You can use the WebBrowser control.
Here's the documentation for using it with a string.
XAML:
<phone:WebBrowser x:Name="webBrowser" />
C#:
string LongText = "blablabla";
webBrowser.NavigateToString(LongText);
Question
In my project .I need to display loog texts,it is some kind of book app project .i know how to read a txt file in c# but it doesn't look efficient to me there should be more easy and customable ways. do you know any other way to display long texts
Solution
You can use the WebBrowser control.
Here's the documentation for using it with a string.
XAML:
<phone:WebBrowser x:Name="webBrowser" />
C#:
string LongText = "blablabla";
webBrowser.NavigateToString(LongText);