How to position cursor after changing selection in TextBox derived

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I'm working on a TextBox control (in C# using .NET 2.0) that allows fields
(like MS Office's mail merge feature). I want the editor to treat the field
as though it was a single character. The user shouldn't be able to delete
part of the field or select only part of the field.

For example, a line might look like: «Image» shot on «Date» at «Resolution»
where «Image» «Date» «Resolution» are all fields.

My control inherits from TextBox. I've overloaded OnKeyDown & OnKeyPress so
that if the user backspaces it deletes the whole field. If they move left or
right, it skips past the entire field. I've also overloaded OnMouseUp so if
they select part of the field with the mouse, it increases the selection to
select all of the field. This all seems to work.

My problem is if the user holds shift and left, I can't get the field
selected and put the cursor to the left of the selection. Similarly if they
select by dragging the mouse right to left, the cursor is on the right side.

A snippet of the OnMouseUp code follows:

int SelectionEnd = SelectionLength + SelectionStart;
for (int i = SelectionStart; i >= 0; i--)
{
if (Text[i] == FieldEnd)
// Found an end tag. SelectionStart isn't in a field.
break;
if (Text[i] == FieldStart)
{
// Found the start tag. Modify selection.
Select(i, SelectionEnd - i);

}
}

I assume I can't do what I want with just Select(start,le n) or
SelectionStart and SelectionLength . I've tried setting start to the end and
using a negative length (like you would do with a RichTextBox , but that
doesn't work. I've even tried setting SelectionStart to the position I want
after setting the selection but still no joy. Any help will be much
appreciated!