The answer to this question lies in how C# Controls work

Controls in Windows Forms are bound to a specific thread and are not thread safe. Therefore, if you are calling a control's method from a different thread, you must use one of the control's invoke methods to marshal the call to the proper thread. This property can be used to determine if you must call an invoke method, which can be useful if you do not know what thread owns a control.

From Control.InvokeRequired

Effectively, what Invoke does is ensure that the code you are calling occurs on the thread that the control "lives on" effectively preventing cross threaded exceptions.

From a historical perspective, in .Net 1.1, this was actually allowed. What it meant is that you could try and execute code on the "GUI" thread from any background thread and this would mostly work. Sometimes it would just cause your app to exit because you were effectively interrupting the GUI thread while it was doing something else. This is the Cross Threaded Exception - imagine trying to update a TextBox while the GUI is painting something else.

  • Which action takes priority?
  • Is it even possible for both to happen at once?
  • What happens to all of the other commands the GUI needs to run?

Effectively, you are interrupting a queue, which can have lots of unforeseen consequences. Invoke is effectively the "polite" way of getting what you want to do into that queue, and this rule was enforced from .Net 2.0 onward via a thrown InvalidOperationException.

To understand what is actually going on behind the scenes, and what is meant by "GUI Thread", it's useful to understand what a Message Pump or Message Loop is.

This is actually already answered in the question "What is a Message Pump" and is recommended reading for understanding the actual mechanism that you are tying into when interacting with controls.

Other reading you may find useful includes:

What's up with Begin Invoke

One of the cardinal rules of Windows GUI programming is that only the thread that created a control can access and/or modify its contents (except for a few documented exceptions). Try doing it from any other thread and you'll get unpredictable behavior ranging from deadlock, to exceptions to a half updated UI. The right way then to update a control from another thread is to post an appropriate message to the application message queue. When the message pump gets around to executing that message, the control will get updated, on the same thread that created it (remember, the message pump runs on the main thread).

and, for a more code heavy overview with a representative sample:

Invalid Cross-thread Operations

// the canonical form (C# consumer)

public delegate void ControlStringConsumer(Control control, string text);  // defines a delegate type

public void SetText(Control control, string text) {
    if (control.InvokeRequired) {
        control.Invoke(new ControlStringConsumer(SetText), new object[]{control, text});  // invoking itself
    } else {
        control.Text=text;      // the "functional part", executing only on the main thread

Once you have an appreciation for InvokeRequired, you may wish to consider using an extension method for wrapping these calls up. This is ably covered in the Stack Overflow question Cleaning Up Code Littered with Invoke Required.

There is also a further write up of what happened historically that may be of interest.

A control or window object in Windows Forms is just a wrapper around a Win32 window identified by a handle (sometimes called HWND). Most things you do with the control will eventually result in a Win32 API call that uses this handle. The handle is owned by the thread that created it (typically the main thread), and shouldn't be manipulated by another thread. If for some reason you need to do something with the control from another thread, you can use Invoke to ask the main thread to do it on your behalf.

For instance, if you want to change the text of a label from a worker thread, you can do something like this:

theLabel.Invoke(new Action(() => theLabel.Text = "hello world from worker thread!"));

If you want to modify a control it must be done in the thread in which the control was created. This Invoke method allows you to execute methods in the associated thread (the thread that owns the control's underlying window handle).

In below sample thread1 throws an exception because SetText1 is trying to modify textBox1.Text from another thread. But in thread2, Action in SetText2 is executed in the thread in which the TextBox was created

private void btn_Click(object sender, EvenetArgs e)
    var thread1 = new Thread(SetText1);
    var thread2 = new Thread(SetText2);

private void SetText1() 
    textBox1.Text = "Test";

private void SetText2() 
    textBox1.Invoke(new Action(() => textBox1.Text = "Test"));