How to truncate a string to fit in a container?

Question

There are a lot of questions (EG: 1, 2, 3, 4, 5) asking how you can truncate a string to a desired amount of characters. But I want a piece of text truncated to fit in a container. (IE: crop the string by it's width in pixels, not characters).

This is easy if you use WPF, but not so much in WinForms...

So: how can you truncate a string to make it fit in a container?

Answer 1

After a day of coding I found a solution and I wanted to share it with the community.

First of all: there is no native truncate function for a string or winforms TextBox. If you use a label, you can use the AutoEllipsis property.

FYI: An ellipsis is a punctuation mark that consist of three dots. IE: …

That's why I made this:

public static class Extensions
{
    /// <summary>
    /// Truncates the TextBox.Text property so it will fit in the TextBox. 
    /// </summary>
    static public void Truncate(this TextBox textBox)
    {
        //Determine direction of truncation
        bool direction = false;
        if (textBox.TextAlign == HorizontalAlignment.Right) direction = true;

        //Get text
        string truncatedText = textBox.Text;

        //Truncate text
        truncatedText = truncatedText.Truncate(textBox.Font, textBox.Width, direction);

        //If text truncated
        if (truncatedText != textBox.Text)
        {
            //Set textBox text
            textBox.Text = truncatedText;

            //After setting the text, the cursor position changes. Here we set the location of the cursor manually.
            //First we determine the position, the default value applies to direction = left.

            //This position is when the cursor needs to be behind the last char. (Example:"…My Text|");
            int position = 0;

            //If the truncation direction is to the right the position should be before the ellipsis
            if (!direction)
            {
                //This position is when the cursor needs to be before the last char (which would be the ellipsis). (Example:"My Text|…");
                position = 1; 
            }

            //Set the cursor position
            textBox.Select(textBox.Text.Length - position, 0);
        }
    }

    /// <summary>
    /// Truncates the string to be smaller than the desired width.
    /// </summary>
    /// <param name="font">The font used to determine the size of the string.</param>
    /// <param name="width">The maximum size the string should be after truncating.</param>
    /// <param name="direction">The direction of the truncation. True for left (…ext), False for right(Tex…).</param>
    static public string Truncate(this string text, Font font, int width, bool direction)
    {
        string truncatedText, returnText;
        int charIndex = 0;
        bool truncated = false;
        //When the user is typing and the truncation happens in a TextChanged event, already typed text could get lost.
        //Example: Imagine that the string "Hello Worl" would truncate if we add 'd'. Depending on the font the output 
        //could be: "Hello Wor…" (notice the 'l' is missing). This is an undesired effect.
        //To prevent this from happening the ellipsis is included in the initial sizecheck.
        //At this point, the direction is not important so we place ellipsis behind the text.
        truncatedText = text + "…";

        //Get the size of the string in pixels.
        SizeF size = MeasureString(truncatedText, font);

        //Do while the string is bigger than the desired width.
        while (size.Width > width)
        {
            //Go to next char
            charIndex++;

            //If the character index is larger than or equal to the length of the text, the truncation is unachievable.
            if (charIndex >= text.Length)
            {
                //Truncation is unachievable!

                //Throw exception so the user knows what's going on.
                throw new IndexOutOfRangeException("The desired width of the string is too small to truncate to.");
            }
            else
            {
                //Truncation is still applicable!

                //Raise the flag, indicating that text is truncated.
                truncated = true;

                //Check which way to text should be truncated to, then remove one char and add an ellipsis.
                if (direction)
                {
                    //Truncate to the left. Add ellipsis and remove from the left.
                    truncatedText = "…" + text.Substring(charIndex);
                }
                else
                {
                    //Truncate to the right. Remove from the right and add the ellipsis.
                    truncatedText = text.Substring(0, text.Length - charIndex) + "…";
                }

                //Measure the string again.
                size = MeasureString(truncatedText, font);
            }
        }

        //If the text got truncated, change the return value to the truncated text.
        if (truncated) returnText = truncatedText;
        else returnText = text;

        //Return the desired text.
        return returnText;
    }

    /// <summary>
    /// Measures the size of this string object.
    /// </summary>
    /// <param name="text">The string that will be measured.</param>
    /// <param name="font">The font that will be used to measure to size of the string.</param>
    /// <returns>A SizeF object containing the height and size of the string.</returns>
    static private SizeF MeasureString(String text, Font font)
    {
        //To measure the string we use the Graphics.MeasureString function, which is a method that can be called from a PaintEventArgs instance.
        //To call the constructor of the PaintEventArgs class, we must pass a Graphics object. We'll use a PictureBox object to achieve this. 
        PictureBox pb = new PictureBox();

        //Create the PaintEventArgs with the correct parameters.
        PaintEventArgs pea = new PaintEventArgs(pb.CreateGraphics(), new System.Drawing.Rectangle());
        pea.Graphics.PageUnit = GraphicsUnit.Pixel;
        pea.Graphics.PageScale = 1;

        //Call the MeasureString method. This methods calculates what the height and width of a string would be, given the specified font.
        SizeF size = pea.Graphics.MeasureString(text, font);

        //Return the SizeF object.
        return size;
    }
}

Usage: This is a class you can copy and paste in the namespace that contains your winforms form. Make sure you include "using System.Drawing;"

This class has two extensions methods, both called Truncate. Basically you can now do this:

public void textBox1_TextChanged(object sender, EventArgs e)
{
    textBox1.Truncate();
}

You can now type something in textBox1 and if needed, it will automatically truncate your string to fit in the textBox and it will add an ellipsis.

Overview: This class currently contains 3 methods:

1. Truncate (extension for TextBox)
2. Truncate (extension for string)
3. MeasureString

Truncate (extension for TextBox)

This method will automatically truncates the TextBox.Text property. The direction of truncation is determent by the TextAlign property. (EG: "Truncation for left alignm…", "…ncation for right alignment".) Please note: this method might need some altering to work with other writing systems such as Hebrew or Arabic.

---

Truncate (extension for string)

In order to use this method you must pass two parameters: a font and a desired width. The font is used to calculate the width of the string and the desired width is used as the maximum width allowed after truncation.

---

MeasureString

This method is private in the code snippet. So if you want to use it, you must change it to public first. This method is used to measure the height and width of the string in pixels. It requires two parameters: the text to be measured and the font of the text.

I hope I helped someone with this. Perhaps there is an other way to do this, I found this answer by Hans Passant, which truncates a ToolTipStatusLabel, which is quite impressive. My .NET skills are nowhere near that of Hans Passant so I haven't managed to convert that code to work with something like a TextBox... But if you did succeeed, or have another solution I would love to see it! :)

Answer 2

I tested the code of Jordy and compared the result with this code of mine, there is no difference, they both trim/truncate fairly OK but not well in some cases, that may be the size measured by MeasureString() is not exact. I know this code is just a simplified version, I post it here if someone cares about it and uses it because it's short and I tested: there is no difference in how exactly this code can trim/truncate string compared with the Jordy's code, of course his code is some kind of full version with 3 methods supported.

public static class TextBoxExtension
{
    public static void Trim(this TextBox text){            
        string txt = text.Text;
        if (txt.Length == 0 || text.Width == 0) return;
        int i = txt.Length;            
        while (TextRenderer.MeasureText(txt + "...", text.Font).Width > text.Width)            
        {
            txt = text.Text.Substring(0, --i);
            if (i == 0) break;
        }
        text.Text = txt + "...";
    }
    //You can implement more methods such as receiving a string with font,... and returning the truncated/trimmed version.
 }