From 898c2a3f9f2aca103d29bf57b14924622b83d80e Mon Sep 17 00:00:00 2001
From: ds5678 <49847914+ds5678@users.noreply.github.com>
Date: Sat, 11 Jul 2026 13:54:32 -0700
Subject: [PATCH] Add Issue3877 test and handle negative dict capacity

Added Issue3877 test to PrettyTestRunner and new test case source to verify dictionary initialization with negative capacity. Updated SwitchOnStringTransform to skip processing when a negative dictionary capacity is detected.
---
 .../PrettyTestRunner.cs                       |  6 ++
 .../TestCases/Pretty/Issue3877.cs             | 65 +++++++++++++++++++
 .../IL/Transforms/SwitchOnStringTransform.cs  |  2 +
 3 files changed, 73 insertions(+)
 create mode 100644 ICSharpCode.Decompiler.Tests/TestCases/Pretty/Issue3877.cs

diff --git a/ICSharpCode.Decompiler.Tests/PrettyTestRunner.cs b/ICSharpCode.Decompiler.Tests/PrettyTestRunner.cs
index 707b1d1a6..e70b58e32 100644
--- a/ICSharpCode.Decompiler.Tests/PrettyTestRunner.cs
+++ b/ICSharpCode.Decompiler.Tests/PrettyTestRunner.cs
@@ -836,6 +836,12 @@ namespace ICSharpCode.Decompiler.Tests
 			await RunForLibrary(cscOptions: cscOptions);
 		}
 
+		[Test]
+		public async Task Issue3877([ValueSource(nameof(roslyn2OrNewerWithNet40Options))] CompilerOptions cscOptions)
+		{
+			await RunForLibrary(cscOptions: cscOptions);
+		}
+
 		[Test]
 		public async Task AssemblyCustomAttributes([ValueSource(nameof(defaultOptions))] CompilerOptions cscOptions)
 		{
diff --git a/ICSharpCode.Decompiler.Tests/TestCases/Pretty/Issue3877.cs b/ICSharpCode.Decompiler.Tests/TestCases/Pretty/Issue3877.cs
new file mode 100644
index 000000000..517de87cc
--- /dev/null
+++ b/ICSharpCode.Decompiler.Tests/TestCases/Pretty/Issue3877.cs
@@ -0,0 +1,65 @@
+using System.Collections.Generic;
+using System.Runtime.CompilerServices;
+
+namespace ICSharpCode.Decompiler.Tests.TestCases.Pretty
+{
+	internal static class Issue3877
+	{
+		[CompilerGenerated]
+		private static Dictionary<string, int> backingField;
+
+		public static void M(string s)
+		{
+			N(0);
+			if (s != null)
+			{
+				if (backingField == null)
+				{
+#if OPT
+					backingField = new Dictionary<string, int>(-87) {
+						{ "string0", 0 },
+						{ "string1", 1 },
+						{ "string2", 2 },
+						{ "string3", 3 },
+						{ "string4", 4 }
+					};
+#else
+					Dictionary<string, int> dictionary = new Dictionary<string, int>(-87);
+					dictionary.Add("string0", 0);
+					dictionary.Add("string1", 1);
+					dictionary.Add("string2", 2);
+					dictionary.Add("string3", 3);
+					dictionary.Add("string4", 4);
+					backingField = dictionary;
+#endif
+				}
+				if (backingField.TryGetValue(s, out var value))
+				{
+					switch (value)
+					{
+						case 0:
+							N(10);
+							break;
+						case 1:
+							N(20);
+							break;
+						case 2:
+							N(30);
+							break;
+						case 3:
+							N(40);
+							break;
+						case 4:
+							N(50);
+							break;
+					}
+				}
+			}
+			N(60);
+		}
+
+		public static void N(int i)
+		{
+		}
+	}
+}
diff --git a/ICSharpCode.Decompiler/IL/Transforms/SwitchOnStringTransform.cs b/ICSharpCode.Decompiler/IL/Transforms/SwitchOnStringTransform.cs
index de0407eb3..df75cb5e4 100644
--- a/ICSharpCode.Decompiler/IL/Transforms/SwitchOnStringTransform.cs
+++ b/ICSharpCode.Decompiler/IL/Transforms/SwitchOnStringTransform.cs
@@ -818,6 +818,8 @@ namespace ICSharpCode.Decompiler.IL.Transforms
 				if (!newObj.Arguments[0].MatchLdcI4(out valuesLength))
 					return false;
 			}
+			if (valuesLength < 0)
+				return false;
 			values = new List<(string, int)>(valuesLength);
 			int i = 0;
 			while (MatchAddCall(dictionaryType, block.Instructions[i + 1], dictVar, out var index, out var value))