Reduce allocation in Activity.{Parent}Id (#86685)

diff --git a/src/libraries/System.Diagnostics.DiagnosticSource/src/System/Diagnostics/Activity.cs b/src/libraries/System.Diagnostics.DiagnosticSource/src/System/Diagnostics/Activity.cs
index 68c3445..db4d5e2 100644 (file)
--- a/src/libraries/System.Diagnostics.DiagnosticSource/src/System/Diagnostics/Activity.cs
+++ b/src/libraries/System.Diagnostics.DiagnosticSource/src/System/Diagnostics/Activity.cs
@@ -225,8 +225,12 @@ namespace System.Diagnostics
                     // Convert flags to binary.
                     Span<char> flagsChars = stackalloc char[2];
                     HexConverter.ToCharsBuffer((byte)((~ActivityTraceFlagsIsSet) & _w3CIdFlags), flagsChars, 0, HexConverter.Casing.Lower);
-                    string id = "00-" + _traceId + "-" + _spanId + "-" + flagsChars.ToString();
-
+                    string id =
+#if NET6_0_OR_GREATER
+                        string.Create(null, stackalloc char[128], $"00-{_traceId}-{_spanId}-{flagsChars}");
+#else
+                        "00-" + _traceId + "-" + _spanId + "-" + flagsChars.ToString();
+#endif
                     Interlocked.CompareExchange(ref _id, id, null);
 
                 }
@@ -253,7 +257,12 @@ namespace System.Diagnostics
                     {
                         Span<char> flagsChars = stackalloc char[2];
                         HexConverter.ToCharsBuffer((byte)((~ActivityTraceFlagsIsSet) & _parentTraceFlags), flagsChars, 0, HexConverter.Casing.Lower);
-                        string parentId = "00-" + _traceId + "-" + _parentSpanId + "-" + flagsChars.ToString();
+                        string parentId =
+#if NET6_0_OR_GREATER
+                            string.Create(null, stackalloc char[128], $"00-{_traceId}-{_parentSpanId}-{flagsChars}");
+#else
+                            "00-" + _traceId + "-" + _parentSpanId + "-" + flagsChars.ToString();
+#endif
                         Interlocked.CompareExchange(ref _parentId, parentId, null);
                     }
                     else if (Parent != null)