Merge pull request #4064 from icsharpcode/exp/ternary-chain-limit

Expand nested conditional operators back into if-else

ICSharpCode.Decompiler.Tests/ILPrettyTestRunner.cs

@@ -51,6 +51,12 @@ namespace ICSharpCode.Decompiler.Tests
 			}
 		}
 
+		[Test]
+		public async Task ConditionalChain()
+		{
+			await Run();
+		}
+
 		[Test]
 		public async Task AnonymousMethodEdgeCases()
 		{

ICSharpCode.Decompiler.Tests/PrettyTestRunner.cs

@@ -457,6 +457,12 @@ namespace ICSharpCode.Decompiler.Tests
 			await RunForLibrary(cscOptions: cscOptions);
 		}
 
+		[Test]
+		public async Task ConditionalOperatorNesting([ValueSource(nameof(roslynOnlyOptions))] CompilerOptions cscOptions)
+		{
+			await RunForLibrary(cscOptions: cscOptions);
+		}
+
 		[Test]
 		public async Task UnsafeCode([ValueSource(nameof(defaultOptions))] CompilerOptions cscOptions)
 		{

ICSharpCode.Decompiler.Tests/TestCases/ILPretty/ConditionalChain.cs

@@ -0,0 +1,27 @@
+using System;
+
+public class ConditionalChain
+{
+	public int Ladder(int n, int[] a, int[] b)
+	{
+		int num;
+		if (n > 40)
+		{
+			num = a[0];
+		}
+		else if (n > 30)
+		{
+			num = b[0];
+		}
+		else if (n > 20)
+		{
+			num = a[1];
+		}
+		else
+		{
+			num = ((n > 10) ? b[1] : a[2]);
+		}
+		Console.WriteLine(num);
+		return num;
+	}
+}

ICSharpCode.Decompiler.Tests/TestCases/ILPretty/ConditionalChain.il

@@ -0,0 +1,119 @@
+
+//  .NET IL Disassembler.  Version 10.0.11
+
+
+
+// Metadata version: v4.0.30319
+.assembly extern System.Runtime
+{
+  .publickeytoken = (B0 3F 5F 7F 11 D5 0A 3A )                         // .?_....:
+  .ver 4:0:0:0
+}
+.assembly extern System.Console
+{
+  .publickeytoken = (B0 3F 5F 7F 11 D5 0A 3A )                         // .?_....:
+  .ver 4:0:0:0
+}
+.assembly ConditionalChain
+{
+  .custom instance void [System.Runtime]System.Runtime.CompilerServices.CompilationRelaxationsAttribute::.ctor(int32) = ( 01 00 08 00 00 00 00 00 ) 
+  .custom instance void [System.Runtime]System.Runtime.CompilerServices.RuntimeCompatibilityAttribute::.ctor() = ( 01 00 01 00 54 02 16 57 72 61 70 4E 6F 6E 45 78   // ....T..WrapNonEx
+                                                                                                                   63 65 70 74 69 6F 6E 54 68 72 6F 77 73 01 )       // ceptionThrows.
+
+  // --- The following custom attribute is added automatically, do not uncomment -------
+  //  .custom instance void [System.Runtime]System.Diagnostics.DebuggableAttribute::.ctor(valuetype [System.Runtime]System.Diagnostics.DebuggableAttribute/DebuggingModes) = ( 01 00 07 01 00 00 00 00 ) 
+}
+.module ConditionalChain.dll
+// MVID: {db181304-d3cd-4187-9018-312550b7e28d}
+.custom instance void [System.Runtime]System.Runtime.CompilerServices.RefSafetyRulesAttribute::.ctor(int32) = ( 01 00 0B 00 00 00 00 00 ) 
+.imagebase 0x10000000
+.file alignment 0x00000200
+.stackreserve 0x00100000
+.subsystem 0x0003       // WINDOWS_CUI
+.corflags 0x00000001    //  ILONLY
+// Image base: 0x0x7fc5d693d000
+
+
+// =============== CLASS MEMBERS DECLARATION ===================
+
+.class public auto ansi beforefieldinit ConditionalChain
+       extends [System.Runtime]System.Object
+{
+  .method public hidebysig instance int32 
+          Ladder(int32 n,
+                 int32[] a,
+                 int32[] b) cil managed
+  {
+    // Code size       58 (0x3a)
+    .maxstack  2
+    .locals init (int32 V_0,
+             int32 V_1)
+    IL_0000:  nop
+    IL_0001:  ldarg.1
+    IL_0002:  ldc.i4.s   40
+    IL_0004:  bgt.s      IL_0029
+
+    IL_0006:  ldarg.1
+    IL_0007:  ldc.i4.s   30
+    IL_0009:  bgt.s      IL_0024
+
+    IL_000b:  ldarg.1
+    IL_000c:  ldc.i4.s   20
+    IL_000e:  bgt.s      IL_001f
+
+    IL_0010:  ldarg.1
+    IL_0011:  ldc.i4.s   10
+    IL_0013:  bgt.s      IL_001a
+
+    IL_0015:  ldarg.2
+    IL_0016:  ldc.i4.2
+    IL_0017:  ldelem.i4
+    IL_0018:  br.s       IL_001d
+
+    IL_001a:  ldarg.3
+    IL_001b:  ldc.i4.1
+    IL_001c:  ldelem.i4
+    IL_001d:  br.s       IL_0022
+
+    IL_001f:  ldarg.2
+    IL_0020:  ldc.i4.1
+    IL_0021:  ldelem.i4
+    IL_0022:  br.s       IL_0027
+
+    IL_0024:  ldarg.3
+    IL_0025:  ldc.i4.0
+    IL_0026:  ldelem.i4
+    IL_0027:  br.s       IL_002c
+
+    IL_0029:  ldarg.2
+    IL_002a:  ldc.i4.0
+    IL_002b:  ldelem.i4
+    IL_002c:  stloc.0
+    IL_002d:  ldloc.0
+    IL_002e:  call       void [System.Console]System.Console::WriteLine(int32)
+    IL_0033:  nop
+    IL_0034:  ldloc.0
+    IL_0035:  stloc.1
+    IL_0036:  br.s       IL_0038
+
+    IL_0038:  ldloc.1
+    IL_0039:  ret
+  } // end of method ConditionalChain::Ladder
+
+  .method public hidebysig specialname rtspecialname 
+          instance void  .ctor() cil managed
+  {
+    // Code size       8 (0x8)
+    .maxstack  8
+    IL_0000:  ldarg.0
+    IL_0001:  call       instance void [System.Runtime]System.Object::.ctor()
+    IL_0006:  nop
+    IL_0007:  ret
+  } // end of method ConditionalChain::.ctor
+
+} // end of class ConditionalChain
+
+
+// =============================================================
+
+// *********** DISASSEMBLY COMPLETE ***********************

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

@@ -0,0 +1,144 @@
+// Copyright (c) 2026 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.
+
+using System;
+using System.Collections.Generic;
+using System.Linq;
+
+namespace ICSharpCode.Decompiler.Tests.TestCases.Pretty
+{
+	public class ConditionalOperatorNesting
+	{
+		private static void Use(object value)
+		{
+		}
+
+		public int ShallowChainStaysInline(int n, int[] a, int[] b)
+		{
+			return ((n > 20) ? a[0] : b[0]) + 1;
+		}
+
+		// One conditional is under the limit and stays where it is, whatever the branch types.
+		public void MismatchedBranchTypesUnderTheLimitStayInline(int n, string s, object o)
+		{
+			Use((n > 10) ? s : o);
+		}
+
+		public void LogicOperatorsAreNotCounted(int n, int[] a)
+		{
+			Use(n > 40 && a[0] > 1 && a[1] > 2 && a[2] > 3 && a[3] > 4);
+		}
+
+		public int FilterKeepsExpression(int n, int[] a, int[] b)
+		{
+			try
+			{
+				throw new Exception();
+			}
+			catch (Exception) when (((n > 40) ? a[0] : ((n > 30) ? b[0] : ((n > 20) ? a[1] : ((n > 10) ? b[1] : a[2])))) > 5)
+			{
+				return 1;
+			}
+		}
+	}
+
+	public class ConditionalOperatorNestingBase
+	{
+		public ConditionalOperatorNestingBase(int x)
+		{
+		}
+	}
+
+	public class ConditionalOperatorNestingCtor : ConditionalOperatorNestingBase
+	{
+		public ConditionalOperatorNestingCtor(int n, int[] a, int[] b)
+			: base((n > 40) ? a[0] : ((n > 30) ? b[0] : ((n > 20) ? a[1] : ((n > 10) ? b[1] : a[2]))))
+		{
+		}
+
+		public ConditionalOperatorNestingCtor(int n, int[] a)
+			: this(n, a, a)
+		{
+		}
+	}
+
+#if CS90
+	public class ConditionalOperatorNestingInit
+	{
+		public int P { get; init; }
+	}
+
+	public record ConditionalOperatorNestingRecord
+	{
+		public int P { get; init; }
+	}
+#endif
+
+	// A nested conditional feeding a construct that has to stay a single expression must not be
+	// expanded: the transform that recognizes the construct matches on the expression, and an
+	// if-else between the statements silently stops it matching - or produces code that does not
+	// compile, as an object initializer assigning an init-only member would.
+	public class ConditionalOperatorNestingSingleExpression
+	{
+		public int[] ArrayInitializer(int n, int a, int b, int c)
+		{
+			return new int[2] {
+				(n > 2) ? a : ((n > 1) ? b : c),
+				5
+			};
+		}
+
+		public IEnumerable<int> Query(int[] xs, int n, int a, int b, int c)
+		{
+			return xs.Where((int x) => x > ((n > 2) ? a : ((n > 1) ? b : c)));
+		}
+#if CS70
+		public int RefLocal(int n, int[] a, int[] b)
+		{
+			ref int reference = ref n > 2 ? ref a[0] : ref n > 1 ? ref b[0] : ref a[1];
+			reference++;
+			return reference;
+		}
+#endif
+#if CS80
+		public int SwitchExpression(int k, int n, int a, int b, int c)
+		{
+			return k switch {
+				1 => (n > 2) ? a : ((n > 1) ? b : c),
+				2 => a,
+				_ => b,
+			};
+		}
+#endif
+#if CS90
+		public ConditionalOperatorNestingInit ObjectInitializer(int n, int a, int b, int c)
+		{
+			return new ConditionalOperatorNestingInit {
+				P = ((n > 2) ? a : ((n > 1) ? b : c))
+			};
+		}
+
+		public ConditionalOperatorNestingRecord WithExpression(ConditionalOperatorNestingRecord i, int n, int a, int b, int c)
+		{
+			return i with {
+				P = ((n > 2) ? a : ((n > 1) ? b : c))
+			};
+		}
+#endif
+	}
+}

ICSharpCode.Decompiler/CSharp/CSharpDecompiler.cs

@@ -174,6 +174,7 @@ namespace ICSharpCode.Decompiler.CSharp
 				new IntroduceDynamicTypeOnLocals(),
 				new IntroduceNativeIntTypeOnLocals(),
 				new IntroduceScopedModifierOnLocals(),
+				new ExpandNestedConditionals(),
 				new AssignVariableNames(),
 			};
 		}

ICSharpCode.Decompiler/IL/ILTypeExtensions.cs

@@ -163,6 +163,45 @@ namespace ICSharpCode.Decompiler.IL
 			}
 		}
 
+		/// <summary>
+		/// Infers the C# type an instruction expects of the child in <paramref name="childIndex"/>,
+		/// i.e. the counterpart to <see cref="InferType"/>: that one asks what a value is, this one
+		/// asks what the position it flows into says it should be.
+		///
+		/// Returns SpecialType.UnknownType where the position names nothing.
+		/// </summary>
+		/// <remarks>
+		/// Where a value's own type is only its stack type - `I4` being `int`, `bool`, `char` and
+		/// every enum at once - the consumer often still knows, because a parameter, a return type
+		/// or a field carries its type in metadata.
+		/// </remarks>
+		public static IType InferExpectedType(this ILInstruction inst, int childIndex, ICompilation? compilation)
+		{
+			switch (inst)
+			{
+				case CallInstruction call:
+					if (childIndex == 0 && call.IsInstanceCall)
+						return call.ConstrainedTo ?? call.Method.DeclaringType;
+					return call.GetParameter(childIndex)?.Type ?? SpecialType.UnknownType;
+				case Leave leave when childIndex == 0:
+					// the value of a leave is a return value only where it leaves the function body
+					var function = leave.Ancestors.OfType<ILFunction>().FirstOrDefault();
+					if (function == null || leave.TargetContainer != function.Body)
+						return SpecialType.UnknownType;
+					return function.Method?.ReturnType ?? SpecialType.UnknownType;
+				case StObj stobj when childIndex == 1:
+					return stobj.Type;
+				case StLoc stloc when childIndex == 0:
+					return stloc.Variable.Type;
+				case IfInstruction ifInst when childIndex == 0:
+					return compilation?.FindType(KnownTypeCode.Boolean) ?? SpecialType.UnknownType;
+				case NewArr newArr:
+					return compilation?.FindType(KnownTypeCode.Int32) ?? SpecialType.UnknownType;
+				default:
+					return SpecialType.UnknownType;
+			}
+		}
+
 		/// <summary>
 		/// Infers the C# type for an IL instruction.
 		/// 

ICSharpCode.Decompiler/IL/Transforms/ExpandNestedConditionals.cs

@@ -0,0 +1,222 @@
+// Copyright (c) 2026 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.
+
+
+using System.Collections.Generic;
+using System.Linq;
+
+using ICSharpCode.Decompiler.TypeSystem;
+
+namespace ICSharpCode.Decompiler.IL.Transforms
+{
+	/// <summary>
+	/// Expands a statement-level assignment of a nested conditional operator back into if-else.
+	///
+	/// `ExpressionTransforms.HandleConditionalOperator` collapses `if (c) a = x; else a = y;` into a
+	/// conditional operator, innermost first, and keeps going for as long as the chain does. A source
+	/// else-if ladder therefore comes back as one expression, however long it was: the sample in
+	/// issue #2027 decompiles to a single 2095-character statement.
+	///
+	/// This undoes the collapse past <see cref="MaxNesting"/> levels, so a statement keeps at most
+	/// that many conditional operators.
+	/// </summary>
+	/// <remarks>
+	/// This runs at the end of the pipeline, not inside ExpressionTransforms, because every transform
+	/// that needs its input to be a single expression has to see the collapsed form first: object and
+	/// collection initializers, `with`, switch expressions, interpolated string handlers, and the
+	/// query lambdas the C# stage later rewrites into clauses. Cutting the chain earlier leaves an
+	/// if-else between the statements they pattern-match on, and they silently stop matching.
+	///
+	/// Only a store to a declared local is expanded. Such a local carries its own type from metadata,
+	/// so the branches can be split without the result widening to the stack type - which is what
+	/// makes a separate type check unnecessary here.
+	/// </remarks>
+	public class ExpandNestedConditionals : IILTransform
+	{
+		/// <summary>
+		/// How many conditional operators a single statement may keep.
+		/// </summary>
+		const int MaxNesting = 1;
+
+		ILTransformContext context;
+
+		public void Run(ILFunction function, ILTransformContext context)
+		{
+			this.context = context;
+			if (context.Settings.AggressiveInlining)
+				return;
+			foreach (var f in function.Descendants.OfType<ILFunction>())
+			{
+				if (!IsExpandableFunction(f))
+					continue;
+				foreach (var block in f.Descendants.OfType<Block>().ToArray())
+				{
+					if (block.Ancestors.OfType<ILFunction>().FirstOrDefault() != f)
+						continue;
+					if (!IsExpandableBlock(block))
+						continue;
+					for (int i = 0; i < block.Instructions.Count; i++)
+					{
+						context.CancellationToken.ThrowIfCancellationRequested();
+						ExtractInto(block, i);
+						Expand(f, block, i);
+					}
+				}
+			}
+		}
+
+		/// <summary>
+		/// A lambda body may have to stay a single expression: the query-expression stage rewrites
+		/// one into a clause, and an expression tree is built from the expression itself. A
+		/// constructor is matched as a whole, so that its leading stores become field initializers
+		/// and a record's primary constructor is recognized.
+		/// </summary>
+		static bool IsExpandableFunction(ILFunction function)
+		{
+			return function.Kind is ILFunctionKind.TopLevelFunction or ILFunctionKind.LocalFunction
+				&& function.Method?.IsConstructor != true;
+		}
+
+		/// <summary>
+		/// Only plain control-flow blocks hold statements. The other block kinds are expressions
+		/// spelled as blocks - an initializer, a named-argument call - and an if-else inside one of
+		/// them is not a statement the C# stage can print.
+		/// </summary>
+		static bool IsExpandableBlock(Block block)
+		{
+			if (block.Kind != BlockKind.ControlFlow)
+				return false;
+			if (ILInlining.IsCatchWhenBlock(block))
+				return false;
+			// A branch body is a block hanging off the if, not off a container; it holds statements
+			// either way. Only a container's own layout reserves blocks for a loop or switch header.
+			if (block.Parent is not BlockContainer container)
+				return true;
+			return container.Kind switch {
+				// the entry point carries the loop condition or the switch value
+				ContainerKind.While or ContainerKind.Switch => block != container.EntryPoint,
+				// and for a for-loop the last block carries the increment
+				ContainerKind.For => block != container.EntryPoint
+					&& block != container.Blocks[container.Blocks.Count - 1],
+				ContainerKind.DoWhile => block != container.Blocks[container.Blocks.Count - 1],
+				_ => true,
+			};
+		}
+
+		/// <summary>
+		/// A conditional that is not stored to a variable - an argument, a return value, a
+		/// condition - has nothing to expand into. ILExtraction gives it one, vetting the move
+		/// through PrepareExtract so the order of evaluation is preserved.
+		/// </summary>
+		/// <remarks>
+		/// Extraction is only done where the position the value flows into names a type. The
+		/// temporary ILExtraction creates is typed from the stack type, and `I4` is `int`, `bool`,
+		/// `char` and every enum at once; the consumer knows better, because a parameter, a return
+		/// type or a field carries its type in metadata.
+		/// </remarks>
+		void ExtractInto(Block block, int pos)
+		{
+			if (block.Instructions[pos] is StLoc { Value: IfInstruction })
+				return;
+			foreach (var inst in block.Instructions[pos].Descendants.OfType<IfInstruction>().ToArray())
+			{
+				if (inst.Parent is StLoc || !ExceedsNesting(inst, MaxNesting))
+					continue;
+				var expected = inst.Parent?.InferExpectedType(inst.ChildIndex, context.TypeSystem);
+				if (expected == null || expected.Kind == TypeKind.Unknown
+					|| expected.GetStackType() != inst.ResultType)
+				{
+					continue;
+				}
+				context.Step("Extract nested conditional operator", inst);
+				var v = inst.Extract(context);
+				if (v != null)
+				{
+					v.Type = expected;
+					context.EndStep(block.Instructions[pos]);
+					return;
+				}
+				context.EndStep(inst);
+			}
+		}
+
+		void Expand(ILFunction function, Block block, int pos)
+		{
+			// Only one branch of a chain nests further, so the expansion walks down it. A chain is
+			// bounded only by the size of the method it came from, hence the loop over recursion.
+			var worklist = new Stack<(Block Block, int Pos)>();
+			worklist.Push((block, pos));
+			while (worklist.Count > 0)
+			{
+				var (current, index) = worklist.Pop();
+				if (current.Instructions[index] is not StLoc { Variable: var v, Value: IfInstruction ifInst } stloc)
+					continue;
+				if (v.Kind is not (VariableKind.Local or VariableKind.StackSlot) || v.Type is ByReferenceType)
+					continue;
+				if (ILInlining.IsInConstructorInitializer(function, stloc))
+					continue;
+				if (!ExceedsNesting(ifInst, MaxNesting))
+					continue;
+
+				context.Step("Expand nested conditional operator", stloc);
+				// HandleConditionalOperator built the conditional by negating the condition and
+				// swapping the branches; undoing that here keeps the source's own polarity, and
+				// makes the expansion the exact inverse of the collapse rather than an equivalent
+				// of it - without which a pretty-test fixture would never reach a fixed point.
+				var condition = ifInst.Condition;
+				var (trueValue, falseValue) = (ifInst.TrueInst, ifInst.FalseInst);
+				while (condition.MatchLogicNot(out var withoutNot))
+				{
+					condition = withoutNot;
+					(trueValue, falseValue) = (falseValue, trueValue);
+				}
+				// Each new store stands where its value did, so the statement keeps a sequence point
+				// of its own instead of inheriting the one the value already carries.
+				var trueStore = new StLoc(v, trueValue).WithILRange(trueValue);
+				var falseStore = new StLoc(v, falseValue).WithILRange(falseValue);
+				var trueBlock = new Block { Instructions = { trueStore } }.WithILRange(trueStore);
+				var falseBlock = new Block { Instructions = { falseStore } }.WithILRange(falseStore);
+				var expanded = new IfInstruction(condition, trueBlock, falseBlock);
+				expanded.AddILRange(ifInst);
+				expanded.AddILRange(stloc);
+				stloc.ReplaceWith(expanded);
+				context.EndStep(expanded);
+
+				// the branches are statements of their own now, so each may need expanding in turn
+				worklist.Push((trueBlock, 0));
+				worklist.Push((falseBlock, 0));
+			}
+		}
+
+		/// <summary>
+		/// Gets whether the branches of <paramref name="inst"/> nest conditional operators
+		/// <paramref name="depth"/> levels deep. The short-circuit logic operators share the
+		/// IfInstruction representation, but render as operator chains without visible nesting.
+		/// </summary>
+		static bool ExceedsNesting(ILInstruction inst, int depth)
+		{
+			if (inst is not IfInstruction ifInst)
+				return false;
+			if (ifInst.MatchLogicAnd(out _, out _) || ifInst.MatchLogicOr(out _, out _))
+				return false;
+			return depth <= 0
+				|| ExceedsNesting(ifInst.TrueInst, depth - 1)
+				|| ExceedsNesting(ifInst.FalseInst, depth - 1);
+		}
+	}
+}