Change textblock from class#.cs windows phone

I want change text of textblock (or property of any object) by a class file. but when i call that class i just know name of object?

example:

<TextBlock VerticalAlignment="Top" HorizontalAlignment="Left" FontSize="25" Margin="25,220,0,0" Name="txbuser">Username :</TextBlock>
 

and i have file class.cs

how can i chan text obj when i just know Name obj is "txbuser"?
thanks all

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