updating inactive form.text from a thread in c#

Question

I'm trying to update form.text from a thread. Basically thread need to update the form.text with the current time. my code looks like below

UpdateText("My Monitor (Last Updated " + DateTime.Now.ToString("HH:mm:ss tt") + ")", Form1.ActiveForm);

and the method as below

    void UpdateText(String s, Control c)
    {
        if (c.InvokeRequired)
        {

            this.BeginInvoke(new MethodInvoker(() => UpdateText(s, c)));

        }
        else
        {
            c.Text = s;
        }
    }

As long as the main application window is active, code works. If the application becomes inactive, then I get the error "Object reference not set to an instance of an object"

Answer 1

You are calling Form1.ActiveForm .

ActiveForm : A Form that represents the currently active form, or null if there is no active form.

If the Form is inactive, naturally this would be null . Use a different reference to your form. If you are calling it from within the form, use this .

Answer 2

Keep the reference of ActiveForm in a private field, and use that field in the Thread

Control activeControl;
private void start_new_thread()
{
    active = Form1.ActiveForm;
    // start work under thread
}

private void work_under_thread(object state)
{

    //blocking works
    // update here
    UpdateText("My Monitor (Last Updated " + DateTime.Now.ToString("HH:mm:ss tt") + ")", activeControl);

}