AnonDCX AnonDCX - 1 year ago 88
C# Question

Singleton: Form instance returning null

I have a windows Forms application with one form and a few classes.

I want to get the values of some textBoxes from the Form1 instance and extract the values.

My first way of achieving this was by using

array to get the form but I realised that using a singleton on the class Form1 would be more efficient as I can have direct access and it would be impossible to make other instances.

Here is how I have set it up:

1. Controls class to gets controls from Form1

class Controls
//Request Form1 instance
private static Form1 form = Form1.GetInstance();

//Sets global values for easy access with getters and null setters
//--Variable 'form' is still null hence I get the NullReferenceException
private TextBox employer = form.Controls["textBoxEmployerName"] as TextBox;
private TextBox role = form.Controls["textBoxRole"] as TextBox;
private TextBox company = form.Controls["textBoxCompanyName"] as TextBox;
private TextBox website = form.Controls["textBoxWebsite"] as TextBox;
private TextBox refNumber = form.Controls["textBoxRefNumber"] as TextBox;
private TextBox reason = form.Controls["textBoxReason"] as TextBox;
private TextBox dateListed = form.Controls["textBoxDateListed"] as TextBox;
private Label charLimit = form.Controls["labelCharsRemaining"] as Label;

public TextBox Employer { get { return employer; } }
public TextBox Role { get { return role; } }
public TextBox Company { get { return company; } }
public TextBox Website { get { return website; } }
public TextBox RefNumber { get { return refNumber; } }
public TextBox Reason { get { return reason; } }
public TextBox DateListed { get { return dateListed; } }
public Label CharLimit { get { return charLimit; } }

2. Singleton set up inside class Form1

public partial class Form1 : Form
private static Form1 theInstance;

public Form1()

//Return instance of Form1
//--This is obviously returning null for some reason
public static Form1 GetInstance()
if (theInstance == null)
theInstance = new Form1();
return theInstance;

As you can probably see I am getting the "NullReferenceException" when I attempt to get the Singleton from class Form1.

The following methods I have used are as follows:

  • Using Windows.OpenForms["Form1"].Controls["--somecontrol--"]

  • Using Windows.ActiveForm

  • Using a Singleton Design Pattern on class Form1

All of these ways are returning null and I cant think of a reason why it is returning null.

Any help would be appreaciated.


Answer Source

I want to get the values of some textBoxes from the Form1 instance and extract the values.

This is where you need to stop and re-think your approach. Forms represent views of your data; however, your data itself needs to be in the model, a separate place independent of the views.

Text boxes need to reflect the state of some model object, such as a Person object that has string properties for employer, company, role, web site, and so on. The form would read from that object's properties, display them in a text box, and then react to text box changes, and save values back to the model Person object.

If you make Person a singleton, or provide some other universal way of accessing it, you would be able to access person's properties from all forms, without accessing the forms themselves.

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