Adding multiselect to Gridview.

<asp:GridView ID="GridView1" runat="server" AllowSorting="True" 

            AutoGenerateColumns="False" DataSourceID="EntityDataSource1" OnRowCommand="GridView1_RowCommand1" AllowCustomPaging="True" AllowPaging="True" CellPadding="4" ForeColor="#333333" GridLines="None">

            <AlternatingRowStyle BackColor="White" />

            <Columns>

                <asp:BoundField DataField="ExecutionStartTime" HeaderText="StartTime" ReadOnly="True" SortExpression="ExecutionStartTime" />

                <asp:BoundField DataField="ExecutionEndTime" HeaderText="EndTime" ReadOnly="True" SortExpression="ExecutionEndTime" />

                <asp:BoundField DataField="ExecutionID" HeaderText="ID" ReadOnly="True" SortExpression="ExecutionID" />

                <%--<asp:BoundField DataField="ExecutionUID" HeaderText="UID" ReadOnly="True" SortExpression="ExecutionUID" />--%>

                <asp:BoundField DataField="ExecutionName" HeaderText="Name" ReadOnly="True" SortExpression="ExecutionName" />

                

            </Columns>

            ....

        </asp:GridView>

        <asp:EntityDataSource ID="EntityDataSource1" runat="server" ConnectionString="name=Test_Manager_DatabaseEntities" DefaultContainerName="Test_Manager_DatabaseEntities" EnableFlattening="False" OrderBy="it.[ExecutionStartTime] DESC" EntitySetName="TestExecutions" Select="it.[ExecutionStartTime], it.[ExecutionEndTime], it.[ExecutionID], it.[ExecutionUID], it.[ExecutionName]" EntityTypeFilter="">

        </asp:EntityDataSource>