public void radiobutton_Checked(object sender, RoutedEventArgs e)
    {
        var rb = (RadioButton)sender;
        var tag = Convert.ToInt32(rb.Tag);
        angle_Offset = (degrees_90 * tag) - Heading;
    }

<TextBox x:Name="Rotate" Text="{Binding ElementName=this, Path=Heading}" TextChanged="textChangedEventHandler" />

var buttons = this.Controls.OfType<RadioButton>()
                       .FirstOrDefault(n => n.Checked);