为什么我用CallMethodAction时报异常？
[code=csharp]
    xmlns:i="http://schemas.microsoft.com/expression/2010/interactivity" 
    xmlns:ei="http://schemas.microsoft.com/expression/2010/interactions"
    xmlns:prism="http://www.codeplex.com/prism"

<Button

Content="登 入" Margin="0,0,0,0" Padding="10,5">
                                        <i:Interaction.Triggers>
                                            <i:EventTrigger EventName="Click">
                                               <ei:CallMethodAction TargetObject="{Binding}" MethodName="UserLogin"/>
                                            </i:EventTrigger>
                                        </i:Interaction.Triggers>
                                    </Button>
[/code]

运行时异常：
An unhandled exception("Unhandled Error in SilverLight The type "CallMethodAction" was found because xmlns:ei="http://schemas.microsoft.com/expression/2010/interactions" is unknown namespace.