Fix #3382: Support compiler-generated throw-helper invocations in switch-expression implicit default-case.

ICSharpCode.Decompiler.Tests/TestCases/Pretty/SwitchExpressions.cs

@@ -17,6 +17,9 @@
 // DEALINGS IN THE SOFTWARE.
 
 using System;
+#if !ROSLYN4
+using System.Runtime.CompilerServices;
+#endif
 
 namespace ICSharpCode.Decompiler.Tests.TestCases.Pretty
 {
@@ -159,5 +162,15 @@ namespace ICSharpCode.Decompiler.Tests.TestCases.Pretty
 				_ => "default",
 			};
 		}
+		public static int Issue3382(StringComparison c)
+		{
+			return c switch {
+				StringComparison.Ordinal => 0,
+				StringComparison.OrdinalIgnoreCase => 1,
+#if !ROSLYN4
+				_ => throw new SwitchExpressionException(c),
+#endif
+			};
+		}
 	}
 }

ICSharpCode.Decompiler/CSharp/CSharpDecompiler.cs

@@ -106,6 +106,7 @@ namespace ICSharpCode.Decompiler.CSharp
 				new SwitchDetection(),
 				new SwitchOnStringTransform(),
 				new SwitchOnNullableTransform(),
+				new SwitchExpressionDefaultCaseTransform(),
 				new SplitVariables(), // split variables once again, because SwitchOnNullableTransform eliminates ldloca 
 				new IntroduceRefReadOnlyModifierOnLocals(),
 				new BlockILTransform { // per-block transforms

ICSharpCode.Decompiler/CSharp/ExpressionBuilder.cs

@@ -4031,10 +4031,13 @@ namespace ICSharpCode.Decompiler.CSharp
 				switchExpr.SwitchSections.Add(ses);
 			}
 
-			var defaultSES = new SwitchExpressionSection();
-			defaultSES.Pattern = new IdentifierExpression("_");
-			defaultSES.Body = TranslateSectionBody(defaultSection);
-			switchExpr.SwitchSections.Add(defaultSES);
+			if (!defaultSection.IsCompilerGeneratedDefaultSection)
+			{
+				var defaultSES = new SwitchExpressionSection();
+				defaultSES.Pattern = new IdentifierExpression("_");
+				defaultSES.Body = TranslateSectionBody(defaultSection);
+				switchExpr.SwitchSections.Add(defaultSES);
+			}
 
 			return switchExpr.WithILInstruction(inst).WithRR(new ResolveResult(resultType));
 

ICSharpCode.Decompiler/ICSharpCode.Decompiler.csproj

@@ -103,6 +103,7 @@
     <Compile Include="CSharp\Annotations.cs" />
     <Compile Include="CSharp\CallBuilder.cs" />
     <Compile Include="CSharp\CSharpLanguageVersion.cs" />
+    <Compile Include="IL\Transforms\SwitchExpressionDefaultCaseTransform.cs" />
     <Compile Include="CSharp\Syntax\Expressions\RecursivePatternExpression.cs" />
     <Compile Include="DecompilationProgress.cs" />
     <Compile Include="Disassembler\IEntityProcessor.cs" />

ICSharpCode.Decompiler/IL/Instructions/SwitchInstruction.cs

@@ -202,6 +202,12 @@ namespace ICSharpCode.Decompiler.IL
 		/// </summary>
 		public bool HasNullLabel { get; set; }
 
+		/// <summary>
+		/// If true, this section only contains a compiler-generated throw helper
+		/// used in a switch expression and will not be visible in the decompiled source code.
+		/// </summary>
+		public bool IsCompilerGeneratedDefaultSection { get; set; }
+
 		/// <summary>
 		/// The set of labels that cause execution to jump to this switch section.
 		/// </summary>

ICSharpCode.Decompiler/IL/Transforms/SwitchExpressionDefaultCaseTransform.cs

@@ -0,0 +1,130 @@
+// Copyright (c) 2025 Siegfried Pammer
+// 
+// Permission is hereby granted, free of charge, to any person obtaining a copy of this
+// software and associated documentation files (the "Software"), to deal in the Software
+// without restriction, including without limitation the rights to use, copy, modify, merge,
+// publish, distribute, sublicense, and/or sell copies of the Software, and to permit persons
+// to whom the Software is furnished to do so, subject to the following conditions:
+// 
+// The above copyright notice and this permission notice shall be included in all copies or
+// substantial portions of the Software.
+// 
+// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED,
+// INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR
+// PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE
+// FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR
+// OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER
+// DEALINGS IN THE SOFTWARE.
+
+#nullable enable
+
+using System;
+using System.Diagnostics.CodeAnalysis;
+using System.Linq;
+
+using ICSharpCode.Decompiler.TypeSystem;
+using ICSharpCode.Decompiler.Util;
+
+namespace ICSharpCode.Decompiler.IL.Transforms
+{
+	// call ThrowInvalidOperationException(...)
+	// leave IL_0000 (ldloc temp)
+	//
+	// -or-
+	//
+	// call ThrowSwitchExpressionException(...)
+	// leave IL_0000 (ldloc temp)
+	// 
+	// -to-
+	//
+	// throw(newobj SwitchExpressionException(...))
+	class SwitchExpressionDefaultCaseTransform : IILTransform
+	{
+		void IILTransform.Run(ILFunction function, ILTransformContext context)
+		{
+			IMethod? FindConstructor(string fullTypeName, params Type[] argumentTypes)
+			{
+				IType exceptionType = context.TypeSystem.FindType(new FullTypeName(fullTypeName));
+				var types = argumentTypes.SelectArray(context.TypeSystem.FindType);
+
+				foreach (var ctor in exceptionType.GetConstructors(m => !m.IsStatic && m.Parameters.Count == argumentTypes.Length))
+				{
+					bool found = true;
+					foreach (var pair in ctor.Parameters.Select(p => p.Type).Zip(types))
+					{
+						if (!NormalizeTypeVisitor.IgnoreNullability.EquivalentTypes(pair.Item1, pair.Item2))
+						{
+							found = false;
+							break;
+						}
+					}
+					if (found)
+						return ctor;
+				}
+
+				return null;
+			}
+
+			IMethod[] exceptionCtorTable = new IMethod[2];
+
+			exceptionCtorTable[0] = FindConstructor("System.InvalidOperationException")!;
+			exceptionCtorTable[1] = FindConstructor("System.Runtime.CompilerServices.SwitchExpressionException", typeof(object))!;
+
+			if (exceptionCtorTable[0] == null && exceptionCtorTable[1] == null)
+				return;
+
+			bool MatchThrowHelperCall(ILInstruction inst, [NotNullWhen(true)] out IMethod? exceptionCtor, out ILInstruction? value)
+			{
+				exceptionCtor = null;
+				value = null;
+				if (inst is not Call call)
+					return false;
+				if (call.Method.DeclaringType.FullName != "<PrivateImplementationDetails>")
+					return false;
+				switch (call.Arguments.Count)
+				{
+					case 0:
+						if (call.Method.Name != "ThrowInvalidOperationException")
+							return false;
+						exceptionCtor = exceptionCtorTable[0];
+						break;
+					case 1:
+						if (call.Method.Name != "ThrowSwitchExpressionException")
+							return false;
+						exceptionCtor = exceptionCtorTable[1];
+						value = call.Arguments[0];
+						break;
+					default:
+						return false;
+				}
+				return exceptionCtor != null;
+			}
+
+			foreach (var block in function.Descendants.OfType<Block>())
+			{
+				if (block.Parent is not BlockContainer container)
+					continue;
+				if (container.EntryPoint is not { IncomingEdgeCount: 1, Instructions: [SwitchInstruction inst] })
+					continue;
+				if (block.Instructions.Count != 2 || block.IncomingEdgeCount != 1 || !block.FinalInstruction.MatchNop())
+					continue;
+				var defaultSection = inst.GetDefaultSection();
+				if (defaultSection.Body != block && (defaultSection.Body is not Branch b || b.TargetBlock != block))
+					continue;
+				if (!MatchThrowHelperCall(block.Instructions[0], out IMethod? exceptionCtor, out ILInstruction? value))
+					continue;
+				if (block.Instructions[1] is not Leave { Value: LdLoc { Variable: { Kind: VariableKind.Local or VariableKind.StackSlot, LoadCount: 1, InitialValueIsInitialized: true } } })
+				{
+					continue;
+				}
+				context.Step("SwitchExpressionDefaultCaseTransform", block.Instructions[0]);
+				var newObj = new NewObj(exceptionCtor);
+				if (value != null)
+					newObj.Arguments.Add(value);
+				block.Instructions[0] = new Throw(newObj).WithILRange(block.Instructions[0]).WithILRange(block.Instructions[1]);
+				block.Instructions.RemoveAt(1);
+				defaultSection.IsCompilerGeneratedDefaultSection = true;
+			}
+		}
+	}
+}