在主窗体上的用户控件中订阅事件时，发生NullReferenceException

Question

On my User Control I have created an event to notify my main form that a tab control must switch its tab. This event must be triggered on a button's click on the user control. (I have a very similar event in another User Control that is working fine)

In my user control:

public delegate void EventHandler(object sender, EventArgs args);
public event EventHandler TabChangeRequested = delegate { };

protected void OnTabChangeRequested()
{
    if (TabChangeRequested != null)
    {
        TabChangeRequested(this, new EventArgs());
    }
}

private void btnBackToSelectType_Click(object sender, EventArgs e)
{
    this.OnTabChangeRequested();
}

In my main form:

public FormMain()
{
    InitializeComponent();
    myUserControl.TabChangeRequested += (sender, args) => { ChangeRunTabToType(); };           
}

private void ChangeRunTabToType()
{
    if (this != null)
        this.ChangeTabIndex(metroTabControlRun, 1);
}

When I run my program I get a NullReferenceException on the line

myUserControl.TabChangeRequested += (sender, args) => { ChangeRunTabToType(); }; 

Does anyone know where it comes from?

Answer 1

If myUserControl isn't instantiated yet (and that's what is null), then I'd recommend moving:

myUserControl.TabChangeRequested += (sender, args) => { ChangeRunTabToType(); };     

to after where you are creating/assigning myUserControl!