c# - Why does the model binder need an empty constructor -

i need some fundamentals here...

i have controller serves view instance of class (at least that's how think works). since giving view new instance of object, why have create newer 1 model binding post back? please @ below example.

[httpget] public actionresult index(){   int hi = 5;   string temp = "yo";   mymodel foo = new mymodel(hi, temp);   return view(foo); } [httppost]  public actionresult index(mymodel foo){   mymodel poo = foo;   if(poo.somestring == laaaa)     return redircttoaction("end", "endcntrl", poo);   else     throw new exception(); }  view: @model myapp.models.mymodel  @html.editorfor(m => m.hi)  <input type="submit" value="hit"/>  model: public class mymodel {  public int hi {get; set;}  public string somestring {get; set;}  public  stuff(int number, string laaaa){   numberforclass = number;   somestring = laaaa;  } } 

why need blank constructor? furthermore, if had parameterless constructor, why poo.somestring change time got redircttoaction("end", "endcntrl", poo)?

why need blank constructor?

because of

[httppost]  public actionresult index(mymodel foo){ ... } 

you asked binder give concrete instance on post, binder needs create object you. original object not persist between , post actions, (some of) properties live on html fields. thats "http stateless" means.

it becomes little more obvious when use lower level

[httppost]  public actionresult index(formcollection collection) {        var foo = new mymodel();       // load properties formcollection } 

why poo.somestring change time got redircttoaction("end", "endcntrl", poo)?

because somestring isn't used in view. blank when new model back. change that:

@model myapp.models.mymodel     @html.hiddenfor(m => m.somestring)   @html.editorfor(m => m.hi)  <input type="submit" value="hit"/> 

this store value hidden field in html , restored on post.