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richTextBox1.Document.Blocks.Clear();
string content = File.ReadAllText(@"e:\testprojekt\L568.h");
DateTime start = DateTime.Now;
richTextBox1.AppendText(content);
TimeSpan diff = DateTime.Now - start;
MessageBox.Show("Benötigte Zeit: " + diff.ToString());                    

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<Window x:Class="WpfApplication1.Window1"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    Title="Window1" Height="300" Width="300">
    <Grid>
        <RichTextBox Margin="0,0,0,63" Name="richTextBox1" HorizontalScrollBarVisibility="Auto" VerticalScrollBarVisibility="Auto"/>
        <Button Height="36" Name="button1" VerticalAlignment="Bottom" Click="button1_Click">Button</Button>
    </Grid>
</Window>                    

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richTextBox1.Document.Blocks.Clear(); 

FlowDocument doc = new FlowDocument();
Paragraph para = new Paragraph();

string content = File.ReadAllText(@"e:\testprojekt\L568.h");

string[] lines = content.Split(new[] { "\r\n" }, StringSplitOptions.None);

foreach (string line in lines)
{
    para.Inlines.Add(new Run(line));
    para.Inlines.Add(new LineBreak());
}

doc.Blocks.Add(para);
richTextBox1.Document = doc;