C#中物件copy前後比對常見錯誤

- 9月 15, 2022

有時可能會將一些表單資料封裝成DTO或者來自EF某張表的record entity
剛好又涉及到要寫複製資料功能進行局部欄位資料異動時候
就會時常發生連動到舊物件資料的問題

有問題寫法

using System;
class Person
{
    public string Name { get; set; }
    public int Age { get; set; }
}

public class Program
{
	public static void Main()
	{
		Person person1 = new Person();
		person1.Age=23;
		person1.Name="Amy";
		Console.WriteLine(String.Format("person1 name:{0} / Age:{1}",person1.Name,person1.Age));
		Person person2 = person1;
		person2.Name = "Ted";
		Console.WriteLine(String.Format("person1 name:{0} / Age:{1}",person1.Name,person1.Age));
		Console.WriteLine(String.Format("person2 name:{0} / Age:{1}",person2.Name,person2.Age));
		Console.ReadLine(); 
	}
}

會觀察到物件複製寫法不對只有異動person2的Name也連動到person1的Name

第一種修正法.重新new並指定欄位

using System;
class Person
{
    public string Name { get; set; }
    public int Age { get; set; }
}

public class Program
{
	public static void Main()
	{
		Person person1 = new Person();
		person1.Age=23;
		person1.Name="Amy";
		Console.WriteLine(String.Format("person1 name:{0} / Age:{1}",person1.Name,person1.Age));
		//Person person2 = person1;
		//第一種.
		Person person2 = new Person(){Name=person1.Name,Age=person1.Age};
		person2.Name = "Ted";
		Console.WriteLine(String.Format("person1 name:{0} / Age:{1}",person1.Name,person1.Age));
		Console.WriteLine(String.Format("person2 name:{0} / Age:{1}",person2.Name,person2.Age));
		Console.ReadLine(); 
	}
}

第二種修正法.透過反射

using System;
using System.Reflection;
class Person
{
    public string Name { get; set; }
    public int Age { get; set; }
}

public class Program
{
	public static void Main()
	{
		Person person1 = new Person();
		person1.Age=23;
		person1.Name="Amy";
		Console.WriteLine(String.Format("person1 name:{0} / Age:{1}",person1.Name,person1.Age));
		//Person person2 = person1;
		//第一種.
		//Person person2 = new Person(){Name=person1.Name,Age=person1.Age};
		
		Type myType = person1.GetType();
		Person person2 = (Person)System.Activator.CreateInstance(myType);
		PropertyInfo[] properties = myType.GetProperties();
		foreach (var property in properties)
        {
            if (property.CanWrite)
            {
                property.SetValue(person2, person1.GetType().GetProperty(property.Name).GetValue(person1, null), null);
            }
        }

		person2.Name = "Ted";
		Console.WriteLine(String.Format("person1 name:{0} / Age:{1}",person1.Name,person1.Age));
		Console.WriteLine(String.Format("person2 name:{0} / Age:{1}",person2.Name,person2.Age));
		Console.ReadLine(); 
	}
}

更省code寫法就是包成extension method

using System;
using System.Collections.Generic;
using System.Reflection;
using System.Text;

namespace Common.CustomExtension
{
    public static class DeepCopyExtension
    {
        public static T GetDeepCopy<T>(this T value)
        {
            Type myType = value.GetType();
            T copyObj = (T)System.Activator.CreateInstance(myType);
            PropertyInfo[] properties = myType.GetProperties();
            foreach (var property in properties)
            {
                if (property.CanWrite)
                {
                    property.SetValue(copyObj, value.GetType().GetProperty(property.Name).GetValue(value, null), null);
                }
            }
            return copyObj;
        }
    }
}

之後只要使用.GetDeepCopy()就可了

Person person2 = person1.GetDeepCopy();

person2.Name = "Ted";

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C#中物件copy前後比對常見錯誤

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