Here I tried this:

<ComboBox Grid.Column="1" ItemsSource="{Binding IpList}" SelectedItem="{Binding SelectedIp , Mode=TwoWay}" IsEnabled ="{Binding IsEnabled}" DropDownOpened = "{Binding HintCommand}">

That's what I got:

DropDownOpened = "{Binding HintCommand}" is not valid. '{Binding HintCommand} is not a valid event handler method name. Only instance methods on the generated code-behind class are valid.

    2 answers 2

    Use Microsoft.Expression.Interactivity
    info : MVVM invoking command on attached event like this:

      <ComboBox Grid.Column="1" ItemsSource="{Binding IpList}" SelectedItem="{Binding SelectedIp , Mode=TwoWay}" IsEnabled ="{Binding IsEnabled}"> <interactivity:Interaction.Triggers> <interactivity:EventTrigger EventName="DropDownOpened"> <!--<cmd:EventToCommand Command="{Binding MyCommand}" />--> </interactivity:EventTrigger> </interactivity:Interaction.Triggers> </ComboBox>

      Who will be interested in:

       xmlns:i="http://schemas.microsoft.com/expression/2010/interactivity" <ComboBox Grid.Column="1" ItemsSource="{Binding IpList}" SelectedItem="{Binding SelectedIp , Mode=TwoWay}" IsEnabled ="{Binding IsEnabled}"> <i:Interaction.Triggers> <i:EventTrigger EventName="DropDownOpened"> <i:InvokeCommandAction Command="{Binding GetInterfacesCommand}"/> </i:EventTrigger> </i:Interaction.Triggers> </ComboBox>

      You need to add microsoft.expression.interactivity.dll to refnrensy