c# - Accessing controls from different forms

Question

I have a main form with some buttons, textboxes, labels, etc. On a second form I would like to copy the text from the main forms textbox onto the second form.

Have tried:

var form = new MainScreen();
TextBox tb= form.Controls["textboxMain"] as TextBox;
textboxSecond.Text = tb.Text;

But it just causes an exception. The main screen textbox is initialised and contains text. When I hover over form I can see all the controls are there.

What am I doing wrong?

Answer 1

Looking at the original code, there are two potential reasons for the NullReferenceException you are getting. First, tb is not defined in the code you provide so I am not sure what that is.

Secondly, TextBox textbox = form.Controls["textboxMain"] as TextBox can return null if the control is not found or is not a TextBox. Controls, by default, are marked with the private accessor, which leads me to suspect that form.Controls[...] will return null for private members.

While marking the controls as internal will potentially fix this issue, it's really not the best way to tackle this situation and will only lead to poor coding habits in the future. private accessors on controls are perfectly fine.

A better way to share the data between the forms would be with public properties. For example, let's say you have a TextBox on your main screen called usernameTextBox and want to expose it publicly to other forms:

public string Username
{
  get { return usernameTextBox.Text; }
  set { usernameTextBox.Text = value; }
}

Then all you would have to do in your code is:

var form = new MainForm();
myTextBox.Text = form.Username; // Get the username TextBox value
form.Username = myTextBox.Text; // Set the username TextBox value

The great part about this solution is that you have better control of how data is stored via properties. Your get and set actions can contain logic, set multiple values, perform validation, and various other functionality.

If you are using WPF I would recommend looking up the MVVM pattern as it allows you to do similar with object states.