Fix handling of parameters with default null

AutomaticInterface/AutomaticInterface/Builder.cs

@@ -1,6 +1,7 @@
 using System;
 using System.Collections.Generic;
 using System.Linq;
+using System.Text;
 using Microsoft.CodeAnalysis;
 using Microsoft.CodeAnalysis.CSharp;
 using Microsoft.CodeAnalysis.CSharp.Syntax;
@@ -155,7 +156,7 @@ private static void AddMethod(InterfaceBuilder codeGenerator, IMethodSymbol meth
 
         var paramResult = new HashSet<string>();
         method
-            .Parameters.Select(x => x.ToDisplayString(FullyQualifiedDisplayFormat))
+            .Parameters.Select(p => GetParameterDisplayString(p, codeGenerator.HasNullable))
             .ToList()
             .ForEach(x => paramResult.Add(x));
 
@@ -226,6 +227,45 @@ 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)
     {
         members

AutomaticInterface/Tests/Methods/Methods.WorksWithMixedOptionalNullParameters.verified.txt

@@ -0,0 +1,20 @@
+//--------------------------------------------------------------------------------------------------
+// <auto-generated>
+//     This code was generated by a tool.
+//
+//     Changes to this file may cause incorrect behavior and will be lost if the code is regenerated.
+// </auto-generated>
+//--------------------------------------------------------------------------------------------------
+
+#nullable enable
+namespace AutomaticInterfaceExample
+{
+    [global::System.CodeDom.Compiler.GeneratedCode("AutomaticInterface", "")]
+    public partial interface IDemoClass
+    {
+        /// <inheritdoc cref="AutomaticInterfaceExample.DemoClass.TryStartTransaction(int?, int, string)" />
+        bool TryStartTransaction(int? param, int param2 = 0, string? data = null);
+
+    }
+}
+#nullable restore

AutomaticInterface/Tests/Methods/Methods.cs

@@ -76,6 +76,29 @@ public bool TryStartTransaction(string data = null)
         await Verify(Infrastructure.GenerateCode(code));
     }
 
+    [Fact]
+    public async Task WorksWithMixedOptionalNullParameters()
+    {
+        const string code = """
+
+            using AutomaticInterface;
+
+            namespace AutomaticInterfaceExample;
+
+            [GenerateAutomaticInterface]
+            public class DemoClass
+            {
+                    public bool TryStartTransaction(int? param, int param2 = 0, string data = null)
+                    {
+            return true;
+            }
+            }
+
+
+            """;
+        await Verify(Infrastructure.GenerateCode(code));
+    }
+
     [Fact]
     public async Task AddsPublicMethodToInterface()
     {