Fix recursive {this} evaluation in Natvis

This PR addresses the issue where using `this` in natvis causes it to
recursively evaluate. Added a check during evaluation if the name is
this, special case to just return as is instead of contuining recursing.

Added tests to handle the case.

Addresses #1391

Author: WardenGnaw

src/MIDebugEngine/Natvis.Impl/Natvis.cs

@@ -1439,6 +1439,13 @@ private string GetExpressionValue(string expression, IVariableInformation variab
             string processedExpr = ReplaceNamesInExpression(expression, variable, scopedNames);
             IVariableInformation expressionVariable = new VariableInformation(processedExpr, variable, _process.Engine, null);
             expressionVariable.SyncEval();
+
+            // Avoid recursive natvis formatting when expression is 'this'
+            if (expression.Trim() == "this")
+            {
+                return expressionVariable.Value;
+            }
+
             return FormatDisplayString(expressionVariable).value;
         }
 

test/CppTests/Tests/NatvisTests.cs

@@ -38,8 +38,8 @@ public NatvisTests(ITestOutputHelper outputHelper) : base(outputHelper)
         private const string NatvisSourceName = "main.cpp";
 
         // These line numbers will need to change if src/natvis/main.cpp changes
-        private const int SimpleClassAssignmentLine = 48;
-        private const int ReturnSourceLine = 57;
+        private const int SimpleClassAssignmentLine = 49;
+        private const int ReturnSourceLine = 61;
 
         [Theory]
         [RequiresTestSettings]
@@ -397,6 +397,47 @@ public void TestThisConditional(ITestSettings settings)
             }
         }
 
+        [Theory]
+        [DependsOnTest(nameof(CompileNatvisDebuggee))]
+        [RequiresTestSettings]
+        public void TestThisInDisplayString(ITestSettings settings)
+        {
+            this.TestPurpose("This test checks that {this} in a DisplayString value does not produce extra braces.");
+            this.WriteSettings(settings);
+
+            IDebuggee debuggee = Debuggee.Open(this, settings.CompilerSettings, NatvisName, DebuggeeMonikers.Natvis.Default);
+
+            using (IDebuggerRunner runner = CreateDebugAdapterRunner(settings))
+            {
+                this.Comment("Configure launch");
+                string visFile = Path.Join(debuggee.SourceRoot, "visualizer_files", "Simple.natvis");
+
+                LaunchCommand launch = new LaunchCommand(settings.DebuggerSettings, debuggee.OutputPath, visFile, false);
+                runner.RunCommand(launch);
+
+                this.Comment("Set Breakpoint");
+                SourceBreakpoints writerBreakpoints = debuggee.Breakpoints(NatvisSourceName, ReturnSourceLine);
+                runner.SetBreakpoints(writerBreakpoints);
+
+                runner.Expects.StoppedEvent(StoppedReason.Breakpoint).AfterConfigurationDone();
+
+                using (IThreadInspector threadInspector = runner.GetThreadInspector())
+                {
+                    IFrameInspector currentFrame = threadInspector.Stack.First();
+
+                    this.Comment("Verifying {this} in DisplayString does not recurse");
+                    var dpPtr = currentFrame.GetVariable("dpPtr");
+
+                    // Natvis: <DisplayString>{{ {this}={*this} }}</DisplayString>
+                    // Expected: "{ 0x<hex_address>=42 }" — {this} is the raw address, {*this} is the dereferenced value
+                    Assert.Matches(@"^\{ 0x[0-9a-fA-F]+=42 \}$", dpPtr.Value);
+                }
+
+                runner.Expects.ExitedEvent(exitCode: 0).TerminatedEvent().AfterContinue();
+                runner.DisconnectAndVerify();
+            }
+        }
+
         [Theory]
         [DependsOnTest(nameof(CompileNatvisDebuggee))]
         [RequiresTestSettings]

test/CppTests/debuggees/natvis/src/DataPoint.h

@@ -0,0 +1,5 @@
+struct DataPoint
+{
+    int value;
+    DataPoint(int v) : value(v) {}
+};

test/CppTests/debuggees/natvis/src/main.cpp

@@ -5,6 +5,7 @@
 #include "SimpleClass.h"
 #include "SimpleMatrix.h"
 #include "SimpleTemplated.h"
+#include "DataPoint.h"
 
 class SimpleDisplayObject
 {
@@ -54,5 +55,8 @@ int main(int argc, char** argv)
     SimpleMap<double, char, int> genericMap(2.71, 'x', 5);
     SimpleMap<int, double, int> intKeyMap(7, 1.618, 3);
 
+    DataPoint dp(42);
+    DataPoint *dpPtr = &dp;
+
     return 0;
 }

test/CppTests/debuggees/natvis/src/visualizer_files/Simple.natvis

@@ -97,4 +97,12 @@
     <DisplayString>{{ size={_size} (int-keyed) }}</DisplayString>
   </Type>
 
+  <Type Name="DataPoint">
+    <DisplayString>{value}</DisplayString>
+  </Type>
+
+  <Type Name="DataPoint *">
+    <DisplayString>{{ {this}={*this} }}</DisplayString>
+  </Type>
+
 </AutoVisualizer>