Add LangVersion support to WholeProjectDecompiler and add NonTrailingNamedArguments setting.

ICSharpCode.Decompiler.Tests/RoundtripAssembly.cs

@@ -145,7 +145,7 @@ namespace ICSharpCode.Decompiler.Tests
 					Stopwatch w = Stopwatch.StartNew();
 					using (var fileStream = new FileStream(file, FileMode.Open, FileAccess.Read)) {
 						PEFile module = new PEFile(file, fileStream, PEStreamOptions.PrefetchEntireImage);
-						UniversalAssemblyResolver resolver = new UniversalAssemblyResolver(file, false, module.Reader.DetectTargetFrameworkId(), PEStreamOptions.PrefetchMetadata);
+						var resolver = new UniversalAssemblyResolver(file, false, module.Reader.DetectTargetFrameworkId(), PEStreamOptions.PrefetchMetadata);
 						resolver.AddSearchDirectory(inputDir);
 						resolver.RemoveSearchDirectory(".");
 						var decompiler = new TestProjectDecompiler(inputDir);

ICSharpCode.Decompiler/CSharp/WholeProjectDecompiler.cs

@@ -57,6 +57,19 @@ namespace ICSharpCode.Decompiler.CSharp
 			}
 		}
 
+		LanguageVersion? languageVersion;
+
+		public LanguageVersion LanguageVersion {
+			get { return languageVersion ?? Settings.GetMinimumRequiredVersion(); }
+			set {
+				var minVersion = Settings.GetMinimumRequiredVersion();
+				if (value < minVersion)
+					throw new InvalidOperationException($"The chosen settings require at least {minVersion}." +
+						$" Please change the DecompilerSettings accordingly.");
+				languageVersion = value;
+			}
+		}
+
 		public IAssemblyResolver AssemblyResolver { get; set; }
 
 		/// <summary>
@@ -148,6 +161,8 @@ namespace ICSharpCode.Decompiler.CSharp
 					}
 				}
 
+				w.WriteElementString("LangVersion", LanguageVersion.ToString().Replace("CSharp", "").Replace('_', '.'));
+
 				w.WriteElementString("AssemblyName", module.Name);
 				bool useTargetFrameworkAttribute = false;
 				LanguageTargets languageTargets = LanguageTargets.None;

ICSharpCode.Decompiler/DecompilerSettings.cs

@@ -84,6 +84,7 @@ namespace ICSharpCode.Decompiler
 			}
 			if (languageVersion < CSharp.LanguageVersion.CSharp7_2) {
 				introduceRefAndReadonlyModifiersOnStructs = false;
+				nonTrailingNamedArguments = false;
 			}
 			if (languageVersion < CSharp.LanguageVersion.CSharp7_3) {
 				//introduceUnmanagedTypeConstraint = false;
@@ -91,6 +92,29 @@ namespace ICSharpCode.Decompiler
 			}
 		}
 
+		public CSharp.LanguageVersion GetMinimumRequiredVersion()
+		{
+			if (tupleComparisons)
+				return CSharp.LanguageVersion.CSharp7_3;
+			if (IntroduceRefAndReadonlyModifiersOnStructs || nonTrailingNamedArguments)
+				return CSharp.LanguageVersion.CSharp7_2;
+			// C# 7.1 missing
+			if (outVariables || tupleTypes || tupleConversions || discards)
+				return CSharp.LanguageVersion.CSharp7;
+			if (awaitInCatchFinally || useExpressionBodyForCalculatedGetterOnlyProperties || nullPropagation
+				|| stringInterpolation || dictionaryInitializers || extensionMethodsInCollectionInitializers)
+				return CSharp.LanguageVersion.CSharp6;
+			if (asyncAwait)
+				return CSharp.LanguageVersion.CSharp5;
+			if (dynamic || namedArguments || optionalArguments)
+				return CSharp.LanguageVersion.CSharp4;
+			if (anonymousTypes || objectCollectionInitializers || automaticProperties || queryExpressions || expressionTrees)
+				return CSharp.LanguageVersion.CSharp3;
+			if (anonymousMethods || liftNullables || yieldReturn || useImplicitMethodGroupConversion)
+				return CSharp.LanguageVersion.CSharp2;
+			return CSharp.LanguageVersion.CSharp1;
+		}
+
 		bool anonymousMethods = true;
 
 		/// <summary>
@@ -688,6 +712,21 @@ namespace ICSharpCode.Decompiler
 			}
 		}
 
+		bool nonTrailingNamedArguments = true;
+
+		/// <summary>
+		/// Gets/Sets whether C# 7.2 non-trailing named arguments should be used.
+		/// </summary>
+		public bool NonTrailingNamedArguments {
+			get { return nonTrailingNamedArguments; }
+			set {
+				if (nonTrailingNamedArguments != value) {
+					nonTrailingNamedArguments = value;
+					OnPropertyChanged();
+				}
+			}
+		}
+
 		bool optionalArguments = true;
 
 		/// <summary>