c# - How to pass the value from child form to parent form? -

i have 2 forms called billingform(parent form) , searchproduct(child form).

billingform code

private void textboxproductno_keydown(object sender, keyeventargs e) {     if(e.keycode==keys.f9)     {         using(searchproduct sp=new searchproduct)         {             sp.showdialog(this);         }     } }  public void updatedtext(string fromchildform) {      textboxproduct.text=fromchildform; } 

searchproduct form code (child form)

private void datagridview1_keydown(object sender,keyeventargs e) {     if(e.keycode==keys.enter)     {         billingform bf=(billingform)this.owner;   //error appear here          bf.updatedtext("hello world");         this.close();     } } 

i getting error message.

an unhandled exception of type 'system.invalidcastexception' occurred in billingsoftware.exe
additional information: unable cast object of type 'billingform.mdiparent' type 'billingform.billingform

try pass parent constructor , use in variable

using(searchproduct sp = new searchproduct(this)) {     sp.showdialog(this); }  //in searchproduct class public billingform myparent {get; private set;}  public searchproduct(billingform parent) {     this.myparent = parent; }  private void datagridview1_keydown(object sender,keyeventargs e) {     if (e.keycode == keys.enter)     {         billingform bf = this.myparent;         bf.updatedtext("hello world");         this.close();     } }