C# 3.0 模擬突變賦值：以擴充方法實現物件屬性批量指派

「突變賦值」概念源自動態語言，允許以單一運算式將多個屬性值批量指派給物件。C# 3.0 雖無原生語法支援，但結合匿名類型與擴充方法，可透過反射實現相近功能。

概念對比

傳統逐一指派

form.Text = "Hello World";
form.Top = 100;
form.Left = 200;

理想突變賦值語法（假想）

form := new { Text = "Hello World", Top = 100, Left = 200 };

C# 3.0 反射實作

using System.Reflection;

public static class MutantExtensions
{
    /// 

    /// 將來源匿名物件的屬性值複製到目標物件同名同型別屬性
    /// 
    public static void MAssign(this object target, object source)
    {
        if (target == null || source == null) return;
        
        Type targetType = target.GetType();
        Type sourceType = source.GetType();
        
        foreach (PropertyInfo srcProp in sourceType.GetProperties())
        {
            if (!srcProp.CanRead) continue;
            
            PropertyInfo targetProp = targetType.GetProperty(srcProp.Name, srcProp.PropertyType);
            if (targetProp == null || !targetProp.CanWrite) continue;
            
            object value = srcProp.GetValue(source, null);
            targetProp.SetValue(target, value, null);
        }
    }
}

使用範例

// WinForms 表單
Form form = new Form();
form.MAssign(new { Text = "Hello World", Top = 100, Left = 200 });

// 任意 POCO 物件
class Person { public string Name { get; set; } public int Age { get; set; } }
Person p = new Person();
p.MAssign(new { Name = "Tom", Age = 25 });

進階增強版本

public static class MutantExtensions
{
    public static void MAssign(this object target, object source, bool ignoreCase = false)
    {
        if (target == null || source == null) return;
        
        var bindingFlags = BindingFlags.Public | BindingFlags.Instance;
        var targetProps = target.GetType().GetProperties(bindingFlags)
            .ToDictionary(p => ignoreCase ? p.Name.ToLower() : p.Name, p => p);
        
        foreach (var srcProp in source.GetType().GetProperties(bindingFlags))
        {
            if (!srcProp.CanRead) continue;
            
            string key = ignoreCase ? srcProp.Name.ToLower() : srcProp.Name;
            if (!targetProps.TryGetValue(key, out var targetProp)) continue;
            if (!targetProp.CanWrite) continue;
            if (targetProp.PropertyType != srcProp.PropertyType) continue; // 嚴格型別匹配
            
            targetProp.SetValue(target, srcProp.GetValue(srcProp, null));
        }
    }
    
    // 僅複製指定屬性
    public static void MAssign(this object target, object source, params string[] propertyNames)
    {
        var props = source.GetType().GetProperties()
            .Where(p => propertyNames.Contains(p.Name, StringComparer.OrdinalIgnoreCase));
        
        foreach (var srcProp in props)
        {
            var targetProp = target.GetType().GetProperty(srcProp.Name, srcProp.PropertyType);
            if (targetProp?.CanWrite == true && srcProp.CanRead)
                targetProp.SetValue(target, srcProp.GetValue(source));
        }
    }
}

效能與限制考量

• 反射開銷：每次呼叫需列舉屬性、查找目標屬性，頻繁呼叫建議快取 PropertyInfo 或使用表達式樹編譯委派
• 型別安全：僅匹配名稱與型別完全相同的屬性，避免隱式轉換錯誤
• 唯讀屬性：目標屬性需可寫入（CanWrite），init 唯讀屬性（C# 9+）在建構後不可設定
• 結構體：傳入結構體會對複本操作，需用 ref 擴充方法（C# 7.2+ ref struct 不支援擴充）

現代替代方案

• AutoMapper：專業物件對物件映射庫，支援慣例配置、扁平化、投影
• C# 9+ record + with 運算式：var newPerson = person with { Name = "New" }; 建立新實例而非突變
• 原生物件初始化器：新建物件時首選 new Person { Name = "Tom", Age = 25 }

此反射擴充方法適合舊版專案維護、動態物件更新、測試資料構建等場景，新專案建議採用不可變模式與專業映射庫。