Write text on specific line in a textbox

I am trying to write text on a specific line in a textbox for example on the forth line)

I've tried :

TextBox1.Lines = TextBox1.Lines.Length

but get this error:

Value of type 'Integer' cannot be converted to '1-dimensional array of String'.

I assume this is beacause TextBox1.Lines is an array of all the text in every line in the textbox.

Multiline is set to true.

How to fix this?

vb.net Code:

Dim lines As String() = myTextBox.Lines
 
lines(3) = myString
myTextBox.Lines = lines

Thanks for the reply,

I get this error:

System.IndexOutOfRangeException was unhandled
Message="Index was outside the bounds of the array."

The textbox where I am trying to write text to is empty.

vb.net Code:

Dim lines As String() = TextBox1.Lines
 
lines(0) = "test"
 
TextBox1.Lines = lines

Well, that's kind of important information. How can you write to the fourth line of a TextBox that has no lines in it? There has to be at least four lines for there to be a fourth line.

If you want to add a new line to a TextBox you can do this:

vb.net Code:

myTextBox.AppendText(Environment.NewLine)

You can also assign any String array you like to the Lines property of a TextBox to overwrite all existing text. The array doesn't have to have come from the Lines property in the first place:

vb.net Code:

Dim lines As String() = {"Line 1", "Line 2", "Line 3"}
 
myTextBox.Lines = lines