为什么字符串列表未更新，而复杂 object 列表已更新

Question

Case 1 - I have a list of string, say

List<string> source = new List<string>();`
source.Add("one");
source.Add("two");
source.Add("three");

If i update the value for "two" using below code, it wont update the value.

Example -

source.ForEach(x => {if (x == "two"){x = "twenty";}});

Case 2 - But when I have a List<ComplexObject> obj = new List<ComplexObject>() with two property Key, DisplayText with value as

1,"One"
2, "two"

And I try to update using below code, the value will be updated

 obj.ForEach(x => {if (x.Key == 2){ x.DisplayText = "Update value for 2";}});

Though both are list but why is the behaviour different?

Answer 1

This problem refers to understanding how parameters works and what is delegate .

The "thing" you pass in ForEach method is function , which has parameter, so it called Action delegate. This mean, that this is simple function with one single parameter. So you need to remember, that "x"(your param name inside ForEach) is not exactly item, that placed inside your list, this is parameter, which have the same behavior with simple parameter of function.

In first case: string, as we know is reference type, but it has value type behavor, so you get no change of source value, because new variable created and chars inside string copied to chars of new variable.

In second case: classes` objects in C# have references to their fileds in heap, so when item of your list proces inside ForEach and pass object like parameter, ComplexObject' object copy all the references to new object. So it's also whole new variable, but references to fields remains the same

ForEach and various methods, which accept delegate works like simple loop with passing each item to this function(it's obviosly much complex, but for general understanding it`s ok).