Error The property "Content" can only be set once, Error The object 'Window' already has a child and cannot add 'Grid'. 'Window' can accept only one child.

Exception Message

Error The property "Content" can only be set once.

Error The object 'Window' already has a child and cannot add 'Grid'. 'Window' can accept only one child.


Cause and Resolution

The Parent <window> </window> can have only one child element. for e.g:-

<window>
     <grid>
    </grid>
    <grid>
    </grid>
</window>

Is wrong instead we should have like this

<window>
    <grid>
            <grid>
            </grid>
            <grid>
            </grid>
   </grid>
</window>

The child element that is added doesnt necessarily be gird it can be stackpanel , dockpanel or canvas etc.