Formatting text in a TextBlock

How do I achieve formatting of a text inside a TextBlock control in my WPF application?

e.g.: I would like to have certain words in bold, others in italic, and some in different colors, like this example:

The reason behind my question is this actual problem:

lblcolorfrom.Content = "Colour From: " + colourChange.ElementAt(3).Value.ToUpper();

I would like the second part of the string to be bold, and I know that I could use two controls (Labels, TextBlocks, etc.) but I'd rather not, due the vast amount of controls already in use.

You need to use Inlines:

<TextBlock.Inlines>
    <Run FontWeight="Bold" FontSize="14" Text="This is WPF TextBlock Example. " />
    <Run FontStyle="Italic" Foreground="Red" Text="This is red text. " />
</TextBlock.Inlines>

With binding:

<TextBlock.Inlines>
    <Run FontWeight="Bold" FontSize="14" Text="{Binding BoldText}" />
    <Run FontStyle="Italic" Foreground="Red" Text="{Binding ItalicText}" />
</TextBlock.Inlines>

You can also bind the other properties:

<TextBlock.Inlines>
    <Run FontWeight="{Binding Weight}"
         FontSize="{Binding Size}"
         Text="{Binding LineOne}" />
    <Run FontStyle="{Binding Style}"
         Foreground="Binding Colour}"
         Text="{Binding LineTwo}" />
</TextBlock.Inlines>

You can bind through converters if you have bold as a boolean (say).

You can do this in XAML easily enough:

<TextBlock>
  Hello <Bold>my</Bold> faithful <Underline>computer</Underline>.<Italic>You rock!</Italic>
</TextBlock>

There are various Inline elements that can help you, for the simplest formatting options you can use Bold, Italic and Underline:

<TextBlock>
    Sample text with <Bold>bold</Bold>, <Italic>italic</Italic> and <Underline>underlined</Underline> words.
</TextBlock>

I think it is worth noting, that those elements are in fact just shorthands for Span elements with various properties set (i.e.: for Bold, the FontWeight property is set to FontWeights.Bold).

This brings us to our next option: the aforementioned Span element.

You can achieve the same effects with this element as above, but you are granted even more possibilities; you can set (among others) the Foreground or the Background properties:

<TextBlock>
    Sample text with <Span FontWeight="Bold">bold</Span>, <Span FontStyle="Italic">italic</Span> and <Span TextDecorations="Underline">underlined</Span> words. <Span Foreground="Blue">Coloring</Span> <Span Foreground="Red">is</Span> <Span Background="Cyan">also</Span> <Span Foreground="Silver">possible</Span>.
</TextBlock>

The Span element may also contain other elements like this:

<TextBlock>
    <Span FontStyle="Italic">Italic <Span Background="Yellow">text</Span> with some <Span Foreground="Blue">coloring</Span>.</Span>
</TextBlock>

There is another element, which is quite similar to Span, it is called Run. The Run cannot contain other inline elements while the Span can, but you can easily bind a variable to the Run's Text property:

<TextBlock>
    Username: <Run FontWeight="Bold" Text="{Binding UserName}"/>
</TextBlock>

Also, you can do the whole formatting from code-behind if you prefer:

TextBlock tb = new TextBlock();
tb.Inlines.Add("Sample text with ");
tb.Inlines.Add(new Run("bold") { FontWeight = FontWeights.Bold });
tb.Inlines.Add(", ");
tb.Inlines.Add(new Run("italic ") { FontStyle = FontStyles.Italic });
tb.Inlines.Add("and ");
tb.Inlines.Add(new Run("underlined") { TextDecorations = TextDecorations.Underline });
tb.Inlines.Add("words.");