Maximum number of lines for a Wrap TextBlock

Question

I have a TextBlock with the following setting:

TextWrapping="Wrap" 

Can I determine the maximum number of lines?

for example consider the following string TextBlock.Text:

This is a very good horse under the blackboard!! 

It currently has been shows like this:

This is a very  good horse under  the blackboard!! 

I need that to become something like:

This is a very  good horse ... 

any solution?

Answer 1

Update (for UWP)

In UWP Apps you don't need this and can use the TextBlock property MaxLines (see MSDN)

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Original Answer:

If you have a specific LineHeight you can calculate the maximum height for the TextBlock.

Example:

TextBlock with maximum 3 lines

<TextBlock    Width="300"   TextWrapping="Wrap"    TextTrimming="WordEllipsis"    FontSize="24"    LineStackingStrategy="BlockLineHeight"   LineHeight="28"   MaxHeight="84">YOUR TEXT</TextBlock> 

This is all that you need to get your requirement working.

How to do this dynamically?

Just create a new control in C#/VB.NET that extends TextBlock and give it a new DependencyProperty int MaxLines.
Then override the OnApplyTemplate() method and set the MaxHeight based on the LineHeight * MaxLines.

That's just a basic explanation on how you could solve this problem!

Answer 2

Based tobi.at's and gt's answer I have created this MaxLines behaviour. Crucially it doesn't depend upon setting the LineHeight property by calculating the line height from the font. You still need to set TextWrapping and TextTrimming for it the TextBox to be render as you would like.

<TextBlock behaviours:NumLinesBehaviour.MaxLines="3" TextWrapping="Wrap" TextTrimming="CharacterEllipsis" Text="Some text here"/> 

There in also a MinLines behaviour which can be different or set to the same number as the MaxLines behaviour to set the number of lines.

public class NumLinesBehaviour : Behavior<TextBlock> {     TextBlock textBlock => AssociatedObject;      public static readonly DependencyProperty MaxLinesProperty =         DependencyProperty.RegisterAttached(             "MaxLines",             typeof(int),             typeof(NumLinesBehaviour),             new PropertyMetadata(default(int), OnMaxLinesPropertyChangedCallback));      public static void SetMaxLines(DependencyObject element, int value)     {         element.SetValue(MaxLinesProperty, value);     }      public static int GetMaxLines(DependencyObject element)     {         return (int)element.GetValue(MaxLinesProperty);     }      private static void OnMaxLinesPropertyChangedCallback(DependencyObject d, DependencyPropertyChangedEventArgs e)     {         TextBlock element = d as TextBlock;         element.MaxHeight = getLineHeight(element) * GetMaxLines(element);     }      public static readonly DependencyProperty MinLinesProperty =         DependencyProperty.RegisterAttached(             "MinLines",             typeof(int),             typeof(NumLinesBehaviour),             new PropertyMetadata(default(int), OnMinLinesPropertyChangedCallback));      public static void SetMinLines(DependencyObject element, int value)     {         element.SetValue(MinLinesProperty, value);     }      public static int GetMinLines(DependencyObject element)     {         return (int)element.GetValue(MinLinesProperty);     }      private static void OnMinLinesPropertyChangedCallback(DependencyObject d, DependencyPropertyChangedEventArgs e)     {         TextBlock element = d as TextBlock;         element.MinHeight = getLineHeight(element) * GetMinLines(element);     }      private static double getLineHeight(TextBlock textBlock)     {         double lineHeight = textBlock.LineHeight;         if (double.IsNaN(lineHeight))             lineHeight = Math.Ceiling(textBlock.FontSize * textBlock.FontFamily.LineSpacing);         return lineHeight;     } }