public class GridFocusedRowChangedEventArgs :
    GridFocusedRowChangedEventArgsBase

<DxGrid Data="GridData" PageSize="5"
        FocusedRowEnabled="true" FocusedRowChanged="Grid_FocusedRowChanged">
    <Columns>
        <DxGridDataColumn FieldName="FirstName" />
        <DxGridDataColumn FieldName="LastName" />
        <DxGridDataColumn FieldName="Title" />
        <DxGridDataColumn FieldName="HireDate" />
    </Columns>
</DxGrid>

<DxFormLayout>
    <DxFormLayoutItem Caption="@MemoCaption" CaptionPosition="CaptionPosition.Vertical" 
                      Visible="@ShowMemo" ColSpanMd="12">
        <DxMemo Text=@Notes Rows="4"/>
    </DxFormLayoutItem>
</DxFormLayout>

@code {
    string MemoCaption { get; set; }
    string Notes { get; set; }
    bool ShowMemo { get; set; }

    void Grid_FocusedRowChanged(GridFocusedRowChangedEventArgs e) {
        if(e.DataItem != null) {
            ShowMemo = true;
            var employee = (Employee)e.DataItem;
            MemoCaption = employee.FirstName + " " + employee.LastName + " details:";
            Notes = employee.Notes;
        }
        else {
            ShowMemo = false;
        }
    }

    object GridData { get; set; }
    protected override async Task OnInitializedAsync() {
        GridData = await NwindDataService.GetEmployeesAsync();
    }
}