From 0416767c391ae705166956347c3a9752aac977f0 Mon Sep 17 00:00:00 2001
From: wuyangfan <yangfan.wu@succaiss.com>
Date: Mon, 25 May 2026 14:13:02 +0800
Subject: [PATCH] fix: use type default for optional DateTime constructor
 params (#954)

When an optional constructor parameter has a null/DBNull default value,
emit destinationType.CreateDefault() instead of Expression.Constant(null).

Co-authored-by: Cursor <cursoragent@cursor.com>
---
 ...ngRecordWithOptionalDateTimeConstructor.cs | 31 +++++++++++++++++++
 src/Mapster/Adapters/BaseClassAdapter.cs      | 27 +++++++---------
 2 files changed, 42 insertions(+), 16 deletions(-)
 create mode 100644 src/Mapster.Tests/WhenMappingRecordWithOptionalDateTimeConstructor.cs

diff --git a/src/Mapster.Tests/WhenMappingRecordWithOptionalDateTimeConstructor.cs b/src/Mapster.Tests/WhenMappingRecordWithOptionalDateTimeConstructor.cs
new file mode 100644
index 00000000..b772dbd7
--- /dev/null
+++ b/src/Mapster.Tests/WhenMappingRecordWithOptionalDateTimeConstructor.cs
@@ -0,0 +1,31 @@
+using System;
+using Microsoft.VisualStudio.TestTools.UnitTesting;
+using Shouldly;
+
+namespace Mapster.Tests
+{
+    [TestClass]
+    public class WhenMappingRecordWithOptionalDateTimeConstructor
+    {
+        [TestMethod]
+        public void Record_With_DateTime_Maps_To_Class_With_Optional_DateTime_Constructor()
+        {
+            var source = new DateTimeFoo(DateTime.Today);
+            var destination = source.Adapt<DateTimeFooDto>();
+
+            destination.Timestamp.ShouldBe(source.Timestamp);
+        }
+
+        public class DateTimeFooDto
+        {
+            public DateTime Timestamp { get; set; }
+
+            public DateTimeFooDto(DateTime timestamp = default)
+            {
+                Timestamp = timestamp;
+            }
+        }
+
+        public record DateTimeFoo(DateTime Timestamp);
+    }
+}
diff --git a/src/Mapster/Adapters/BaseClassAdapter.cs b/src/Mapster/Adapters/BaseClassAdapter.cs
index 6d12a934..57687d02 100644
--- a/src/Mapster/Adapters/BaseClassAdapter.cs
+++ b/src/Mapster/Adapters/BaseClassAdapter.cs
@@ -225,23 +225,9 @@ protected Expression CreateInstantiationExpression(Expression source, ClassMappi
                 Expression defaultConst;
                 Expression getter;
 
-#if NETSTANDARD2_0
-                try
-                {
-                    defaultConst = parameterInfo.IsOptional && parameterInfo.DefaultValue != null
-                    ? Expression.Constant(parameterInfo.DefaultValue, member.DestinationMember.Type)
-                    : parameterInfo.ParameterType.CreateDefault();
-                }
-                catch (FormatException)
-                {
-                    defaultConst = parameterInfo.ParameterType.CreateDefault();
-                }
-
-#else
-                defaultConst = parameterInfo.IsOptional && parameterInfo.DefaultValue != null
-                    ? Expression.Constant(parameterInfo.DefaultValue, member.DestinationMember.Type)
+                defaultConst = parameterInfo.IsOptional
+                    ? CreateOptionalParameterDefault(parameterInfo, member.DestinationMember.Type)
                     : parameterInfo.ParameterType.CreateDefault();
-#endif
                 
                 if (member.Getter == null)
                 {
@@ -289,6 +275,15 @@ protected Expression CreateInstantiationExpression(Expression source, ClassMappi
             return Expression.New(classConverter.ConstructorInfo!, arguments);
         }
 
+        static Expression CreateOptionalParameterDefault(ParameterInfo parameterInfo, Type destinationType)
+        {
+            var defaultValue = parameterInfo.DefaultValue;
+            if (defaultValue == null || defaultValue is DBNull)
+                return destinationType.CreateDefault();
+
+            return Expression.Constant(defaultValue, destinationType);
+        }
+
         protected virtual ClassModel GetConstructorModel(ConstructorInfo ctor, bool breakOnUnmatched)
         {
             return new ClassModel