fix: resolve CS8607 for ValueObject with nullable value type underlying

src/Vogen/GenerateCodeForHashCodes.cs

@@ -1,3 +1,5 @@
+using Analyzer.Utilities.Extensions;
+
 namespace Vogen;
 
 public static class GenerateCodeForHashCodes
@@ -11,6 +13,13 @@ public static string GenerateGetHashCodeForAClass(VoWorkItem item)
 
         string itemUnderlyingType = item.UnderlyingTypeFullNameWithGlobalAlias;
 
+        // When the underlying type is a nullable value type (e.g. ushort?), EqualityComparer<T>.Default.GetHashCode
+        // has [DisallowNull] on its parameter in .NET 10+, causing CS8607. Use null-safe approach instead.
+        // Wrap the expression in parentheses to ensure correct precedence with the ^ operator.
+        string hashCodeExpression = item.UnderlyingType.IsNullableValueType()
+            ? "(Value is null ? 0 : Value.GetHashCode())"
+            : $"global::System.Collections.Generic.EqualityComparer<{itemUnderlyingType}>.Default.GetHashCode(Value)";
+
         return
             $$"""
 
@@ -20,7 +29,7 @@ public static string GenerateGetHashCodeForAClass(VoWorkItem item)
                           {
                               global::System.Int32 hash = (global::System.Int32) 2166136261;
                               hash = (hash * 16777619) ^ GetType().GetHashCode();
-                              hash = (hash * 16777619) ^ global::System.Collections.Generic.EqualityComparer<{{itemUnderlyingType}}>.Default.GetHashCode(Value);
+                              hash = (hash * 16777619) ^ {{hashCodeExpression}};
                               return hash;
                           }
                         }
@@ -36,12 +45,18 @@ public static string GenerateForAStruct(VoWorkItem item)
 
         string itemUnderlyingType = item.UnderlyingTypeFullNameWithGlobalAlias;
 
+        // When the underlying type is a nullable value type (e.g. ushort?), EqualityComparer<T>.Default.GetHashCode
+        // has [DisallowNull] on its parameter in .NET 10+, causing CS8607. Use null-safe approach instead.
+        string hashCodeExpression = item.UnderlyingType.IsNullableValueType()
+            ? "Value is null ? 0 : Value.GetHashCode()"
+            : $"global::System.Collections.Generic.EqualityComparer<{itemUnderlyingType}>.Default.GetHashCode(Value)";
+
         return
             $$"""
 
                         public readonly override global::System.Int32 GetHashCode()
                         {
-                          return global::System.Collections.Generic.EqualityComparer<{{itemUnderlyingType}}>.Default.GetHashCode(Value);
+                          return {{hashCodeExpression}};
                         }
               """;
     }

tests/SnapshotTests/BugFixes/Bug914_NullableValueType_GetHashCode.cs

@@ -0,0 +1,40 @@
+using System.Threading.Tasks;
+using Vogen;
+
+namespace SnapshotTests.BugFixes;
+
+// See https://github.com/SteveDunn/Vogen/issues/914
+// ValueObject with a nullable value type underlying (e.g. ushort?) generated CS8607 because
+// EqualityComparer<T>.Default.GetHashCode has [DisallowNull] in .NET 10+.
+public class Bug914Tests
+{
+    [Theory]
+    [InlineData("partial struct")]
+    [InlineData("readonly partial struct")]
+    [InlineData("partial class")]
+    [InlineData("partial record class")]
+    [InlineData("partial record struct")]
+    [InlineData("readonly partial record struct")]
+    public async Task NullableValueType_underlying_generates_null_safe_GetHashCode(string type)
+    {
+        var source = $$"""
+                     using Vogen;
+
+                     namespace ConsumerTests;
+
+                     [ValueObject<ushort?>]
+                     internal {{type}} Speed
+                     {
+                         private const ushort UnknownSpeed = 255;
+
+                         private static ushort? NormalizeInput(ushort? input) =>
+                             input == UnknownSpeed ? null : input;
+                     }
+                     """;
+
+        await new SnapshotRunner<ValueObjectGenerator>()
+            .WithSource(source)
+            .CustomizeSettings(s => s.UseHashedParameters(type))
+            .RunOnAllFrameworks();
+    }
+}