Use DisplayParts instead of regex

Author: paramamue

AutomaticInterface/AutomaticInterface/Builder.cs

@@ -1,7 +1,7 @@
 using System;
 using System.Collections.Generic;
 using System.Linq;
-using System.Text.RegularExpressions;
+using System.Text;
 using Microsoft.CodeAnalysis;
 using Microsoft.CodeAnalysis.CSharp;
 using Microsoft.CodeAnalysis.CSharp.Syntax;
@@ -10,9 +10,6 @@ namespace AutomaticInterface;
 
 public static class Builder
 {
-    private static Regex ParamWithDefaultValueRegex { get; } =
-    new(pattern: @"^(?<type>\S+)(?<rest>\s+\S+\s+=.+)$", options: RegexOptions.Compiled);
-
     private static string InheritDoc(ISymbol source) =>
         $"/// <inheritdoc cref=\"{source.ToDisplayString().Replace('<', '{').Replace('>', '}')}\" />"; // we use inherit doc because that should be able to fetch documentation from base classes.
 
@@ -159,19 +156,7 @@ private static void AddMethod(InterfaceBuilder codeGenerator, IMethodSymbol meth
 
         var paramResult = new HashSet<string>();
         method
-            .Parameters.Select(x =>
-            {
-                var dispString = x.ToDisplayString(FullyQualifiedDisplayFormat);
-                if (x.HasExplicitDefaultValue && x.ExplicitDefaultValue is null && x.NullableAnnotation != NullableAnnotation.Annotated)
-                {
-                    var match = ParamWithDefaultValueRegex.Match(dispString);
-                    if (match.Success)
-                    {
-                        dispString = $"{match.Groups["type"].Value}?{match.Groups["rest"].Value}";
-                    }
-                }
-                return dispString;
-            })
+            .Parameters.Select(GetParameterDisplayString)
             .ToList()
             .ForEach(x => paramResult.Add(x));
 
@@ -242,6 +227,35 @@ private static bool IsNullable(ITypeSymbol typeSymbol)
         return false;
     }
 
+    private static string GetParameterDisplayString(IParameterSymbol param)
+    {
+        var paramParts = param.ToDisplayParts(FullyQualifiedDisplayFormat);
+        var typeSb = new StringBuilder();
+        var restSb = new StringBuilder();
+        var isInsideType = true;
+        // The part before the first space is the parameter type
+        foreach (var part in paramParts)
+        {
+            if (isInsideType && part.Kind == SymbolDisplayPartKind.Space)
+            {
+                isInsideType = false;
+            }
+            if (isInsideType)
+            {
+                typeSb.Append(part.ToString());
+            } else
+            {
+                restSb.Append(part.ToString());
+            }
+        }
+        // If this parameter has default value null, we need to force the nullable annotation
+        if (param.HasExplicitDefaultValue && param.ExplicitDefaultValue is null && param.NullableAnnotation != NullableAnnotation.Annotated)
+        {
+            typeSb.Append('?');
+        }
+        return typeSb.Append(restSb).ToString();
+    }
+
     private static void AddEventsToInterface(List<ISymbol> members, InterfaceBuilder codeGenerator)
     {
         members