edit #1 ...
Assuming that Form1 creates an instance of Form2, and that the instance of Form2 needs to access a TextBox on the instance of Form1:
In Form2:
Create a Public Property of Type 'TextBox:
public TextBox tbFileName { get; set; }
In Form1:
private Form2 f2 = new Form2();
You need to inject the reference to the Textbox on Form1 into the Property of Type Textbox in the instance of Form2:
private void Form1_Load(object sender, EventArgs e)
{
f2.tbFileName = nameOfYourTextBox;
}
Now, when you need to access Form1's TextBox
in Form2:
string currentTextInForm1 = tbFileName.Text;