How to update textbox in form1 from form2?
// specifically typecasting the TYPE of form being passed in,
// not just a generic form.  We need to see the exposed elements
Form1 CalledFrom;
// Ensure to do the : this() to make sure default behavior
// to initialize the controls on the form are created too.
public Form2(Form1 viaParameter) : this()
{
  CalledFrom = viaParameter;
}

private void btnOnForm2_Click(object sender, EventArgs e)
{
  CalledFrom.ValuesByProperty = this.txtOnForm2.Text;
  MessageBox.Show( "Check form 1 textbox" );

  string GettingBack = CalledFrom.ValuesByProperty;
  MessageBox.Show( GettingBack );

  CalledFrom.SetViaMethod( "forced value, not from textbox" );
  MessageBox.Show( "Check form 1 textbox" );

  GettingBack = CalledFrom.GetViaMethod();
  MessageBox.Show( GettingBack );
}

private void button1_Click(object sender, EventArgs e)
{
   Form2 oFrm = new Form2(this);
   oFrm.Show();
}

public string ValuesByProperty
{
  get { return this.textBox1.Text; }
  set { this.textBox1.Text = value; }
}

public void SetViaMethod(string newValue)
{ this.textBox1.Text = newValue; }

public string GetViaMethod()
{ return this.textBox1.Text; }

public partial class YourFirstForm
{
    // example to expose a method on first form and pass IN a value
    public void SetMyObject( string ValueFromSecondForm )
    {
       this.txtBox1.Text = ValueFromSecondForm;
    }

    // example via a property you are trying to set... identical in results
    public string ViaSetAsProperty
    { set { this.txtBox1.Text = value; } }

    // Now, the reverse, lets expose some things from form 1 to the second...
    public string GetMyObjectText()
    {
       return this.txtBox1.Text;
    }

    // or via a GETTER property...
    public string GettingText
    { get { return this.txtBox1.Text; } }


    // However, if you want to allow both set and get to form 1's values,
    // do as a single property with both getter / setter exposed..
    public string TextContent
    {  get { return this.txtBox1.Text; }
       set { this.txtBox1.text = value; }
    }
}

public YourSecondForm( Form passedInVar )
{
   // preserve the first form
   ObjRefToFirstForm = passedInVar;
}

// Now, however you want to handle... via a button click, the text change event, etc
public void SendDataToForm1()
{
   // via calling the METHOD on form 1 that is public
   ObjRefToFirstForm.SetMyObj( this.SomeOtherTextbox.Text );

   // via a SETTER on form 1
   ObjRefToFirstForm.ViaSetAsProperty = this.SomeOtherTextbox.Text;


   // sample to GET something from form 1 via method
   string TestGet = ObjRefToFirstForm.GetMyObjectText();

   // or from the exposed property
   TestGet = ObjRefToFirstForm.GettingText;


   // Now, try via the one property that has both a setter and getter
   ObjRefToFirstForm.TextContent = this.SomeOtherTextbox.Text;
   TestGet = ObjRefToFirstForm.TextContent;
}

AddEntryWindow addEntryWindow = new AddEntryWindow();
addEntryWindow.ShowDialog(this);

foreach (AddEntry list in addedEntry)
{
     // Displaying and formating the output in text box in MainWindow.
     ((MainWindow)owner).txtDisplayFileContent.Text += txtUserName.Text;
}