Autoscale Font in a TextBox Control so that its as big as possible and still fits in text area bounds

Question

I need a TextBox or some type of Multi-Line Label control which will automatically adjust the font-size to make it as large as possible and yet have the entire message fit inside the bounds of the text area.

I wanted to see if anyone had implemented a user control like this before developing my own.

Example application: have a TextBox which will be half of the area on a windows form. When a message comes in which is will be approximately 100-500 characters it will put all the text in the control and set the font as large as possible. An implementation which uses Mono Supported .NET libraries would be a plus.

If know one has implemented a control already... If someone knows how to test if a given text completely fits inside the text area that would be useful for if I roll my own control.

Edit: I ended up writing an extension to RichTextBox. I will post my code shortly once i've verified that all the kinks are worked out.

Answer 1

I had to solve the same basic problem. The iterative solutions above were very slow. So, I modified it with the following. Same idea. Just uses calculated ratios instead of iterative. Probably, not quite as precise. But, much faster.

For my one-off need, I just threw an event handler on the label holding my text.

    private void PromptLabel_TextChanged(object sender, System.EventArgs e)
    {
        if (PromptLabel.Text.Length == 0)
        {
            return;
        }

        float height = PromptLabel.Height * 0.99f;
        float width = PromptLabel.Width * 0.99f;

        PromptLabel.SuspendLayout();

        Font tryFont = PromptLabel.Font;
        Size tempSize = TextRenderer.MeasureText(PromptLabel.Text, tryFont);

        float heightRatio = height / tempSize.Height;
        float widthRatio = width / tempSize.Width;

        tryFont = new Font(tryFont.FontFamily, tryFont.Size * Math.Min(widthRatio, heightRatio), tryFont.Style);

        PromptLabel.Font = tryFont;
        PromptLabel.ResumeLayout();
    }

Answer 2

I haven't seen an existing control to do this, but you can do it the hard way by using a RichTextBox and the TextRenderer's MeasureText method and repeatedly resizing the font. It's inefficient, but it works.

This function is an event handler for the 'TextChanged' event on a RichTextBox.

An issue I've noticed:

When typing, the text box will scroll to the current caret even if scrollbars are disabled. This can result in the top line or left side getting chopped off until you move back up or left with the arrow keys. The size calculation is correct assuming you can get the top line to display at the top of the text box. I included some scrolling code that helps sometimes (but not always).

This code assumes word wrap is disabled. It may need modification if word wrap is enabled.

---

The code:

[DllImport("user32.dll")]
public static extern int SendMessage(IntPtr hWnd, uint wMsg, int wParam, uint lParam);

private static uint EM_LINEINDEX = 0xbb;

private void richTextBox1_TextChanged(object sender, EventArgs e)
{
    // If there's no text, return
    if (richTextBox1.TextLength == 0) return;

    // Get height and width, we'll be using these repeatedly
    int height = richTextBox1.Height;
    int width = richTextBox1.Width;

    // Suspend layout while we mess with stuff
    richTextBox1.SuspendLayout();

    Font tryFont = richTextBox1.Font;
    Size tempSize = TextRenderer.MeasureText( richTextBox1.Text, richTextBox1.Font);

    // Make sure it isn't too small first
    while (tempSize.Height < height || tempSize.Width < width)
    {
        tryFont = new Font(tryFont.FontFamily, tryFont.Size + 0.1f, tryFont.Style);
        tempSize = TextRenderer.MeasureText(richTextBox1.Text, tryFont);
    }

    // Now make sure it isn't too big
    while (tempSize.Height > height || tempSize.Width > width)
    {
        tryFont = new Font(tryFont.FontFamily, tryFont.Size - 0.1f, tryFont.Style);
        tempSize = TextRenderer.MeasureText(richTextBox1.Text, tryFont);
    }

    // Swap the font
    richTextBox1.Font = tryFont;

    // Resume layout
    richTextBox1.ResumeLayout();

    // Scroll to top (hopefully)
    richTextBox1.ScrollToCaret();
    SendMessage(richTextBox1.Handle, EM_LINEINDEX, -1, 0);
}