From fb44ad24562c3ed2b24bc9dee8a919e71998d7ba Mon Sep 17 00:00:00 2001
From: Andreas Gullberg Larsen <andreas.larsen84@gmail.com>
Date: Sat, 1 Aug 2026 21:29:41 +0200
Subject: [PATCH] Describe Modular numeric format strings

---
 .../QuantityEmitter.cs                        | 13 +++++++++++--
 .../GeneratedQuantityTests.cs                 | 19 +++++++++++++++++++
 .../Metadata/QuantityDescriptor.cs            |  1 +
 3 files changed, 31 insertions(+), 2 deletions(-)

diff --git a/UnitsNet.Modular/UnitsNet.Modular.Generator/QuantityEmitter.cs b/UnitsNet.Modular/UnitsNet.Modular.Generator/QuantityEmitter.cs
index 01e0b7083b..809c7428f0 100644
--- a/UnitsNet.Modular/UnitsNet.Modular.Generator/QuantityEmitter.cs
+++ b/UnitsNet.Modular/UnitsNet.Modular.Generator/QuantityEmitter.cs
@@ -10,6 +10,10 @@ namespace UnitsNet.Modular.Generator;
 
 internal static class QuantityEmitter
 {
+    private const string NumericFormatStringSyntaxAttribute =
+        "global::System.Diagnostics.CodeAnalysis.StringSyntax(" +
+        "global::System.Diagnostics.CodeAnalysis.StringSyntaxAttribute.NumericFormat)";
+
     public static string Emit(
         QuantitySelection selection,
         IReadOnlyList<EmittedQuantityRelation> relationships,
@@ -222,8 +226,13 @@ public static string Emit(
         writer.AppendLine("    public override int GetHashCode() => BaseValue.GetHashCode();");
         writer.AppendLine("    public override string ToString() => ToString(null, null);");
         writer.AppendLine("    public string ToString(global::System.IFormatProvider? formatProvider) => ToString(null, formatProvider);");
-        writer.AppendLine("    public string ToString(string? format) => ToString(format, null);");
-        writer.AppendLine("    public string ToString(string? format, global::System.IFormatProvider? formatProvider) =>");
+        writer.AppendLine("    public string ToString(");
+        writer.Append("        [").Append(NumericFormatStringSyntaxAttribute).AppendLine("]");
+        writer.AppendLine("        string? format) => ToString(format, null);");
+        writer.AppendLine("    public string ToString(");
+        writer.Append("        [").Append(NumericFormatStringSyntaxAttribute).AppendLine("]");
+        writer.AppendLine("        string? format,");
+        writer.AppendLine("        global::System.IFormatProvider? formatProvider) =>");
         writer.AppendLine("        global::UnitsNet.Modular.SourceGen.QuantityOperations.Format(_value, Unit, format, formatProvider, Metadata);");
         writer.AppendLine();
         if (quantity.IsLogarithmic)
diff --git a/UnitsNet.Modular/UnitsNet.Modular.Tests/GeneratedQuantityTests.cs b/UnitsNet.Modular/UnitsNet.Modular.Tests/GeneratedQuantityTests.cs
index 2b46c2ecfc..bece36f27e 100644
--- a/UnitsNet.Modular/UnitsNet.Modular.Tests/GeneratedQuantityTests.cs
+++ b/UnitsNet.Modular/UnitsNet.Modular.Tests/GeneratedQuantityTests.cs
@@ -1,5 +1,7 @@
 // Licensed under MIT No Attribution, see LICENSE file at the root.
 
+using System.Diagnostics.CodeAnalysis;
+using System.Reflection;
 using System.Text.Json;
 using Fictional.Measurements;
 using UnitsNet;
@@ -10,6 +12,23 @@ namespace UnitsNet.Modular.Tests;
 
 public sealed class GeneratedQuantityTests
 {
+    [Fact]
+    public void FormattingApis_IdentifyNumericFormatSyntax()
+    {
+        MethodInfo quantityToString = typeof(Length).GetMethod(nameof(Length.ToString), [typeof(string)])!;
+        StringSyntaxAttribute? quantitySyntax = quantityToString
+            .GetParameters()[0]
+            .GetCustomAttribute<StringSyntaxAttribute>();
+
+        MethodInfo descriptorFormat = typeof(IQuantityDescriptor).GetMethod(nameof(IQuantityDescriptor.Format))!;
+        StringSyntaxAttribute? descriptorSyntax = descriptorFormat
+            .GetParameters()[1]
+            .GetCustomAttribute<StringSyntaxAttribute>();
+
+        Assert.Equal(StringSyntaxAttribute.NumericFormat, quantitySyntax?.Syntax);
+        Assert.Equal(StringSyntaxAttribute.NumericFormat, descriptorSyntax?.Syntax);
+    }
+
     [Fact]
     public void GeneratedRegistry_SystemTextJsonRoundTripsBuiltInAndCustomQuantities()
     {
diff --git a/UnitsNet.Modular/UnitsNet.Modular/Metadata/QuantityDescriptor.cs b/UnitsNet.Modular/UnitsNet.Modular/Metadata/QuantityDescriptor.cs
index b8e45d38a1..88c24ad51b 100644
--- a/UnitsNet.Modular/UnitsNet.Modular/Metadata/QuantityDescriptor.cs
+++ b/UnitsNet.Modular/UnitsNet.Modular/Metadata/QuantityDescriptor.cs
@@ -89,6 +89,7 @@ bool TryParse(
     /// <summary>Formats a generated quantity after validating its concrete type.</summary>
     string Format(
         IQuantity<double> quantity,
+        [System.Diagnostics.CodeAnalysis.StringSyntax(System.Diagnostics.CodeAnalysis.StringSyntaxAttribute.NumericFormat)]
         string? format = null,
         IFormatProvider? formatProvider = null);
 }