Validate uses of display-class variable copies before removing them

ICSharpCode.Decompiler/IL/Transforms/TransformDisplayClassUsage.cs

@@ -170,18 +170,42 @@ namespace ICSharpCode.Decompiler.IL.Transforms
 					}
 				}
 			}
+		}
+
+		bool ValidateDisplayClassUses(ILVariable v, DisplayClass displayClass, bool readOnly = false)
+		{
+			foreach (var ldloc in v.LoadInstructions) {
+				if (!ValidateUse(displayClass, ldloc))
+					return false;
+			}
+			foreach (var ldloca in v.AddressInstructions) {
+				if (!ValidateUse(displayClass, ldloca))
+					return false;
+			}
+			return true;
 
-			bool ValidateDisplayClassUses(ILVariable v, DisplayClass displayClass)
+			bool ValidateUse(DisplayClass container, ILInstruction use)
 			{
-				foreach (var ldloc in v.LoadInstructions) {
-					if (!ValidateUse(displayClass, ldloc))
-						return false;
-				}
-				foreach (var ldloca in v.AddressInstructions) {
-					if (!ValidateUse(displayClass, ldloca))
+				IField field;
+				switch (use.Parent) {
+					case LdFlda ldflda when ldflda.MatchLdFlda(out _, out field):
+						var keyField = (IField)field.MemberDefinition;
+						if (!container.VariablesToDeclare.TryGetValue(keyField, out VariableToDeclare variable) || variable == null) {
+							if (readOnly)
+								return false;
+							variable = AddVariable(container, null, field);
+						}
+						container.VariablesToDeclare[keyField] = variable;
+						return variable != null;
+					case StObj stobj when stobj.MatchStObj(out var target, out ILInstruction value, out _) && value == use:
+						if (target.MatchLdFlda(out var load, out field) && load.MatchLdLocRef(out var otherVariable) && displayClasses.TryGetValue(otherVariable, out var otherDisplayClass)) {
+							if (otherDisplayClass.VariablesToDeclare.TryGetValue((IField)field.MemberDefinition, out var declaredVar))
+								return declaredVar.CanPropagate;
+						}
+						return true;
+					default:
 						return false;
 				}
-				return true;
 			}
 		}
 
@@ -285,8 +309,7 @@ namespace ICSharpCode.Decompiler.IL.Transforms
 		private Block FindDisplayStructInitBlock(ILVariable v)
 		{
 			var root = v.Function.Body;
-			var block = Visit(root)?.Ancestors.OfType<Block>().FirstOrDefault();
-			return block;
+			return Visit(root)?.Ancestors.OfType<Block>().FirstOrDefault();
 
 			ILInstruction Visit(ILInstruction inst)
 			{
@@ -417,28 +440,6 @@ namespace ICSharpCode.Decompiler.IL.Transforms
 			return variable;
 		}
 
-		bool ValidateUse(DisplayClass container, ILInstruction use)
-		{
-			IField field;
-			switch (use.Parent) {
-				case LdFlda ldflda when ldflda.MatchLdFlda(out _, out field):
-					var keyField = (IField)field.MemberDefinition;
-					if (!container.VariablesToDeclare.TryGetValue(keyField, out VariableToDeclare variable) || variable == null) {
-						variable = AddVariable(container, null, field);
-					}
-					container.VariablesToDeclare[keyField] = variable;
-					return variable != null;
-				case StObj stobj when stobj.MatchStObj(out var target, out ILInstruction value, out _) && value == use:
-					if (target.MatchLdFlda(out var load, out field) && load.MatchLdLocRef(out var otherVariable) && displayClasses.TryGetValue(otherVariable, out var displayClass)) {
-						if (displayClass.VariablesToDeclare.TryGetValue((IField)field.MemberDefinition, out var declaredVar))
-							return declaredVar.CanPropagate;
-					}
-					return true;
-				default:
-					return false;
-			}
-		}
-
 		private void Transform(ILFunction function)
 		{
 			VisitILFunction(function);
@@ -630,8 +631,7 @@ namespace ICSharpCode.Decompiler.IL.Transforms
 					}
 					return;
 				}
-				// TODO : this is dangerous!
-				if (inst.Value.MatchLdLocRef(out var otherVariable) && displayClasses.TryGetValue(otherVariable, out displayClass)) {
+				if (inst.Value.MatchLdLocRef(out var otherVariable) && displayClasses.TryGetValue(otherVariable, out displayClass) && ValidateDisplayClassUses(inst.Variable, displayClass)) {
 					instructionsToRemove.Add(inst);
 					displayClasses.Add(inst.Variable, displayClass);
 				}