Move Builder.GetParameterDisplayString into the RoslynExtensions class; refactor

This centralises the rendering logic plus simplifies the implementation to use `ITypeSymbol.WithNullableAnnotation` and hands the rendering off to the default `ToDisplayString` implementation

Author: simonmckenzie

AutomaticInterface/AutomaticInterface/Builder.cs

@@ -1,7 +1,6 @@
 using System;
 using System.Collections.Generic;
 using System.Linq;
-using System.Text;
 using Microsoft.CodeAnalysis;
 using Microsoft.CodeAnalysis.CSharp;
 using Microsoft.CodeAnalysis.CSharp.Syntax;
@@ -163,7 +162,13 @@ List<string> generatedInterfaceNames
 
         var paramResult = new HashSet<string>();
         method
-            .Parameters.Select(p => GetParameterDisplayString(p, codeGenerator.HasNullable))
+            .Parameters.Select(p =>
+                p.ToDisplayString(
+                    FullyQualifiedDisplayFormat,
+                    codeGenerator.HasNullable,
+                    generatedInterfaceNames
+                )
+            )
             .ToList()
             .ForEach(x => paramResult.Add(x));
 
@@ -234,45 +239,6 @@ private static bool IsNullable(ITypeSymbol typeSymbol)
         return false;
     }
 
-    private static string GetParameterDisplayString(
-        IParameterSymbol param,
-        bool nullableContextEnabled
-    )
-    {
-        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 and we're enabling the nullable context, we need to force the nullable annotation if there isn't one already
-        if (
-            param.HasExplicitDefaultValue
-            && param.ExplicitDefaultValue is null
-            && param.NullableAnnotation != NullableAnnotation.Annotated
-            && param.Type.IsReferenceType
-            && nullableContextEnabled
-        )
-        {
-            typeSb.Append('?');
-        }
-        return typeSb.Append(restSb).ToString();
-    }
-
     private static void AddEventsToInterface(
         List<ISymbol> members,
         InterfaceBuilder codeGenerator,

AutomaticInterface/AutomaticInterface/RoslynExtensions.cs

@@ -1,4 +1,5 @@
-using System.Collections.Generic;
+using System;
+using System.Collections.Generic;
 using System.Linq;
 using System.Text;
 using System.Text.RegularExpressions;
@@ -85,8 +86,36 @@ List<string> generatedInterfaceNames
         public static string ToDisplayString(
             this IParameterSymbol symbol,
             SymbolDisplayFormat displayFormat,
+            bool nullableContextEnabled,
             List<string> generatedInterfaceNames
-        ) => ToDisplayString((ISymbol)symbol, displayFormat, generatedInterfaceNames);
+        )
+        {
+            string? RenderTypeSymbolWithNullableAnnotation(SymbolDisplayPart part) =>
+                part.Symbol is ITypeSymbol typeSymbol
+                    ? typeSymbol
+                        .WithNullableAnnotation(NullableAnnotation.Annotated)
+                        .ToDisplayString(displayFormat)
+                    : null;
+
+            // Special case for reference parameters with default value null (e.g. string x = null) - the nullable
+            // context isn't applied automatically, so it must be forced explicitly
+            var forceNullableAnnotation =
+                nullableContextEnabled
+                && symbol
+                    is {
+                        Type.IsReferenceType: true,
+                        HasExplicitDefaultValue: true,
+                        ExplicitDefaultValue: null
+                    }
+                && symbol.NullableAnnotation != NullableAnnotation.Annotated;
+
+            return ToDisplayString(
+                symbol,
+                displayFormat,
+                generatedInterfaceNames,
+                forceNullableAnnotation ? RenderTypeSymbolWithNullableAnnotation : null
+            );
+        }
 
         public static string ToDisplayString(
             this ITypeSymbol symbol,
@@ -100,7 +129,8 @@ List<string> generatedInterfaceNames
         private static string ToDisplayString(
             this ISymbol symbol,
             SymbolDisplayFormat displayFormat,
-            List<string> generatedInterfaceNames
+            List<string> generatedInterfaceNames,
+            Func<SymbolDisplayPart, string?>? customRenderDisplayPart = null
         )
         {
             var displayStringBuilder = new StringBuilder();
@@ -122,7 +152,8 @@ List<string> generatedInterfaceNames
                 }
                 else
                 {
-                    displayStringBuilder.Append(part);
+                    var customRender = customRenderDisplayPart?.Invoke(part);
+                    displayStringBuilder.Append(customRender ?? part.ToString());
                 }
             }