How can I access values on the UI thread?

Question

I have no issues invoking actions on UI controls through BeginInvoke and such, however, that's a void method. I need to get a value from a textbox for use in a different thread.

How can I accomplish this?

Answer 1

You can pass a delegate to Invoke that points to a method which can set an instance variable to hold the text value, e.g.

    public Form1()
    {
        InitializeComponent();

        Invoke(new Action(SetTextboxTextVariable));
    }

    private string _text;

    private void SetTextboxTextVariable()
    {
        _text = txtBox.Text;
    } 

This covers the basic idea, synchronizing and notifying other threads is up to you. :)

Update

Or to do it in one step:

    public Form1()
    {
        InitializeComponent();

        string text = GetText();
    }

    private string GetText()
    {
        string text;

        Invoke(new Action(text = txtBox.Text));

        return text;
    }

Answer 2

The simplest solution is to capture a local variable in a closure.

String text;
textBox.Invoke(() => text = textBox.Text);

The compiler will generate some code that is much like chibacity's solution - the local variable becomes a field of a compiler-generated class.

UPDATE

This does not work - the lambda expression is not assignable to Delegate. This problem can be solved using an extension method.

internal void ExecuteOnOwningThread(this Control control, Action action)
{
    if (control.InvokeRequired)
    {
        control.Invoke(action);
    }
    else
    {
        action();
    }
}

The usage is then as follows.

String text;
textBox.ExecuteOnOwningThread(() => text = textBox.Text);

It is possible to stuff multiple statements into the lambda expression.

textBox.ExecuteOnOwningThread(() =>
{
    DoStuff();
    text = textBox.Text
    DoOtherStuff();
});

But as chibacity already mentioned in a comment it may be better to explicitly write a method. Beyond a certain point using lambda expressions will adversely affect the readability of the code. And using lambda expressions is of course very prone to introducing repeated code.

Answer 3

The Control.Invoke() method returns the value of the invoked method. Which makes it easy:

        string value = (string)this.Invoke(new Func<string>(() => textBox1.Text));

Or a bit more explicit:

    private void backgroundWorker1_DoWork(object sender, DoWorkEventArgs e) {
        var dlg = new Func<string>(GetText);
        string value = (string)this.Invoke(dlg);
    }
    private string GetText() {
        return textBox1.Text;
    }