resources are not working properly

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sir ! i've written the following code in wpf 4.0 for string demo :

MainWindow.xaml
<window x:class="WpfApplication1.MainWindow" xmlns:x="#unknown">
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        xmlns:sys="clr-namespace:System;assembly=mscorlib"
        Title="MainWindow" Height="350" Width="525"
        removed="{DynamicResource windowsBackgroundBrush}">
    <window.resources>
        <sys:string x:key="ComboBoxTitle" xmlns:sys="#unknown">items</sys:string>
        <x:array x:key="ComboBoxItems" type="sys:String">
            <sys:string xmlns:sys="#unknown">item#1</sys:string>
            <sys:string xmlns:sys="#unknown">item#2</sys:string>
            <sys:string xmlns:sys="#unknown">item#3</sys:string>
        </x:array>
        <lineargradientbrush x:key="windowsBackgroundBrush">
            <gradientstop offset="0" color="Chocolate" />
            <gradientstop offset="1" color="Aqua" />       
            </lineargradientbrush>
    </window.resources>
    <stackpanel margin="10" removed="PowderBlue">
        <Label Content="{StaticResource ComboBoxTitle}"></Label>
        <combobox itemssource="{StaticResource ComboBoxItems}"></combobox>
    </stackpanel>
    

</window>

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but exception generated is :
Cannot convert the value in attribute 'ItemsSource' to object of type 'System.Collections.IEnumerable'. 'System.Windows.Markup.ArrayExtension' is not a valid value for property 'ItemsSource'. Error at object 'System.Windows.Controls.ComboBox' in markup file 'WpfApplication1;component/mainwindow.xaml' Line 21 Position 19.
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but if i write <x:array extension="" xmlns:x="#unknown"> then it's not accepting!
what could be the cause of this exception