您需要确保你的用户控件尚未确定这是在该用户的XAML文件宽度。只是删除了宽度=...，从它，你是好去。

编辑：这是我测试代码它：

的 SOUserAnswerTest.xaml：的

 <用户控件X：类=WpfApplication1.SOAnswerTest
的xmlns =htt​​p://schemas.microsoft.com/winfx/2006/xaml/presentation
的xmlns：X =HTTP ：//schemas.microsoft.com/winfx/2006/xaml
 HEIGHT =300> 
<网格和GT; 
< Grid.ColumnDefinitions> 
< ColumnDefinition NAME =LeftSideMenuWIDTH =100/> 
< ColumnDefinition名称=中WIDTH =*/> 
< ColumnDefinition NAME =RightSideMenuWIDTH =90/> 
< /Grid.ColumnDefinitions> 
将; TextBlock的Grid.Column =0>一种与所述; / TextBlock的> 
< TextBlock的Grid.Column =1> B< / TextBlock的> 
< TextBlock的Grid.Column =2> C< / TextBlock的> 
< /网格和GT; 
< /用户控件> 
 

的 Window1.xaml：的

 <窗​​口x：类=WpfApplication1.Window1
的xmlns =htt​​p://schemas.microsoft.com/winfx/2006/xaml/presentation 
的xmlns：X =http://schemas.microsoft.com/winfx/2006/xaml
的xmlns：地方=CLR的命名空间：WpfApplication1
标题=窗口1 HEIGHT =300WIDTH =415> 
<网格和GT; 

<局部：SOAnswerTest Grid.Column =0Grid.Row =5Grid.ColumnSpan =2/&GT; 
&LT; /网格和GT; 
&LT; /窗GT; 
 

I have a Window with my user control and I would like to make usercontrol width equals window width. How to do that?

The user control is a horizontal menu and contains a grid with three columns:

<ColumnDefinition Name="LeftSideMenu" Width="433"/>
<ColumnDefinition Name="Middle" Width="*"/>
<ColumnDefinition Name="RightSideMenu" Width="90"/>

That is the reason I want the window width, to stretch the user control to 100% width, with the second column relative.

EDIT:

I am using a grid, there is the code for Window:

<Window x:Class="TCI.Indexer.UI.Operacao"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    xmlns:tci="clr-namespace:TCI.Indexer.UI.Controles"
    Title=" " MinHeight="550" MinWidth="675" Loaded="Load" ResizeMode="NoResize" WindowStyle="None" WindowStartupLocation="CenterScreen" WindowState="Maximized" Focusable="True"
    x:Name="windowOperacao">
    <Canvas x:Name="canv">
        <Grid>
            <tci:Status x:Name="ucStatus"/> <!-- the control which I want to stretch in width -->
        </Grid>
    </Canvas>
</Window>

You need to make sure your usercontrol hasn't set it's width in the usercontrol's xaml file. Just delete the Width="..." from it and you're good to go!

EDIT: This is the code I tested it with:

SOUserAnswerTest.xaml:

<UserControl x:Class="WpfApplication1.SOAnswerTest"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    Height="300">
    <Grid>
    	<Grid.ColumnDefinitions>
    		<ColumnDefinition Name="LeftSideMenu" Width="100"/>
    		<ColumnDefinition Name="Middle" Width="*"/>
    		<ColumnDefinition Name="RightSideMenu" Width="90"/>
    	</Grid.ColumnDefinitions>
    	<TextBlock Grid.Column="0">a</TextBlock>
    	<TextBlock Grid.Column="1">b</TextBlock>
    	<TextBlock Grid.Column="2">c</TextBlock>
    </Grid>
</UserControl>

Window1.xaml:

<Window x:Class="WpfApplication1.Window1"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    xmlns:local="clr-namespace:WpfApplication1"
    Title="Window1" Height="300" Width="415">
    <Grid>

    	<local:SOAnswerTest Grid.Column="0" Grid.Row="5" Grid.ColumnSpan="2"/>
    </Grid>
</Window>