Implement natvis 'na' modifier and $T substitution by SachinM123 · Pull Request #1612 · microsoft/MIEngine

SachinM123 · 2026-07-21T22:45:51Z

Add support for the natvis 'na' (no-address) modifier to strip MI's leading address prefix from string values. This requires tracking the format specifier through variable initialization and applying cleanup at value retrieval points.

Also implement template parameter substitution ($T1, $T2, etc.) within natvis brace expressions before format specifier extraction.

Add support for the natvis 'na' (no-address) modifier to strip MI's leading address prefix from string values. This requires tracking the format specifier through variable initialization and applying cleanup at value retrieval points. Also implement template parameter substitution ($T1, $T2, etc.) within natvis brace expressions before format specifier extraction.

gregg-miskelly · 2026-07-22T05:10:51Z

            TypeName = results.TryFindString("type");
            Value = results.TryFindString("value");
+            // Diagnostic: log raw tuple child value before any cleanup
+            _debuggedProcess.Logger?.WriteLine(LogLevel.Verbose, FormattableString.Invariant($"TupleCtor: name={Name}, exp={results.TryFindString("exp")}, type={TypeName}, format={_format}, formatHasNa={_formatHasNa}, rawValue={results.TryFindString("value")}"));


These should already be in the MI log. Is there value in keeping them?

gregg-miskelly · 2026-07-22T05:11:32Z

+            _debuggedProcess.Logger?.WriteLine(LogLevel.Verbose, FormattableString.Invariant($"TupleCtor: name={Name}, exp={results.TryFindString("exp")}, type={TypeName}, format={_format}, formatHasNa={_formatHasNa}, rawValue={results.TryFindString("value")}"));
+            // Only strip the leading MI address prefix ("0x... \"\"") when the natvis
+            // format included the 'na' modifier. 
+            if (_formatHasNa && !string.IsNullOrEmpty(Value) && Regex.IsMatch(Value, "^0x[0-9a-fA-F]+\\s+"))


"

You want a non-space character after the spaces

gregg-miskelly · 2026-07-22T05:12:37Z

+            _debuggedProcess.Logger?.WriteLine(LogLevel.Verbose, FormattableString.Invariant($"TupleCtor: name={Name}, exp={results.TryFindString("exp")}, type={TypeName}, format={_format}, formatHasNa={_formatHasNa}, rawValue={results.TryFindString("value")}"));
+            // Only strip the leading MI address prefix ("0x... \"\"") when the natvis
+            // format included the 'na' modifier. 
+            if (_formatHasNa && !string.IsNullOrEmpty(Value) && Regex.IsMatch(Value, "^0x[0-9a-fA-F]+\\s+"))


"

Since we will be using the Regex often, probably better to move it to a static variable. See s_addressPattern for an example, though I don't think you need to bother making this a Lazy

Copilot

Pull request overview

This PR extends MIEngine’s NatVis evaluator to (1) support the na (no-address) modifier by stripping MI’s leading 0x... address prefix from certain evaluated values, and (2) perform $T1, $T2, … template-parameter substitution inside NatVis brace expressions before parsing format specifiers.

Changes:

Add $Tn macro substitution over full {...} brace expressions in Natvis display-string formatting.
Detect and apply na address-prefix stripping during NatVis display-string formatting and during variable evaluation/initialization by tracking an _formatHasNa flag.
Update NatVis string “cleanup” helpers and related comments around address-prefix stripping behavior.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 4 comments.

File	Description
src/MIDebugEngine/Natvis.Impl/Natvis.cs	Adds `$Tn` substitution in brace expressions and introduces `na` detection/cleanup in display-string formatting; updates format-specifier helpers and string cleanup docs.
src/MIDebugEngine/Engine.Impl/Variables.cs	Tracks `na` in parsed format specifiers and attempts to strip MI’s address prefix at variable construction/evaluation time.

+                        bool hasNa = HasNaModifier(rawExpr);
                        string spec = ExtractFormatSpecifier(rawExpr);
                        string exprValue = GetExpressionValue(rawExpr, variable, scopedNames, intrinsics);


        internal static string CleanUtf16StringValue(string value)
        {
            if (string.IsNullOrEmpty(value)) return value;
            // Strip leading "0x<hex> " address prefix emitted by GDB/LLDB.
            value = s_addressPrefix.Replace(value, "");
-            // Strip surrounding u"..." or U"..." quotes.
-            if (value.Length >= 3 &&
-                (value.StartsWith("u\"", StringComparison.Ordinal) || value.StartsWith("U\"", StringComparison.Ordinal)))
-            {
-                value = value.EndsWith("\"", StringComparison.Ordinal)
-                    ? value.Substring(2, value.Length - 3)
-                    : value.Substring(2);
-            }
            return value;
        }


+        /// This method strips the leading address prefix that GDB/LLDB emits ("0x... ").
+        /// It does NOT remove surrounding quotes; callers should decide whether quotes
+        /// should be removed based on the caller's context.


            TypeName = results.TryFindString("type");
            Value = results.TryFindString("value");
+            // Diagnostic: log raw tuple child value before any cleanup
+            _debuggedProcess.Logger?.WriteLine(LogLevel.Verbose, FormattableString.Invariant($"TupleCtor: name={Name}, exp={results.TryFindString("exp")}, type={TypeName}, format={_format}, formatHasNa={_formatHasNa}, rawValue={results.TryFindString("value")}"));
+            // Only strip the leading MI address prefix ("0x... \"\"") when the natvis
+            // format included the 'na' modifier. 
+            if (_formatHasNa && !string.IsNullOrEmpty(Value) && Regex.IsMatch(Value, "^0x[0-9a-fA-F]+\\s+"))
+            {
+                // Recognize typical GDB prefix: 0x<hex> <string>
+                string before = Value;
+                Value = Regex.Replace(Value, "^0x[0-9a-fA-F]+\\s+", "");
+                _debuggedProcess.Logger?.WriteLine(LogLevel.Verbose, FormattableString.Invariant($"TupleCtor: stripped address prefix: before={before}, after={Value}"));
+            }


gregg-miskelly · 2026-07-22T05:14:05Z

+                // Recognize typical GDB prefix: 0x<hex> <string>
+                string before = Value;
+                Value = Regex.Replace(Value, "^0x[0-9a-fA-F]+\\s+", "");
+                _debuggedProcess.Logger?.WriteLine(LogLevel.Verbose, FormattableString.Invariant($"TupleCtor: stripped address prefix: before={before}, after={Value}"));


_debuggedProcess

Remove

gregg-miskelly · 2026-07-22T05:18:50Z

            string expFS = exp.Substring(lastComma + 1).Trim();
+            // Detect whether the natvis 'na' modifier is present in the original format specifier.
+            // We must detect this before we strip modifiers below.
+            _formatHasNa = expFS.IndexOf("na", StringComparison.Ordinal) >= 0;


_formatHasNa

The other outputs from this method are return as out params/return value, so I you want this as an out bool formatNa to go along them them.

gregg-miskelly · 2026-07-22T05:20:08Z

                        }
                        Value = results.TryFindString("value");
+                        // If natvis requested 'na', strip MI's leading address prefix
+                        if (_formatHasNa && !string.IsNullOrEmpty(Value) && Regex.IsMatch(Value, "^0x[0-9a-fA-F]+\\s+"))


"^0x[0-9a-fA-F]+\s+"

Same suggestions on the other usage of the Regex

Actually, better yet: can you extract this into a helper method so you don't repeat it three times?

gregg-miskelly · 2026-07-22T05:27:32Z

+                        // expression (this covers both the expression and any trailing format specifier)
+                        if (scopedNames != null)
+                        {
+                            rawExpr = Regex.Replace(rawExpr, "\\$T\\d+", (Match mt) =>


"\$T\d+"

nit: this would be easier to read as @"\$T\d+"

gregg-miskelly · 2026-07-22T05:32:07Z

+                                return mt.Value;
+                            });
+                        }
+                        bool hasNa = HasNaModifier(rawExpr);


bool hasNa = HasNaModifier(rawExpr);

Instead of duplicating the work of ExtractFormatSpecifier, have ExtractFormatSpecifier return this as an out param

gregg-miskelly · 2026-07-22T05:32:32Z

-                        if (spec == "sub" || spec == "su")
-                            exprValue = CleanUtf16StringValue(exprValue);
-                        else if (spec == "sb")
-                            exprValue = CleanAsciiStringValue(exprValue);


Did you mean to loose this sub/su/sb code?

Actually, is su needed? Variabel.cs line 477 already handles su correctly.

gregg-miskelly · 2026-07-22T05:42:41Z

-                value = value.EndsWith("\"", StringComparison.Ordinal)
-                    ? value.Substring(2, value.Length - 3)
-                    : value.Substring(2);
-            }


,sub is supposed to remove the quotes/prefix

Though this changes looks like it is correct for ,su

gregg-miskelly · 2026-07-22T05:47:13Z

+                                    return replacement;
+                                return mt.Value;
+                            });
+                        }


If I am reading the code correctly, this is already at least partially done by ReplaceNamesInExpression, which is called by GetExpressionValue, so I don't think you want to do this over the entire expression.

Can you trace through why ReplaceNamesInExpression isn't doing this already? (lines 1713-1717).

gregg-miskelly · 2026-07-22T05:50:46Z

Can you add a test for your changes? You may also need to adjust a test baseline for su since the code was doing the wrong thing there.

gregg-miskelly requested a review from Copilot July 22, 2026 05:07

Copilot started reviewing on behalf of gregg-miskelly July 22, 2026 05:08 View session

gregg-miskelly reviewed Jul 22, 2026

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Copilot AI reviewed Jul 22, 2026

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gregg-miskelly reviewed Jul 22, 2026

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SachinM123 commented Jul 21, 2026

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gregg-miskelly Jul 22, 2026 • edited Loading Uh oh! There was an error while loading. Please reload this page.

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Pull request overview

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gregg-miskelly Jul 22, 2026 • edited Loading Uh oh! There was an error while loading. Please reload this page.

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gregg-miskelly commented Jul 22, 2026

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gregg-miskelly Jul 22, 2026 •

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