增强 ExtendableConverter 对属性忽略条件的支持

支持 DefaultIgnoreCondition 及属性级 JsonIgnore(Condition)，序列化时可跳过默认值或空值属性，扩展字段也遵循全局忽略条件。新增 IsDefaultValue 方法，完善单元测试覆盖，优化 JsonDocument 释放。

diff --git a/NewLife.Core/Serialization/ExtendableConverter.cs b/NewLife.Core/Serialization/ExtendableConverter.cs
index acf36ae..affe131 100644
--- a/NewLife.Core/Serialization/ExtendableConverter.cs
+++ b/NewLife.Core/Serialization/ExtendableConverter.cs
@@ -12,6 +12,8 @@ namespace NewLife.Serialization;
 /// 序列化时先输出类型的公共属性，再将 <see cref="IExtend.Items"/> 中的扩展字段追加到同一 JSON 对象层级。
 /// 反序列化时按属性名匹配 JSON 字段并赋值给对应成员，无法匹配的多余字段写入 <see cref="IExtend.Items"/> 字典。
 /// 属性名匹配遵循 <see cref="JsonSerializerOptions.PropertyNamingPolicy"/> 与 <see cref="JsonSerializerOptions.PropertyNameCaseInsensitive"/> 配置。
+/// 序列化时遵循 <see cref="JsonSerializerOptions.DefaultIgnoreCondition"/> 及属性级 <see cref="JsonIgnoreAttribute"/> 条件，
+/// 正确跳过默认值或空值属性。
 /// </remarks>
 public class ExtendableConverter : JsonConverter<Object>
 {
@@ -30,7 +32,8 @@ public class ExtendableConverter : JsonConverter<Object>
         if (!typeof(IExtend).IsAssignableFrom(typeToConvert))
             return JsonSerializer.Deserialize(ref reader, typeToConvert, options)!;
 
-        var jsonRoot = JsonDocument.ParseValue(ref reader).RootElement;
+        using var jsonDoc = JsonDocument.ParseValue(ref reader);
+        var jsonRoot = jsonDoc.RootElement;
         var obj = Activator.CreateInstance(typeToConvert);
         if (obj is IExtend extendable)
         {
@@ -61,21 +64,53 @@ public class ExtendableConverter : JsonConverter<Object>
 
         writer.WriteStartObject();
 
+        var ignoreCondition = options.DefaultIgnoreCondition;
         var propMap = SerialHelper.GetJsonPropertyMap(value.GetType(), options);
         foreach (var (jsonName, prop) in propMap)
         {
             if (!prop.CanRead) continue;
+
+            var propValue = prop.GetValue(value);
+
+            // 属性级 [JsonIgnore(Condition=...)] 优先于全局 DefaultIgnoreCondition
+            var attrCondition = prop.GetCustomAttribute<JsonIgnoreAttribute>()?.Condition;
+            var condition = attrCondition is JsonIgnoreCondition.WhenWritingNull or JsonIgnoreCondition.WhenWritingDefault
+                ? attrCondition.Value
+                : ignoreCondition;
+
+            if (condition == JsonIgnoreCondition.WhenWritingNull && propValue is null) continue;
+            if (condition == JsonIgnoreCondition.WhenWritingDefault && IsDefaultValue(propValue, prop.PropertyType)) continue;
+
             writer.WritePropertyName(jsonName);
-            JsonSerializer.Serialize(writer, prop.GetValue(value), prop.PropertyType, options);
+            JsonSerializer.Serialize(writer, propValue, prop.PropertyType, options);
         }
 
-        foreach (var kvp in extendable.Items)
+        var items = extendable.Items;
+        if (items != null)
         {
-            writer.WritePropertyName(kvp.Key);
-            JsonSerializer.Serialize(writer, kvp.Value, typeof(Object), options);
+            foreach (var kvp in items)
+            {
+                // 扩展字段也遵循全局忽略条件，Object 类型的默认值为 null
+                if (ignoreCondition is JsonIgnoreCondition.WhenWritingNull or JsonIgnoreCondition.WhenWritingDefault && kvp.Value is null) continue;
+
+                writer.WritePropertyName(kvp.Key);
+                JsonSerializer.Serialize(writer, kvp.Value, typeof(Object), options);
+            }
         }
 
         writer.WriteEndObject();
     }
+
+    /// <summary>判断值是否为类型的默认值</summary>
+    /// <param name="value">属性值</param>
+    /// <param name="type">属性声明类型</param>
+    /// <returns></returns>
+    private static Boolean IsDefaultValue(Object? value, Type type)
+    {
+        if (value is null) return true;
+        if (!type.IsValueType) return false;
+
+        return value.Equals(Activator.CreateInstance(type));
+    }
 }
 #endif
\ No newline at end of file

diff --git a/XUnitTest.Core/Serialization/ExtendableConverterTests.cs b/XUnitTest.Core/Serialization/ExtendableConverterTests.cs
index 11df48d..7ef3511 100644
--- a/XUnitTest.Core/Serialization/ExtendableConverterTests.cs
+++ b/XUnitTest.Core/Serialization/ExtendableConverterTests.cs
@@ -1,5 +1,6 @@
 using System.Text;
 using System.Text.Json;
+using System.Text.Json.Serialization;
 using NewLife.Data;
 using NewLife.Serialization;
 using Xunit;
@@ -139,6 +140,70 @@ public class ExtendableConverterTests
         Assert.Contains("\"Address\":null", json.Replace(" ", ""));
     }
 
+    [Fact(DisplayName = "Write WhenWritingDefault 跳过 null 和默认值属性")]
+    public void Write_WhenWritingDefault_SkipsDefaultValues()
+    {
+        var opts = new JsonSerializerOptions
+        {
+            DefaultIgnoreCondition = JsonIgnoreCondition.WhenWritingDefault,
+            Converters = { new ExtendableConverter() }
+        };
+
+        // Name 为空字符串（非 null，不是默认值）、Age 为 0（默认值）、Address 为 null（默认值）
+        var model = new ComplexModel { Name = "Bob", Address = null };
+        model.Items["extra"] = "value";
+        model.Items["empty"] = null;
+
+        var json = JsonSerializer.Serialize(model, opts);
+
+        // Name 有值应保留
+        Assert.Contains("\"Name\"", json);
+        // Address 为 null（引用类型默认值）应跳过
+        Assert.DoesNotContain("\"Address\"", json);
+        // Items 中有值的保留
+        Assert.Contains("\"extra\"", json);
+        // Items 中 null 值应跳过
+        Assert.DoesNotContain("\"empty\"", json);
+    }
+
+    [Fact(DisplayName = "Write WhenWritingDefault 跳过值类型默认值")]
+    public void Write_WhenWritingDefault_SkipsValueTypeDefaults()
+    {
+        var opts = new JsonSerializerOptions
+        {
+            DefaultIgnoreCondition = JsonIgnoreCondition.WhenWritingDefault,
+            Converters = { new ExtendableConverter() }
+        };
+
+        var model = new SimpleModel { Name = "Alice", Age = 0 };
+
+        var json = JsonSerializer.Serialize(model, opts);
+
+        // Name 有值应保留
+        Assert.Contains("\"Name\"", json);
+        // Age 为 0（Int32 默认值）应跳过
+        Assert.DoesNotContain("\"Age\"", json);
+    }
+
+    [Fact(DisplayName = "Write WhenWritingNull 仅跳过 null 不跳过值类型默认值")]
+    public void Write_WhenWritingNull_SkipsOnlyNull()
+    {
+        var opts = new JsonSerializerOptions
+        {
+            DefaultIgnoreCondition = JsonIgnoreCondition.WhenWritingNull,
+            Converters = { new ExtendableConverter() }
+        };
+
+        var model = new ComplexModel { Name = "Bob", Address = null };
+
+        var json = JsonSerializer.Serialize(model, opts);
+
+        // Name 有值应保留
+        Assert.Contains("\"Name\"", json);
+        // Address 为 null 应跳过
+        Assert.DoesNotContain("\"Address\"", json);
+    }
+
     [Fact(DisplayName = "Write 遵循 PropertyNamingPolicy 命名策略")]
     public void Write_NamingPolicy_Applied()
     {