c# Windows forms 'Object reference not set to an instance of an object'

Question

I come to ask because i'm new using windows forms and i need some help.

I have a Windows Forms project with 2 forms. The MainForm and the InnerForm.

I'm trying to access to a TableLayoutPanel in MainForm from InnerForm to add new rows in this tablelayoutpanel with some actions that happen in the InnerForm.

I have the next code:

MainForm:

 public partial class MainForm : Form
 {
    public MainForm()
    {
        InitializeComponent();

        TableLayoutPanel panel = tableLayoutPanel1;

        panel.ColumnCount = 4;
        panel.RowCount = 1;
        panel.RowStyles.Add(new RowStyle(SizeType.AutoSize, 0));
        panel.Controls.Add(new Label() { Text = "Tag/ID" }, 0, 0);
        panel.Controls.Add(new Label() { Text = "Tipo" }, 1, 0);
        panel.Controls.Add(new Label() { Text = "Acción" }, 2, 0);
        panel.Controls.Add(new Label() { Text = "Ejecutar" }, 3, 0);
    }

    private void AddInnerForm(string url)
    {
        var inner = new InnerForm(url)
        // more code
    }

    public void agregarRow(ConsoleMessageEvents args){
        // some action with tableLayoutPanel1
    }
 }

InnerForm:

 public partial class InnerForm : UserControl
 {
    MainForm theMain;

    public InnerForm(MainForm main)
    {
        theMain = main;
    }

    public InnerForm(string url)
    {
        InitializeComponent();
        // more code
    }

    private void OnBrowserConsoleMessage(object sender, ConsoleMessageEventArgs args)
    {
       theMain.agregaRow(args);
    }
 }

but when I'm debugging the program I get this error:

An unhandled exception of type 'System.NullReferenceException' occurred in Project.exe

Additional information: Object reference not set to an instance of an object.

in this line in InnerForm:

private void OnBrowserConsoleMessage(object sender, ConsoleMessageEventArgs args)
{
   **theMain.agregaRow(args)**;
}

What is the problem here?

possible duplicate of What does "Object reference not set to an instance of an object" mean? — Dour High Arch
– Dour High Arch, Commented Mar 27, 2015 at 16:12
possible duplicate of What is a NullReferenceException and how do I fix it? — C-Pound Guru
– C-Pound Guru, Commented Mar 29, 2015 at 18:44

Radin Gospodinov · Accepted Answer · 2015-03-27 14:53:57Z

2

That should fix it. The field theMain was not initialized, also you need to call InitializeComponenets in every constructos to create the child controls.

   public partial class InnerForm : UserControl
 {
    MainForm theMain;

    public InnerForm(MainForm main)
    {
        InitializeComponent();
        theMain = main;
    }

    public InnerForm(string url, MainForm mainForm)
    {
        this.theMain = mainForm;
        InitializeComponent();
        // more code
    }

    private void OnBrowserConsoleMessage(object sender, ConsoleMessageEventArgs args)
    {
       theMain.agregaRow(args);
    }
 }

And in MainForm:

private void AddInnerForm(string url)
    {
        var inner = new InnerForm(url, this)
        // more code
    }

answered Mar 27, 2015 at 14:53

Radin Gospodinov

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Comments

Andrey Korneyev · Accepted Answer · 2015-03-27 14:50:19Z

1

From your code it easy to see what you're initializing theMain variable in this constructor:

public InnerForm(MainForm main)
{
    theMain = main;
}

But in your code var inner = new InnerForm(url) you're using another constructor which is not initializing this variable - so theMain is left null and you're getting that exception when trying to access its method.

public InnerForm(string url)
{
    InitializeComponent();
    // more code
}

answered Mar 27, 2015 at 14:50

Andrey Korneyev

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