Find longest initial substring which renders in a limited width

I have a long string, ex: "Please help me to solve thi开发者_如何转开发s problem." This string is so long to fit in a width of 100 pixels. I need to get a substring of this string and substring will fit in 100 pixels. Ex: substring "Please help me to sol" is fit in 100 pixels.

Please help me how to estimate a substring like this. Thanks.

My application is Win Forms and C#.

As usual, the Win32 API has a function designed exactly for this: GetTextExtentExPoint

P/invoke declaration: http://pinvoke.net/default.aspx/gdi32/GetTextExtentExPoint.html

Looks like you could use TextRenderer::MeasureText().

this should work... not really tested and no optimisations, though

var g = Graphics.FromImage(someimage); // or any from hwnd etc... should use your panel/whatever
var f = new Font("YOURFONT", 12f, FontStyle.Regular, GraphicsUnit.Pixel); // replace with your vals
var yourtext = "yourtextyourtextyourtextyourtextyourtext";
while (g.MeasureString(yourtext, f).Width > 100)
    yourtext = yourtext.Remove(yourtext.Length - 2, 1);

If all you are looking for is a good estimate, you might first measure the width of a representative string to determine the average character width of your font once. A good representative might not be just the alphabet; you might look for a character frequency histogram to get a better average width.

string Representative = "abcdefghijklmnopqrstuvwxyz";
float CharacterWidth;

using(Bitmap b = new Bitmap(0, 0))
using(Graphics g = Graphics.FromImage(b))
using(Font f = /* some font definition */)
{
    CharacterWidth = g.MeasureString(Representative, f).Width / Representative.Length;
}

Then use that to estimate how many characters would fit within N pixels.

string Text = ...

int DisplayWidth = 100;

int FitLength = Math.Min(Text.Length, (int)(DisplayWidth / CharacterWidth));
string FitText = Text.Substring(0, FitLength);