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.

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)
 		{

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)
+		{
+		}
+	}
+}

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))