Getting a string index based on a pixel offset

I am custom drawing a text box and now I am implementing the part where the user can click on a line and have the cursor move to where he clicked.

I know how to get the row of text he clicked on开发者_StackOverflow中文版 because the character height is constant across all fonts, but not the column, because I'm not sure how to say "get me all the text that can be drawn before this amount of pixels," and because character width is not consistent unless you're using a fixed-width font, which is not a guarantee.

So I have the point from which I'm drawing the string (0) then I have the point that the user clicked. How do I get the string index of the character they clicked on?

Extra info: I am drawing the text by storing the lines in a List then iterating the list and using Graphics.DrawString on each line.

There is no simple method to find the character at a pixel.

However you can find the pixels that a string will fill. Use the Graphics.MeasureCharacterRanges method. You can perform a binary search on your string until you find the string where MeasureCharacterRanges returns your cursor position.

Note: You might see the Graphics.MeasureString method and be tempted to use that. DON'T! That method doesn't return accurate measurements. I can't remember why, but you will do your head in if you try!