 		private void FilterSettings_PropertyChanged(object sender, PropertyChangedEventArgs e)
 		{
-			if (e.PropertyName == "Language" || e.PropertyName == "LanguageVersion") {
+			if (e.PropertyName == "Language") {
 				ActiveTabPage.Language = sessionSettings.FilterSettings.Language;
+				if (sessionSettings.FilterSettings.Language.HasLanguageVersions) {
+					sessionSettings.FilterSettings.LanguageVersion = ActiveTabPage.LanguageVersion;
+				}
+			} else if (e.PropertyName == "LanguageVersion") {
 				ActiveTabPage.LanguageVersion = sessionSettings.FilterSettings.LanguageVersion;
 			}
 		}

ILSpy/FilterSettings.cs

@@ -141,7 +141,6 @@ namespace ICSharpCode.ILSpy
 			set {
 				if (language != value) {
 					language = value;
-					LanguageVersion = language.LanguageVersions.LastOrDefault();
 					OnPropertyChanged();
 				}
 			}

ILSpy/Languages/Language.cs

@@ -33,7 +33,7 @@ using SRM = System.Reflection.Metadata;
 
 namespace ICSharpCode.ILSpy
 {
-	public struct LanguageVersion : IEquatable<LanguageVersion>
+	public class LanguageVersion
 	{
 		public string Version { get; }
 		public string DisplayName { get; }
@@ -44,23 +44,10 @@ namespace ICSharpCode.ILSpy
 			this.DisplayName = name ?? version.ToString();
 		}
 
-		public bool Equals(LanguageVersion other)
-		{
-			return other.Version == this.Version && other.DisplayName == this.DisplayName;
-		}
-
-		public override bool Equals(object obj)
-		{
-			return obj is LanguageVersion version && Equals(version);
-		}
-
-		public override int GetHashCode()
+		public override string ToString()
 		{
-			return unchecked(982451629 * Version.GetHashCode() + 982451653 * DisplayName.GetHashCode());
+			return $"[LanguageVersion DisplayName={DisplayName}, Version={Version}]";
 		}
-
-		public static bool operator ==(LanguageVersion lhs, LanguageVersion rhs) => lhs.Equals(rhs);
-		public static bool operator !=(LanguageVersion lhs, LanguageVersion rhs) => !lhs.Equals(rhs);
 	}
 
 	/// <summary>

ILSpy/ViewModels/TabPageModel.cs

@@ -17,21 +17,36 @@
 // DEALINGS IN THE SOFTWARE.
 
 using System;
+using System.Collections.Generic;
+using System.Linq;
 using System.Threading.Tasks;
-using System.Windows;
 using ICSharpCode.ILSpy.TextView;
 
 namespace ICSharpCode.ILSpy.ViewModels
 {
 	public class TabPageModel : PaneModel
 	{
+		private readonly Dictionary<Language, LanguageVersion> languageVersionHistory = new Dictionary<Language, LanguageVersion>();
+
 		private Language language;
 		public Language Language {
 			get => language;
 			set {
 				if (language != value) {
+					if (language != null && language.HasLanguageVersions) {
+						languageVersionHistory[language] = languageVersion;
+					}
 					language = value;
 					RaisePropertyChanged(nameof(Language));
+					if (language.HasLanguageVersions) {
+						if (languageVersionHistory.TryGetValue(value, out var version)) {
+							LanguageVersion = version;
+						} else {
+							LanguageVersion = Language.LanguageVersions.Last();
+						}
+					} else {
+						LanguageVersion = default;
+					}
 				}
 			}
 		}
@@ -42,6 +57,9 @@ namespace ICSharpCode.ILSpy.ViewModels
 			set {
 				if (languageVersion != value) {
 					languageVersion = value;
+					if (language.HasLanguageVersions) {
+						languageVersionHistory[language] = languageVersion;
+					}
 					RaisePropertyChanged(nameof(LanguageVersion));
 				}
 			}