src/NLog/Internal/StringBuilderExt.cs (2)

424-435: Prefer AppendSpan (no temp buffer) and avoid C# 12 collection expressions

The collection expression Span<char> buffer = [ ... ]; requires C# 12 and might break older LangVersion targets. Also, using AppendSpan(4) avoids the extra copy from a temporary span into StringBuilder.

Proposed replacement within this block:

-                Span<char> buffer =
-                [
-                    (char)(((number / 1000) % 10) + '0'),
-                    (char)(((number / 100) % 10) + '0'),
-                    (char)(((number / 10) % 10) + '0'),
-                    (char)((number % 10) + '0'),
-                ];
-                builder.Append(buffer); // Single Append instead of many
+                var dest = builder.AppendSpan(4);
+                dest[0] = (char)(((number / 1000) % 10) + '0');
+                dest[1] = (char)(((number / 100) % 10) + '0');
+                dest[2] = (char)(((number / 10) % 10) + '0');
+                dest[3] = (char)((number % 10) + '0');

If AppendSpan isn’t available for all targeted TFMs, alternatively use a non-C#12 form:

-                Span<char> buffer = [ /* digits */ ];
-                builder.Append(buffer);
+                Span<char> buffer = stackalloc char[4];
+                buffer[0] = (char)(((number / 1000) % 10) + '0');
+                buffer[1] = (char)(((number / 100) % 10) + '0');
+                buffer[2] = (char)(((number / 10) % 10) + '0');
+                buffer[3] = (char)((number % 10) + '0');
+                builder.Append(buffer);

---

415-443: Clarify input domain (expects 0..9999); negative values render incorrect digits

The fallback digit math will produce garbage for negatives (e.g., -1 yields '/000'). Current call sites seem to pass non-negative values (year, fraction), but make the contract explicit.

Consider adding a debug-time guard near the start of the method:

 internal static void Append4DigitsZeroPadded(this StringBuilder builder, int number)
 {
+    System.Diagnostics.Debug.Assert(number >= 0 && number < 10000, "Append4DigitsZeroPadded expects 0..9999");

Alternatively, document the assumption in XML comments if you want zero runtime impact in Release.

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