MeasureString always thinks whitespace will fit

Question

I\'m trying to do some hit testing on strings (I want to get the char index from the x offset), but I\'m hitting issues with measure string.

This is essentially the cod

Answer 1

I Really don't like this solution but so far its the only working solution I have. Its based on the solution from @FSDaniel, but I've noticed that the results from Graphics.MeasureString & Graphics.DrawString drift out even if you pass the same StringFormat to them.

The only consistent measuring I've managed to achive using the TextRenderer with the TextFormatFlags.TextBoxControl flag set.

This is a nasty solution which could be improved a little (performance wise) by hunting for a result, i.e. start in the middle of the string if its too big try 3/4 of the string, if thats to small try 5/8 etc until you have a result.

This is far from the best solution, so if anyone has anything better, please post it!

    protected override void OnPaint(PaintEventArgs e)
    {
        string sample = "abc                       defXXXXXXXXXXXXiiiiiiiX";

        TextFormatFlags flags = TextFormatFlags.NoPadding | TextFormatFlags.TextBoxControl | TextFormatFlags.SingleLine | TextFormatFlags.NoPrefix;

        TextRenderer.DrawText(e.Graphics, sample, this.Font, Point.Empty, Color.Black, flags);
        e.Graphics.DrawLine(Pens.Red, trackBar1.Value, 0, trackBar1.Value, 100);

        string measuredString = sample;
        for (int i = 0; i < sample.Length; i++)
        {
            Size size = TextRenderer.MeasureText(e.Graphics, sample.Substring(0, i+1), this.Font, new Size(10000000, 1000000), flags);
            if (size.Width > trackBar1.Value)
            {
                textBox1.Text = "[" + sample.Substring(0,i+1) + "]";
                break;
            }
        }

        base.OnPaint(e);
    }

Answer 2

The problem is that the length of "abc" and "abc " are the same. Since you don't print out the trailing spaces the graphical representation is the same. I would add a character at the end and then measure the string and remove the length of the added character.

Swith your OnPaint to this and it seems to work:

    protected override void OnPaint(PaintEventArgs e) {
        string sample = "abc                       def";

        e.Graphics.DrawString(sample, this.Font, Brushes.Black, PointF.Empty);
        e.Graphics.DrawLine(Pens.Red, trackBar1.Value, 0, trackBar1.Value, 100);

        StringFormat sf = new StringFormat(StringFormatFlags.MeasureTrailingSpaces | StringFormatFlags.NoWrap | StringFormatFlags.LineLimit);
        sf.Trimming = StringTrimming.Character;

        var underscoreWidth = e.Graphics.MeasureString("_", this.Font).Width;

        for (int i = 0; i < sample.Length; i++) {
            var s = sample.Substring(0, i + 1) + "_";
            var size = e.Graphics.MeasureString(s, this.Font).Width - underscoreWidth;
            if (size > trackBar1.Value) {
                if (s.Length > 0) {
                    var ok = s.Substring(0, s.Length - 2);
                    textBox1.Text = "[" + ok + "]";
                    base.OnPaint(e);
                    return;
                }
            }
        }

        textBox1.Text = "[" + sample + "]";
        base.OnPaint(e);
    }