multiline textbox auto adjust it's height according to the amount of text

Try this following code:

public partial class Form1 : Form
{
     private const int EM_GETLINECOUNT = 0xba;
     [DllImport("user32", EntryPoint = "SendMessageA", CharSet = CharSet.Ansi, SetLastError = true, ExactSpelling = true)]
     private static extern int SendMessage(int hwnd, int wMsg, int wParam, int lParam);


     public Form1()
     {
        InitializeComponent();
     }

     private void textBox1_TextChanged(object sender, EventArgs e)
     {
        var numberOfLines = SendMessage(textBox1.Handle.ToInt32(), EM_GETLINECOUNT, 0, 0);
        this.textBox1.Height = (textBox1.Font.Height + 2) * numberOfLines;
     }
} 

There doesn't seem to be any functionality to do this built in to the TextBox class, but the Font class has a Height property that returns the number of pixels between baselines.

It is also possible to find out how many lines the text in the TextBox occupies, as described in this blog post (warning: it's not exactly elegant).

Once you've obtained this information, you should be able to make the TextChanged handler set the height of the TextBox accordingly using some simple maths.