From ffe7e7b3f1e697fe0e6930fe61f2e961b98fa5ea Mon Sep 17 00:00:00 2001
From: DualFroz <me@dualfroz.com>
Date: Sat, 10 Oct 2026 13:03:54 +0200
Subject: [PATCH] Fix #3702: Don't rewrite GetValueOrDefault() to == true in ?.
 chains

Turning bool?.GetValueOrDefault() into `== true` ends an enclosing
null-conditional chain, so `c?.Flag.GetValueOrDefault()` became
`(c?.Flag == true)` of type bool instead of bool?. A following `?? false`
then fails to compile (CS0019), and a following `.ToString()` yields
"False" instead of null when c is null. Keep the call when its target is
part of a ?. chain.
---
 .../TestCases/Correctness/NullPropagation.cs  | 22 +++++++++++++++++++
 .../TestCases/Pretty/NullPropagation.cs       | 11 ++++++++++
 ICSharpCode.Decompiler/CSharp/CallBuilder.cs  |  4 +++-
 .../Transforms/IntroduceQueryExpressions.cs   |  2 +-
 4 files changed, 37 insertions(+), 2 deletions(-)

diff --git a/ICSharpCode.Decompiler.Tests/TestCases/Correctness/NullPropagation.cs b/ICSharpCode.Decompiler.Tests/TestCases/Correctness/NullPropagation.cs
index 90a83bd..14e31dd 100644
--- a/ICSharpCode.Decompiler.Tests/TestCases/Correctness/NullPropagation.cs
+++ b/ICSharpCode.Decompiler.Tests/TestCases/Correctness/NullPropagation.cs
@@ -7,6 +7,7 @@ namespace ICSharpCode.Decompiler.Tests.TestCases.Correctness
 		static void Main()
 		{
 			new NullPropagation().TestNotCoalescing();
+			new NullPropagation().TestNullableBoolInChain();
 #if CS72
 			new NullPropagation().TestRefStructNotCoalescing();
 #endif
@@ -15,6 +16,7 @@ namespace ICSharpCode.Decompiler.Tests.TestCases.Correctness
 		class MyClass
 		{
 			public string Text;
+			public bool? Flag;
 		}
 
 		void TestNotCoalescing()
@@ -29,6 +31,26 @@ namespace ICSharpCode.Decompiler.Tests.TestCases.Correctness
 			return c != null ? c.Text : "Hello";
 		}
 
+		void TestNullableBoolInChain()
+		{
+			Console.WriteLine("TestNullableBoolInChain:");
+			foreach (MyClass c in new[] { null, new MyClass(), new MyClass { Flag = false }, new MyClass { Flag = true } })
+			{
+				Console.WriteLine(EqualsTrue(c));
+				Console.WriteLine(GetValueOrDefaultToString(c) ?? "<null>");
+			}
+		}
+
+		bool EqualsTrue(MyClass c)
+		{
+			return c?.Flag == true;
+		}
+
+		string GetValueOrDefaultToString(MyClass c)
+		{
+			return c?.Flag.GetValueOrDefault().ToString();
+		}
+
 #if CS72
 		ref struct ByRefLikeStruct
 		{
diff --git a/ICSharpCode.Decompiler.Tests/TestCases/Pretty/NullPropagation.cs b/ICSharpCode.Decompiler.Tests/TestCases/Pretty/NullPropagation.cs
index 063dab6..c2f9201 100644
--- a/ICSharpCode.Decompiler.Tests/TestCases/Pretty/NullPropagation.cs
+++ b/ICSharpCode.Decompiler.Tests/TestCases/Pretty/NullPropagation.cs
@@ -32,6 +32,7 @@ namespace ICSharpCode.Decompiler.Tests.TestCases.Pretty
 			public readonly MyStruct ReadonlyStructField;
 			public string Text;
 			public MyClass Field;
+			public bool? NullableBool;
 			public MyClass Property { get; set; }
 			public MyClass this[int index] => null;
 			public MyClass Method(int arg)
@@ -380,5 +381,15 @@ namespace ICSharpCode.Decompiler.Tests.TestCases.Pretty
 			return x?.ToString();
 #endif
 		}
+
+		private static bool Issue3702(MyClass c)
+		{
+			return c?.NullableBool.GetValueOrDefault() ?? false;
+		}
+
+		private static string Issue3702ToString(MyClass c)
+		{
+			return c?.NullableBool.GetValueOrDefault().ToString();
+		}
 	}
 }
diff --git a/ICSharpCode.Decompiler/CSharp/CallBuilder.cs b/ICSharpCode.Decompiler/CSharp/CallBuilder.cs
index bad908d..0371d7a 100644
--- a/ICSharpCode.Decompiler/CSharp/CallBuilder.cs
+++ b/ICSharpCode.Decompiler/CSharp/CallBuilder.cs
@@ -26,6 +26,7 @@ using System.Text;
 
 using ICSharpCode.Decompiler.CSharp.Resolver;
 using ICSharpCode.Decompiler.CSharp.Syntax;
+using ICSharpCode.Decompiler.CSharp.Transforms;
 using ICSharpCode.Decompiler.IL;
 using ICSharpCode.Decompiler.IL.Transforms;
 using ICSharpCode.Decompiler.Semantics;
@@ -594,7 +595,8 @@ namespace ICSharpCode.Decompiler.CSharp
 			if (settings.LiftNullables && method.Name == "GetValueOrDefault"
 				&& method.DeclaringType.IsKnownType(KnownTypeCode.NullableOfT)
 				&& method.DeclaringType.TypeArguments[0].IsKnownType(KnownTypeCode.Boolean)
-				&& argumentList.Length == 0)
+				&& argumentList.Length == 0
+				&& !IntroduceQueryExpressions.IsNullConditional(target))
 			{
 				argumentList.CheckNoNamedOrOptionalArguments();
 				return new BinaryOperatorExpression(
diff --git a/ICSharpCode.Decompiler/CSharp/Transforms/IntroduceQueryExpressions.cs b/ICSharpCode.Decompiler/CSharp/Transforms/IntroduceQueryExpressions.cs
index c9c5064..57580b1 100644
--- a/ICSharpCode.Decompiler/CSharp/Transforms/IntroduceQueryExpressions.cs
+++ b/ICSharpCode.Decompiler/CSharp/Transforms/IntroduceQueryExpressions.cs
@@ -380,7 +380,7 @@ namespace ICSharpCode.Decompiler.CSharp.Transforms
 			}
 		}
 
-		bool IsNullConditional(Expression target) => target switch {
+		internal static bool IsNullConditional(Expression target) => target switch {
 			UnaryOperatorExpression { Operator: UnaryOperatorType.NullConditional } => true,
 			MemberReferenceExpression member => IsNullConditional(member.Target),
 			InvocationExpression invocation => IsNullConditional(invocation.Target),