Fix #1602: Recognize Roslyn empty string case block.

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

@@ -1168,5 +1168,29 @@ namespace ICSharpCode.Decompiler.Tests.TestCases.Pretty
 					throw new Exception();
 			}
 		}
+
+		public static int Issue1602(string x)
+		{
+			switch (x) {
+				case null:
+					return 0;
+				case "":
+					return -1;
+				case "A":
+					return 65;
+				case "B":
+					return 66;
+				case "C":
+					return 67;
+				case "D":
+					return 68;
+				case "E":
+					return 69;
+				case "F":
+					return 70;
+				default:
+					throw new ArgumentOutOfRangeException();
+			}
+		}
 	}
 }

ICSharpCode.Decompiler/IL/Transforms/SwitchOnStringTransform.cs

@@ -820,8 +820,13 @@ namespace ICSharpCode.Decompiler.IL.Transforms
 				// extract target block
 				if (!section.Body.MatchBranch(out Block target))
 					return false;
-				if (!MatchRoslynCaseBlockHead(target, switchValueLoad.Variable, out ILInstruction targetOrLeave, out Block currentExitBlock, out string stringValue))
+				string stringValue;
+				if (MatchRoslynEmptyStringCaseBlockHead(target, switchValueLoad.Variable, out ILInstruction targetOrLeave, out Block currentExitBlock)) {
+					stringValue = "";
+				} else if (!MatchRoslynCaseBlockHead(target, switchValueLoad.Variable, out targetOrLeave, out currentExitBlock, out stringValue)) {
 					return false;
+				}
+
 				if (exitOrDefaultBlock != null && exitOrDefaultBlock != currentExitBlock)
 					return false;
 				exitOrDefaultBlock = currentExitBlock;
@@ -902,7 +907,7 @@ namespace ICSharpCode.Decompiler.IL.Transforms
 
 		/// <summary>
 		/// Matches (and the negated version):
-		/// if (call op_Equality(ldloc V_0, ldstr "Fifth case")) br body
+		/// if (call op_Equality(ldloc switchValueVar, stringValue)) br body
 		/// br exit
 		/// </summary>
 		bool MatchRoslynCaseBlockHead(Block target, ILVariable switchValueVar, out ILInstruction bodyOrLeave, out Block defaultOrExitBlock, out string stringValue)
@@ -938,6 +943,72 @@ namespace ICSharpCode.Decompiler.IL.Transforms
 			return false;
 		}
 
+		/// <summary>
+		/// Block target(incoming: 1) {
+		/// 	if (comp.o(ldloc switchValueVar == ldnull)) br exit
+		/// 	br lengthCheckBlock
+		/// }
+		/// 
+		/// Block lengthCheckBlock(incoming: 1) {
+		/// 	if (logic.not(call get_Length(ldloc switchValueVar))) br body
+		/// 	br exit
+		/// }
+		/// </summary>
+		bool MatchRoslynEmptyStringCaseBlockHead(Block target, ILVariable switchValueVar, out ILInstruction bodyOrLeave, out Block defaultOrExitBlock)
+		{
+			bodyOrLeave = null;
+			defaultOrExitBlock = null;
+			if (target.Instructions.Count != 2 || target.IncomingEdgeCount != 1)
+				return false;
+			if (!target.Instructions[0].MatchIfInstruction(out var nullComparisonCondition, out var exitBranch))
+				return false;
+			if (!nullComparisonCondition.MatchCompEqualsNull(out var arg) || !arg.MatchLdLoc(switchValueVar))
+				return false;
+			if (!target.Instructions[1].MatchBranch(out Block lengthCheckBlock))
+				return false;
+			if (lengthCheckBlock.Instructions.Count != 2 || lengthCheckBlock.IncomingEdgeCount != 1)
+				return false;
+			if (!lengthCheckBlock.Instructions[0].MatchIfInstruction(out var lengthCheckCondition, out var exitBranch2))
+				return false;
+			ILInstruction bodyBranch;
+			if (lengthCheckCondition.MatchLogicNot(out arg)) {
+				bodyBranch = exitBranch2;
+				exitBranch2 = lengthCheckBlock.Instructions[1];
+				lengthCheckCondition = arg;
+			} else {
+				bodyBranch = lengthCheckBlock.Instructions[1];
+			}
+			if (!(exitBranch2.MatchBranch(out defaultOrExitBlock) || exitBranch2.MatchLeave(out _)))
+				return false;
+			if (!MatchStringLengthCall(lengthCheckCondition, switchValueVar))
+				return false;
+			if (!(exitBranch.MatchBranch(out defaultOrExitBlock) && exitBranch2.MatchBranch(defaultOrExitBlock)))
+				return false;
+			if (bodyBranch.MatchLeave(out _)) {
+				bodyOrLeave = bodyBranch;
+				return true;
+			}
+			if (bodyBranch.MatchBranch(out var bodyBlock)) {
+				bodyOrLeave = bodyBlock;
+				return true;
+			}
+			return false;
+		}
+
+		/// <summary>
+		/// call get_Length(ldloc switchValueVar)
+		/// </summary>
+		bool MatchStringLengthCall(ILInstruction inst, ILVariable switchValueVar)
+		{
+			return inst is Call call
+				&& call.Method.DeclaringType.IsKnownType(KnownTypeCode.String)
+				&& call.Method.IsAccessor
+				&& call.Method.AccessorKind == System.Reflection.MethodSemanticsAttributes.Getter
+				&& call.Method.AccessorOwner.Name == "Length"
+				&& call.Arguments.Count == 1
+				&& call.Arguments[0].MatchLdLoc(switchValueVar);
+		}
+
 		/// <summary>
 		/// Matches 'stloc(targetVar, call ComputeStringHash(ldloc switchValue))'
 		/// </summary>