[cdac] Fix getting free object size in GetObjectData (#106505)

Missed that free objects also have their component count set - explicitly at the array base offset for num components. This updates the cDAC implementation of `ISOSDacInterface::GetObjectData` to correctly handle free objects with a non-zero component count.

Author: elinor-fung

src/coreclr/debug/daccess/request.cpp

@@ -2632,6 +2632,7 @@ DWORD DACGetNumComponents(TADDR addr, ICorDebugDataTarget* target)
     // This expects that the first member after the MethodTable pointer (from Object)
     // is a 32-bit integer representing the number of components.
     // This holds for ArrayBase and StringObject - see coreclr/vm/object.h
+    // Free objects also have a component count set at this offset- see SetFree in coreclr/gc/gc.cpp
     addr += sizeof(size_t); // Method table pointer
     ULONG32 returned = 0;
     DWORD Value = 0;

src/native/managed/cdacreader/src/Contracts/Object_1.cs

@@ -52,6 +52,9 @@ string IObject.GetStringValue(TargetPointer address)
             throw new ArgumentException("Address does not represent a string object", nameof(address));
 
         Data.String str = _target.ProcessedData.GetOrAdd<Data.String>(address);
+        if (str.StringLength == 0)
+            return string.Empty;
+
         Span<byte> span = stackalloc byte[(int)str.StringLength * sizeof(char)];
         _target.ReadBuffer(str.FirstChar, span);
         return new string(MemoryMarshal.Cast<byte, char>(span));
@@ -85,7 +88,7 @@ public TargetPointer GetArrayData(TargetPointer address, out uint count, out Tar
         else
         {
             // Single-dimensional, zero-based - doesn't have bounds
-            boundsStart = address + (ulong)arrayTypeInfo.Fields["m_NumComponents"].Offset;
+            boundsStart = address + (ulong)arrayTypeInfo.Fields[Data.Array.FieldNames.NumComponents].Offset;
             lowerBounds = _target.ReadGlobalPointer(Constants.Globals.ArrayBoundsZero);
         }
 

src/native/managed/cdacreader/src/Data/Array.cs

@@ -12,8 +12,13 @@ public Array(Target target, TargetPointer address)
     {
         Target.TypeInfo type = target.GetTypeInfo(DataType.Array);
 
-        NumComponents = target.Read<uint>(address + (ulong)type.Fields["m_NumComponents"].Offset);
+        NumComponents = target.Read<uint>(address + (ulong)type.Fields[FieldNames.NumComponents].Offset);
     }
 
     public uint NumComponents { get; init; }
+
+    internal static class FieldNames
+    {
+        internal const string NumComponents = $"m_{nameof(NumComponents)}";
+    }
 }

src/native/managed/cdacreader/src/Legacy/SOSDacImpl.cs

@@ -408,6 +408,12 @@ public unsafe int GetObjectData(ulong objAddr, DacpObjectData* data)
             if (runtimeTypeSystemContract.IsFreeObjectMethodTable(handle))
             {
                 data->ObjectType = DacpObjectType.OBJ_FREE;
+
+                // Free objects have their component count explicitly set at the same offset as that for arrays
+                // Update the size to include those components
+                Target.TypeInfo arrayTypeInfo = _target.GetTypeInfo(DataType.Array);
+                ulong numComponentsOffset = (ulong)_target.GetTypeInfo(DataType.Array).Fields[Data.Array.FieldNames.NumComponents].Offset;
+                data->Size += _target.Read<uint>(objAddr + numComponentsOffset) * data->dwComponentSize;
             }
             else if (mt == _stringMethodTable)
             {