Set TextBox Height

• Download user control - 11.11 KB
• Download test project - 23.28 KB

Introduction

This article describes how to set the height of a single-line textbox control. The TextBox control in C# (and VB.NET) ignores the height property and adjusts it to fit the font height. By calculating the exact font size needed, we can set the TextBox to a desired height.

Background

I had a C# project that required a single-line textbox with adjustable height. I found many examples of how to adjust the width, but nothing on how to dynamically change the height of a textbox. I did not want to use multi-line because I wanted to keep the auto-complete features of the single-line textbox.

Using the Code

Single-line textbox height is set by the size of the font, not the TextBox.Height property. This makes it difficult if you are looking for an exact height. Luckily, the font property uses a float for the font size (emSize). You can use fractions of fonts to fine-tune the textbox height.

The calculation the textbox uses to determine its height is:

Height = ( Font Size * Font Line Spacing / Font Em Height ) + 7

• Font Size - It is easiest to measure font size in pixels, so you do not have to factor in screen dpi.
• Font Line Spacing - The distance, in design units, between two consecutive lines of text.
• Font Em Height - Height, in design units of the font's widest letter - typically the letter M.

Textboxes have a 3-pixel lower and 4-pixel upper white space around the font height. Therefore, the calculation increases the height by 7 pixels.

We can reverse this calculation to obtain the font size needed for a desired height:

Font Size = ( height - 7 ) * Font Em Height / Font Line Spacing

This method will return a font object that will set the size of your textbox:

private Font GetFontForTextBoxHeight(int TextBoxHeight, Font OriginalFont)
{
    // What is the target size of the textbox?
    float desiredheight = (float)TextBoxHeight;

    // Set the font from the existing TextBox font.
    // We use the fnt = new Font(...) method so we can ensure that
    //  we're setting the GraphicsUnit to Pixels.  This avoids all
    //  the DPI conversions between point & pixel.
    Font fnt = new Font(OriginalFont.FontFamily,
                        OriginalFont.Size,
                        OriginalFont.Style,
                        GraphicsUnit.Pixel);

    // TextBoxes never size below 8 pixels. This consists of the
    // 4 pixels above & 3 below of whitespace, and 1 pixel line of
    // greeked text.
    if (desiredheight < 8)
        desiredheight = 8;

    // Determine the Em sizes of the font and font line spacing
    // These values are constant for each font at the given font style.
    // and screen DPI.
    float FontEmSize = fnt.FontFamily.GetEmHeight(fnt.Style);
    float FontLineSpacing = fnt.FontFamily.GetLineSpacing(fnt.Style);

    // emSize is the target font size.  TextBoxes have a total of
    // 7 pixels above and below the FontHeight of the font.
    float emSize = (desiredheight - 7) * FontEmSize / FontLineSpacing;
    
    // Create the font, with the proper size.
    fnt = new Font(fnt.FontFamily, emSize, fnt.Style, GraphicsUnit.Pixel);

    return fnt;
}

Whenever you have to set a textbox size, set the font property using the above method:

YourTextBox.Font = GetFontForTextBoxHeight(DesiredHeight, YourTextBox.Font);  

Using the Control

The adjustable Height Textbox is a user control that you can add to your form. It behaves the same as a regular TextBox except:

• You can set the height in the design view via the Size_AdjustableHeight property.
• The control, if in single-line mode, will respond to docking and anchoring.

To use the control:

1. Right-click on your toolbox in Design view.
2. Click Browse. Navigate to AdjustableHeightTextbox.dll. Hit OK.
3. Add the control to your form.

Enjoy!

Points of Interest

• I learned that the .NET TextBox code is basically a wrapper for the old MFC textbox control. That is why it is not very customizable.
• The GetFontForTextBoxHeight() routine, with some tweaking, could be used for a ComboBox control. The height calculations are close, but still off by a couple of pixels.

There is a lack of information on the inner workings of the textbox control (as with most of the standard .NET controls). I hope this article has saved you hours of searching.

History

• 2008.09.07 - Posted original article
• 2008.09.10 - Added a user control version of the textbox and a test project

This article, along with any associated source code and files, is licensed under The Code Project Open License (CPOL)