Question

How do we call a form1 class event from another class present in form1.cs?

public partial class form1 : form
{
// an event to change the text of a textbox
}

public class A
{
sendtext()
{
//text to be sent to the texbox
// the text is created as a output of a thread
}

Answer 1

You should create an event in A class and subscribe to it from form1.

class A
{
   public event Action<string> TextReady;

   private OnTextReady(string text)
   {
      var ev = TextReady;
      if(ev!=null) ev(text);        
   }
}

class Form1
{
   private _a = new A();
   public Form1()
   {
      _a.TextReady+= (text)=> textBox.Text = text;
   }
}

You will probably stumble into cross-thread limitations, but that’s another question, just write a comment if you will.