How to notify an item has selected from data grid in wpf and using mvvm? -

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i trying notify item selected data grid because opening modal window while selecting item data grid. editing selected item in modal window , due not want raisedpropertychanged mechanism selected item because when try modify selected item, opens modal window. trying use event trigger fix issue getting errors. below related code:

viewmodel:

  public observablecollection<student> sitems {   {     return ssitems;   }  set {    ssitems = value;    raisepropertychanged( "sitems" );   } }  private studentsinformation studentinformation;    public studentsinformation studentinformationobject {      {        return studentinformation;      }      set {        studentinformation = value;        raisepropertychanged( "studentinformationobject" );      }    }    public relaycommand<student> selectionchangedcommand {       get;       set;     }

these lines of code in constructor:

selectionchangedcommand = new relaycommand<student>(           item => {             if( item != null ) {               messengerinstance.send<student>( item, "selectedstudent" );             }           } );

this collection bonded datagarid.

view:

 <datagrid x:name="datagrid" grid.row="1" margin="5"                               isreadonly="false"  columnheaderheight="30"                               itemssource="{binding path=sitems}" autogeneratecolumns="false"                                selecteditem="{binding path=selectedstudentobject, mode=twoway, updatesourcetrigger=propertychanged}">                         <datagrid.columns>                             <!--<datagridcheckboxcolumn header="select" binding="{binding path=myselect, updatesourcetrigger=propertychanged}" isreadonly="false" />-->                             <datagridtextcolumn header="name" binding="{binding name}"></datagridtextcolumn>                             <datagridtextcolumn header="enrollment" binding="{binding enrollment}"></datagridtextcolumn>                             <datagridtextcolumn header="score" binding="{binding score}"></datagridtextcolumn>                             <datagridtextcolumn header="comment" binding="{binding comment}"></datagridtextcolumn>                         </datagrid.columns>                         <i:eventtrigger eventname="selectionchanged">                             <cmd:eventtocommand command="{binding selectionchangedcommand}"                                                 commandparameter="{binding selecteditem}" />                         </i:eventtrigger>                     </datagrid>

if remove trigger section datagrid populated desired data. if include trigger code error message:

items collection must empty before using itemssource.

i know there other ways fix kind of things. using mvvm light toolkit.

that event trigger should elsewhere. should placed in interaction.triggers

use this:

<datagrid...>     <i:interaction.triggers>         <i:eventtrigger eventname="selectionchanged">             <cmd:eventtocommand command="{binding selectionchangedcommand}"                                commandparameter="{binding selecteditem}" />         </i:eventtrigger>     </i:interaction.triggers> </datagrid>

update

you should use 

<cmd:eventtocommand x:name="selectionchanged"                          command="{binding selectionchangedcommand}"                          passeventargstocommand="true" />

and modify command in vm. 

relaycommand<selectionchangedeventargs> selectionchangedcommand{get; private set;}

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