diff --git a/ICSharpCode.Decompiler.Tests/Helpers/Tester.cs b/ICSharpCode.Decompiler.Tests/Helpers/Tester.cs
index 499e82c4c3..2ffb392acd 100644
--- a/ICSharpCode.Decompiler.Tests/Helpers/Tester.cs
+++ b/ICSharpCode.Decompiler.Tests/Helpers/Tester.cs
@@ -575,7 +575,16 @@ public static async Task<CompilerResults> CompileCSharp(string sourceFileName, C
 			{
 				string depSourcePath = Path.GetFullPath(Path.Combine(
 					Path.GetDirectoryName(sourceFileName), match.Groups[1].Value));
-				var depResults = await CompileCSharp(depSourcePath, flags | CompilerOptions.Library).ConfigureAwait(false);
+				// Compile the dependency next to the main output, so that the assembly resolver
+				// finds it when the main assembly is decompiled.
+				string depOutputFileName = null;
+				if (outputFileName != null)
+				{
+					depOutputFileName = Path.Combine(
+						Path.GetDirectoryName(Path.GetFullPath(outputFileName)),
+						Path.GetFileNameWithoutExtension(depSourcePath) + GetSuffix(flags | CompilerOptions.Library) + ".dll");
+				}
+				var depResults = await CompileCSharp(depSourcePath, flags | CompilerOptions.Library, depOutputFileName).ConfigureAwait(false);
 				dependencyAssemblies.Add(Path.GetFullPath(depResults.PathToAssembly));
 			}
 
diff --git a/ICSharpCode.Decompiler.Tests/ICSharpCode.Decompiler.Tests.csproj b/ICSharpCode.Decompiler.Tests/ICSharpCode.Decompiler.Tests.csproj
index d38245d468..f6cb1035db 100644
--- a/ICSharpCode.Decompiler.Tests/ICSharpCode.Decompiler.Tests.csproj
+++ b/ICSharpCode.Decompiler.Tests/ICSharpCode.Decompiler.Tests.csproj
@@ -221,6 +221,12 @@
     <None Include="TestCases\ILPretty\Unsafe.cs" />
     <Compile Remove="TestCases\ILPretty\WeirdEnums.cs" />
     <None Include="TestCases\ILPretty\WeirdEnums.cs" />
+    <Compile Remove="TestCases\Pretty\ClosedHierarchies.cs" />
+    <None Include="TestCases\Pretty\ClosedHierarchies.cs" />
+    <Compile Remove="TestCases\Pretty\ClosedHierarchiesCrossAssembly.cs" />
+    <None Include="TestCases\Pretty\ClosedHierarchiesCrossAssembly.cs" />
+    <Compile Remove="TestCases\Pretty\ClosedHierarchiesCrossAssembly.dep.cs" />
+    <None Include="TestCases\Pretty\ClosedHierarchiesCrossAssembly.dep.cs" />
     <Compile Remove="TestCases\Pretty\IndexRangeTest.cs" />
     <None Include="TestCases\Pretty\IndexRangeTest.cs" />
     <Compile Remove="TestCases\Pretty\MetadataAttributes.cs" />
diff --git a/ICSharpCode.Decompiler.Tests/PrettyTestRunner.cs b/ICSharpCode.Decompiler.Tests/PrettyTestRunner.cs
index f68632b758..ad7823a001 100644
--- a/ICSharpCode.Decompiler.Tests/PrettyTestRunner.cs
+++ b/ICSharpCode.Decompiler.Tests/PrettyTestRunner.cs
@@ -663,6 +663,18 @@ public async Task ExtensionEverything([ValueSource(nameof(roslyn5OrNewerOptions)
 			await RunForLibrary(cscOptions: cscOptions | CompilerOptions.Preview | CompilerOptions.NullableEnable);
 		}
 
+		[Test]
+		public async Task ClosedHierarchies([ValueSource(nameof(roslyn5OrNewerOptions))] CompilerOptions cscOptions)
+		{
+			await RunForLibrary(cscOptions: cscOptions | CompilerOptions.Preview);
+		}
+
+		[Test]
+		public async Task ClosedHierarchiesCrossAssembly([ValueSource(nameof(roslyn5OrNewerOptions))] CompilerOptions cscOptions)
+		{
+			await RunForLibrary(cscOptions: cscOptions | CompilerOptions.Preview);
+		}
+
 		[Test]
 		public async Task NullPropagation([ValueSource(nameof(roslynOnlyOptions))] CompilerOptions cscOptions)
 		{
diff --git a/ICSharpCode.Decompiler.Tests/TestCases/Pretty/ClosedHierarchies.cs b/ICSharpCode.Decompiler.Tests/TestCases/Pretty/ClosedHierarchies.cs
new file mode 100644
index 0000000000..e17d9fd899
--- /dev/null
+++ b/ICSharpCode.Decompiler.Tests/TestCases/Pretty/ClosedHierarchies.cs
@@ -0,0 +1,74 @@
+namespace ICSharpCode.Decompiler.Tests.TestCases.Pretty
+{
+	internal class ClosedHierarchies
+	{
+		public closed class Animal
+		{
+			public string Name { get; }
+
+			protected Animal()
+			{
+				Name = "unnamed";
+			}
+
+			protected Animal(string name)
+			{
+				Name = name;
+			}
+		}
+
+		public class Cat : Animal
+		{
+		}
+
+		public sealed class Dog : Animal
+		{
+			public Dog()
+				: base("Dog")
+			{
+			}
+		}
+
+		public closed class Job
+		{
+			public required string Title { get; set; }
+		}
+
+		public sealed class CompileJob : Job
+		{
+		}
+
+		public closed class Tree<T>
+		{
+		}
+
+		public sealed class Leaf<U> : Tree<U>
+		{
+		}
+
+		public sealed class ArrayLeaf<V> : Tree<V[]>
+		{
+		}
+	}
+	public closed record JobStatus;
+	internal record JobStatusCanceled : JobStatus;
+	public record JobStatusQueued : JobStatus;
+	public record JobStatusRunning(int PercentComplete) : JobStatus;
+	internal closed class State
+	{
+	}
+	internal sealed class StateActive : State
+	{
+	}
+}
+#if !EXPECTED_OUTPUT
+// The .NET 11 preview 5 BCL does not ship ClosedAttribute yet; the compiler requires
+// every assembly using the 'closed' modifier to declare it (matched by full name).
+namespace System.Runtime.CompilerServices
+{
+	[AttributeUsage(AttributeTargets.Class, AllowMultiple = false, Inherited = false)]
+	internal sealed class ClosedAttribute : Attribute
+	{
+	}
+}
+#endif
diff --git a/ICSharpCode.Decompiler.Tests/TestCases/Pretty/ClosedHierarchiesCrossAssembly.cs b/ICSharpCode.Decompiler.Tests/TestCases/Pretty/ClosedHierarchiesCrossAssembly.cs
new file mode 100644
index 0000000000..d1e729f65e
--- /dev/null
+++ b/ICSharpCode.Decompiler.Tests/TestCases/Pretty/ClosedHierarchiesCrossAssembly.cs
@@ -0,0 +1,9 @@
+// #dependency ClosedHierarchiesCrossAssembly.dep.cs
+using CrossAssemblyClosed;
+
+namespace ICSharpCode.Decompiler.Tests.TestCases.Pretty
+{
+	public closed record LocalMessage;
+	public record LocalTextMessage(string Text) : LocalMessage;
+	public record Sedan(int Doors) : Car(Doors);
+}
diff --git a/ICSharpCode.Decompiler.Tests/TestCases/Pretty/ClosedHierarchiesCrossAssembly.dep.cs b/ICSharpCode.Decompiler.Tests/TestCases/Pretty/ClosedHierarchiesCrossAssembly.dep.cs
new file mode 100644
index 0000000000..46e8318a39
--- /dev/null
+++ b/ICSharpCode.Decompiler.Tests/TestCases/Pretty/ClosedHierarchiesCrossAssembly.dep.cs
@@ -0,0 +1,17 @@
+namespace CrossAssemblyClosed
+{
+	public closed record Vehicle;
+	public record Car(int Doors) : Vehicle;
+	public sealed record Truck(double PayloadTons) : Vehicle;
+}
+
+// Public so that the main test assembly can use the 'closed' modifier without
+// declaring its own copy of the attribute (the shape the BCL will provide once
+// ClosedAttribute ships).
+namespace System.Runtime.CompilerServices
+{
+	[AttributeUsage(AttributeTargets.Class, AllowMultiple = false, Inherited = false)]
+	public sealed class ClosedAttribute : Attribute
+	{
+	}
+}
diff --git a/ICSharpCode.Decompiler.Tests/TestCases/Pretty/Issue3684.cs b/ICSharpCode.Decompiler.Tests/TestCases/Pretty/Issue3684.cs
index 631830e39d..b18a71d38f 100644
--- a/ICSharpCode.Decompiler.Tests/TestCases/Pretty/Issue3684.cs
+++ b/ICSharpCode.Decompiler.Tests/TestCases/Pretty/Issue3684.cs
@@ -15,7 +15,7 @@ public class DerivedClass : BaseClass, IInterface
 		{
 			T IInterface.Convert<T>(T input)
 			{
-				return ((BaseClass)this).Convert<T>(input);
+				return Convert(input);
 			}
 		}
 	}
diff --git a/ICSharpCode.Decompiler/CSharp/CSharpDecompiler.cs b/ICSharpCode.Decompiler/CSharp/CSharpDecompiler.cs
index 0fd1294a1d..3c21fdbf71 100644
--- a/ICSharpCode.Decompiler/CSharp/CSharpDecompiler.cs
+++ b/ICSharpCode.Decompiler/CSharp/CSharpDecompiler.cs
@@ -1703,6 +1703,12 @@ EntityDeclaration DoDecompile(ITypeDefinition typeDef, DecompileRun decompileRun
 				{
 					RemoveAttribute(typeDecl, KnownAttribute.Required);
 				}
+				if (settings.ClosedHierarchies && RemoveAttribute(typeDecl, KnownAttribute.Closed))
+				{
+					// closed classes are implicitly abstract
+					typeDecl.Modifiers |= Modifiers.Closed;
+					typeDecl.Modifiers &= ~Modifiers.Abstract;
+				}
 				if (typeDecl.ClassType == ClassType.Enum)
 				{
 					Debug.Assert(typeDef.Kind == TypeKind.Enum);
@@ -2007,6 +2013,10 @@ EntityDeclaration DoDecompile(IMethod method, DecompileRun decompileRun, ITypeRe
 				{
 					RemoveObsoleteAttribute(methodDecl, "Constructors of types with required members are not supported in this version of your compiler.");
 				}
+				if (method.IsConstructor && settings.ClosedHierarchies)
+				{
+					RemoveCompilerFeatureRequiredAttribute(methodDecl, "ClosedClasses");
+				}
 				return methodDecl;
 
 				bool IsTypeHierarchyKnown(IType type)
diff --git a/ICSharpCode.Decompiler/CSharp/ProjectDecompiler/ProjectFileWriterDefault.cs b/ICSharpCode.Decompiler/CSharp/ProjectDecompiler/ProjectFileWriterDefault.cs
index 4447be86dd..8aeb2d409f 100644
--- a/ICSharpCode.Decompiler/CSharp/ProjectDecompiler/ProjectFileWriterDefault.cs
+++ b/ICSharpCode.Decompiler/CSharp/ProjectDecompiler/ProjectFileWriterDefault.cs
@@ -97,7 +97,10 @@ public void Write(
 				}
 
 				w.WriteElementString("OutputType", outputType);
-				w.WriteElementString("LangVersion", project.LanguageVersion.ToString().Replace("CSharp", "").Replace('_', '.'));
+				// C# 15 is still in preview; the compiler only accepts -langversion:preview for it.
+				w.WriteElementString("LangVersion", project.LanguageVersion >= LanguageVersion.CSharp15_0
+					? "preview"
+					: project.LanguageVersion.ToString().Replace("CSharp", "").Replace('_', '.'));
 				w.WriteElementString("CheckForOverflowUnderflow", project.CheckForOverflowUnderflow ? "true" : "false");
 
 				w.WriteElementString("AssemblyName", module.Name);
diff --git a/ICSharpCode.Decompiler/CSharp/ProjectDecompiler/ProjectFileWriterSdkStyle.cs b/ICSharpCode.Decompiler/CSharp/ProjectDecompiler/ProjectFileWriterSdkStyle.cs
index 0f79025246..aa21c67a42 100644
--- a/ICSharpCode.Decompiler/CSharp/ProjectDecompiler/ProjectFileWriterSdkStyle.cs
+++ b/ICSharpCode.Decompiler/CSharp/ProjectDecompiler/ProjectFileWriterSdkStyle.cs
@@ -190,7 +190,10 @@ static void WriteDesktopExtensions(XmlTextWriter xml, ProjectType projectType)
 
 		static void WriteProjectInfo(XmlTextWriter xml, IProjectInfoProvider project)
 		{
-			xml.WriteElementString("LangVersion", project.LanguageVersion.ToString().Replace("CSharp", "").Replace('_', '.'));
+			// C# 15 is still in preview; the compiler only accepts -langversion:preview for it.
+			xml.WriteElementString("LangVersion", project.LanguageVersion >= LanguageVersion.CSharp15_0
+				? "preview"
+				: project.LanguageVersion.ToString().Replace("CSharp", "").Replace('_', '.'));
 			xml.WriteElementString("AllowUnsafeBlocks", TrueString);
 			xml.WriteElementString("CheckForOverflowUnderflow", project.CheckForOverflowUnderflow ? TrueString : FalseString);
 
diff --git a/ICSharpCode.Decompiler/CSharp/RecordDecompiler.cs b/ICSharpCode.Decompiler/CSharp/RecordDecompiler.cs
index 84a80cc185..d62c6b8752 100644
--- a/ICSharpCode.Decompiler/CSharp/RecordDecompiler.cs
+++ b/ICSharpCode.Decompiler/CSharp/RecordDecompiler.cs
@@ -200,7 +200,7 @@ bool IsAutoSetter(IMethod method, [NotNullWhen(true)] out IField? field)
 				IMethod? chainedCtor = (IMethod?)FindChainedCtor(body)?.MemberDefinition;
 				ctorChainMap[method] = chainedCtor;
 
-				if (chainedCtor != null && chainedCtor.DeclaringTypeDefinition!.Equals(recordTypeDef))
+				if (chainedCtor != null && recordTypeDef.Equals(chainedCtor.DeclaringTypeDefinition))
 				{
 					continue;
 				}
@@ -233,7 +233,7 @@ bool IsAutoSetter(IMethod method, [NotNullWhen(true)] out IField? field)
 					// follow this rule.
 					// we don't have to check the full chain, because the C# compiler enforces that
 					// there are no loops in the ctor call graph.
-					if (target == null || !target.DeclaringTypeDefinition!.Equals(recordTypeDef))
+					if (target == null || !recordTypeDef.Equals(target.DeclaringTypeDefinition))
 					{
 						guessedPrimaryCtor = null;
 						break;
@@ -522,6 +522,12 @@ private bool IsAllowedAttribute(IAttribute attribute)
 			{
 				case "System.Runtime.CompilerServices.CompilerGeneratedAttribute":
 					return true;
+				case "System.Runtime.CompilerServices.CompilerFeatureRequiredAttribute":
+					// closed records carry CompilerFeatureRequired("ClosedClasses") on all constructors
+					return settings.ClosedHierarchies
+						&& attribute.FixedArguments.Length == 1
+						&& attribute.FixedArguments[0].Value is string feature
+						&& feature == "ClosedClasses";
 				default:
 					return false;
 			}
diff --git a/ICSharpCode.Decompiler/CSharp/Syntax/Modifiers.cs b/ICSharpCode.Decompiler/CSharp/Syntax/Modifiers.cs
index d286f21cf8..b9869dc685 100644
--- a/ICSharpCode.Decompiler/CSharp/Syntax/Modifiers.cs
+++ b/ICSharpCode.Decompiler/CSharp/Syntax/Modifiers.cs
@@ -59,6 +59,7 @@ public enum Modifiers
 		Async = 0x10000,
 		Ref = 0x20000,
 		Required = 0x40000,
+		Closed = 0x80000,
 
 		VisibilityMask = Private | Internal | Protected | Public,
 
@@ -76,7 +77,7 @@ public static class CSharpModifiers
 			Modifiers.Public, Modifiers.Private, Modifiers.Protected, Modifiers.Internal,
 			Modifiers.New,
 			Modifiers.Unsafe,
-			Modifiers.Static, Modifiers.Abstract, Modifiers.Virtual, Modifiers.Sealed, Modifiers.Override,
+			Modifiers.Static, Modifiers.Abstract, Modifiers.Virtual, Modifiers.Sealed, Modifiers.Closed, Modifiers.Override,
 			Modifiers.Required, Modifiers.Readonly, Modifiers.Volatile,
 			Modifiers.Ref,
 			Modifiers.Extern, Modifiers.Partial, Modifiers.Const,
@@ -126,6 +127,8 @@ public static string GetModifierName(Modifiers modifier)
 					return "ref";
 				case Modifiers.Required:
 					return "required";
+				case Modifiers.Closed:
+					return "closed";
 				case Modifiers.Any:
 					// even though it's used for pattern matching only, 'any' needs to be in this list to be usable in the AST
 					return "any";
diff --git a/ICSharpCode.Decompiler/CSharp/Transforms/EscapeInvalidIdentifiers.cs b/ICSharpCode.Decompiler/CSharp/Transforms/EscapeInvalidIdentifiers.cs
index 67b2dab919..61ee14271e 100644
--- a/ICSharpCode.Decompiler/CSharp/Transforms/EscapeInvalidIdentifiers.cs
+++ b/ICSharpCode.Decompiler/CSharp/Transforms/EscapeInvalidIdentifiers.cs
@@ -186,6 +186,7 @@ public class RemoveEmbeddedAttributes : DepthFirstAstVisitor, IAstTransform
 			"System.Runtime.CompilerServices.CompilerFeatureRequiredAttribute",
 			"System.Runtime.CompilerServices.RequiredMemberAttribute",
 			"System.Runtime.CompilerServices.IsExternalInit",
+			"System.Runtime.CompilerServices.ClosedAttribute",
 		};
 
 		public override void VisitTypeDeclaration(TypeDeclaration typeDeclaration)
diff --git a/ICSharpCode.Decompiler/DecompilerSettings.cs b/ICSharpCode.Decompiler/DecompilerSettings.cs
index 955003893e..3717e75e54 100644
--- a/ICSharpCode.Decompiler/DecompilerSettings.cs
+++ b/ICSharpCode.Decompiler/DecompilerSettings.cs
@@ -177,10 +177,16 @@ public void SetLanguageVersion(CSharp.LanguageVersion languageVersion)
 				extensionMembers = false;
 				firstClassSpanTypes = false;
 			}
+			if (languageVersion < CSharp.LanguageVersion.CSharp15_0)
+			{
+				closedHierarchies = false;
+			}
 		}
 
 		public CSharp.LanguageVersion GetMinimumRequiredVersion()
 		{
+			if (closedHierarchies)
+				return CSharp.LanguageVersion.CSharp15_0;
 			if (extensionMembers || firstClassSpanTypes)
 				return CSharp.LanguageVersion.CSharp14_0;
 			if (paramsCollections)
@@ -2194,6 +2200,24 @@ public bool ExtensionMembers {
 			}
 		}
 
+		bool closedHierarchies = true;
+
+		/// <summary>
+		/// Gets/Sets whether the closed modifier should be reconstructed on closed hierarchies.
+		/// </summary>
+		[Category("C# 15.0 / VS 2026")]
+		[Description("DecompilerSettings.ClosedHierarchies")]
+		public bool ClosedHierarchies {
+			get { return closedHierarchies; }
+			set {
+				if (closedHierarchies != value)
+				{
+					closedHierarchies = value;
+					OnPropertyChanged();
+				}
+			}
+		}
+
 		bool firstClassSpanTypes = true;
 
 		/// <summary>
diff --git a/ICSharpCode.Decompiler/TypeSystem/Implementation/KnownAttributes.cs b/ICSharpCode.Decompiler/TypeSystem/Implementation/KnownAttributes.cs
index 52a20c3375..227ae1afe1 100644
--- a/ICSharpCode.Decompiler/TypeSystem/Implementation/KnownAttributes.cs
+++ b/ICSharpCode.Decompiler/TypeSystem/Implementation/KnownAttributes.cs
@@ -120,11 +120,14 @@ public enum KnownAttribute
 
 		// C# 14 attributes:
 		ExtensionMarker,
+
+		// C# 15 attributes:
+		Closed,
 	}
 
 	public static class KnownAttributes
 	{
-		internal const int Count = (int)KnownAttribute.ExtensionMarker + 1;
+		internal const int Count = (int)KnownAttribute.Closed + 1;
 
 		static readonly TopLevelTypeName[] typeNames = new TopLevelTypeName[Count]{
 			default,
@@ -200,6 +203,8 @@ public static class KnownAttributes
 			new TopLevelTypeName("System.Runtime.CompilerServices", "InlineArrayAttribute"),
 			// C# 14 attributes:
 			new TopLevelTypeName("System.Runtime.CompilerServices", "ExtensionMarkerAttribute"),
+			// C# 15 attributes:
+			new TopLevelTypeName("System.Runtime.CompilerServices", "ClosedAttribute"),
 		};
 
 		public static ref readonly TopLevelTypeName GetTypeName(this KnownAttribute attr)
diff --git a/ILSpy/Properties/Resources.resx b/ILSpy/Properties/Resources.resx
index 7e34703366..609dfd1dda 100644
--- a/ILSpy/Properties/Resources.resx
+++ b/ILSpy/Properties/Resources.resx
@@ -360,6 +360,9 @@ Are you sure you want to continue?</value>
   <data name="DecompilerSettings.CheckedOperators" xml:space="preserve">
     <value>User-defined checked operators</value>
   </data>
+  <data name="DecompilerSettings.ClosedHierarchies" xml:space="preserve">
+    <value>Use the closed modifier on closed hierarchies</value>
+  </data>
   <data name="DecompilerSettings.CovariantReturns" xml:space="preserve">
     <value>Covariant return types</value>
   </data>