The RadToolBar does not support center-alignment of it's items. They can be left-aligned or right-aligned. You can check the Item Alignment
article for more information.
However there is a way to solve this situation. You can set margin to the first centered button inside the tool bar. This margin will be dynamically calculated on re-size. Here is an example:
sender, SizeChangedEventArgs e)
RadToolBar toolBar = sender
toolBarWidth = toolBar.ActualWidth;
var children = toolBar.Items;
buttonsTotalWidth = 0;
buttonsTotalWidth += button.ActualWidth;
sideConstants = 20;
freeSpace = toolBarWidth - buttonsTotalWidth - sideConstants;
offset = freeSpace / 2;
Thickness(offset, 0, 0, 0);
You can check this approach in the sample project attached.
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