Select Textbox Content With Mouse Position?

Here is what I am trying to do: I've added a .SelectAll() method in the MouseDown event of a textbox and it works well.

Now when the user double clicks, I'd like to unselect the text and set the position where the cursor is.

I don't know if I'am making myself clear here so here is an example:

Let's say this is the following text in the textbox: "this is just a test, please do not panic."

User clicks in the textbox, all the text becomes selected.

Let's assume the position of the curosr is just before the "j"

The user double click so the text becomes unselected and the cursor position automatically changes to the j.

I know I have to use the .Select(start, length) method with 0 as length value, but how do I find the start value depending of the mouse position ?