How does the delegate know which thread to run the method on?

The following sample program creates a form with a textbox and a button.
When the button is pressed, it writes "Written by the main thread" to the textbox
starts a second thread,
the second thread waits 2 seconds, then writes "Written by the background thread" to the textbox via:

line 50:    string text = "Written by the background thread.";
line 56:    SetTextCallback d = new SetTextCallback(SetText);
line 57:    this.Invoke (d, new object[] { text + " (Invoke)" });

Questions:
1. How does the delegate know which thread to run the SetText method on?

2. on line 57, Is the new object[] part necessary?

using System;
using System.ComponentModel;
using System.Threading;
using System.Windows.Forms;

namespace CrossThreadDemo
{
    public class Form1 : Form
    {
        // This delegate enables asynchronous calls for setting
        // the text property on a TextBox control.
        delegate void SetTextCallback(string text);

        // This thread is used to demonstrate thread-safe way
        // to call a Windows Forms control.
        private Thread demoThread = null;
        private TextBox textBox1;
        private Button setTextSafeBtn;

        public Form1()
        {
            InitializeComponent();
        }

        // This event handler creates a thread that calls a
        // Windows Forms control in a thread-safe way.
        private void setTextSafeBtn_Click(
            object sender,
            EventArgs e)
        {
            // Create a background thread and start it.
            this.demoThread = new Thread(new ThreadStart(this.ThreadProcSafe));
            this.demoThread.Start();

            // Continue in the main thread.  Set a textbox value
            // that will be overwritten by demoThread.
            textBox1.Text = "Written by the main thread.";
        }

        // If the calling thread is different from the thread that
        // created the TextBox control, this method passes in the
        // the SetText method to the SetTextCallback delegate and
        // passes in the delegate to the Invoke method.
        private void ThreadProcSafe()
        {
            // Wait two seconds to simulate some background work
            // being done.
            Thread.Sleep(2000);

            string text = "Written by the background thread.";
            // Check if this method is running on a different thread
            // than the thread that created the control.
            if (this.textBox1.InvokeRequired)
            {
                // It's on a different thread, so use Invoke.
                SetTextCallback d = new SetTextCallback(SetText);
                this.Invoke (d, new object[] { text + " (Invoke)" });
            }
            else
            {
                // It's on the same thread, no need for Invoke
                this.textBox1.Text = text + " (No Invoke)";
            }
        }

        // This method is passed in to the SetTextCallBack delegate
        // to set the Text property of textBox1.
        private void SetText(string text)
        {
            this.textBox1.Text = text;
        }


        #region Windows Form Designer generated code
        private void InitializeComponent()
        {
            this.textBox1 = new System.Windows.Forms.TextBox();
            this.setTextSafeBtn = new System.Windows.Forms.Button();
            this.SuspendLayout();
            //
            // textBox1
            //
            this.textBox1.Location = new System.Drawing.Point(12, 12);
            this.textBox1.Name = "textBox1";
            this.textBox1.Size = new System.Drawing.Size(360, 20);
            this.textBox1.TabIndex = 0;
            //
            // setTextSafeBtn
            //
            this.setTextSafeBtn.Location = new System.Drawing.Point(96, 55);
            this.setTextSafeBtn.Name = "setTextSafeBtn";
            this.setTextSafeBtn.TabIndex = 2;
            this.setTextSafeBtn.Text = "Safe Call";
            this.setTextSafeBtn.Click += new System.EventHandler(this.setTextSafeBtn_Click);
            //
            // Form1
            //
            this.ClientSize = new System.Drawing.Size(388, 96);
            this.Controls.Add(this.setTextSafeBtn);
            this.Controls.Add(this.textBox1);
            this.Name = "Form1";
            this.Text = "Form1";
            this.ResumeLayout(false);
            this.PerformLayout();
        }
        #endregion

        [STAThread]
        static void Main()
        {
            Application.EnableVisualStyles();
            Application.Run(new Form1());
        }
    }
}

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