Exporting DataGrid to Excel missing reference

Question

Currently attempting to export datagrid to an excel file.Right now as my code is the only thing that gets exported are the Headers. Below is the datagrid

<Page x:Class="ABSCCoastRefining.EndOfYear"
  xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
  xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
  xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006" 
  xmlns:d="http://schemas.microsoft.com/expression/blend/2008" 
  xmlns:local="clr-namespace:ABSCCoastRefining"
  mc:Ignorable="d" 
  d:DesignHeight="450" d:DesignWidth="800"
  Title="EndOfYear">
<Page.Resources>
    <CollectionViewSource x:Key="ItemCollectionViewSources" CollectionViewType="{x:Type ListCollectionView}"/>
</Page.Resources>

<Grid Background="#FFD1F1D1" Margin="0,-10,0,10">
    <TextBlock Text="End Of Year" FontSize="18" FontWeight="Bold" VerticalAlignment="Top" HorizontalAlignment="Left" Margin="22,10,0,0" Width="124" Background="White"/>
    <DataGrid x:Name="dgEndOfYear" 
        ItemsSource="{Binding}" 
        DataContext="{StaticResource ItemCollectionViewSources}"
        Margin="65,112,285,237" 
        Background="#FFE0F0E0"
        AutoGenerateColumns="False"
        CanUserAddRows="False"
        CanUserDeleteRows="False"
        HorizontalAlignment="Center" 
        VerticalContentAlignment="Center" 
        IsReadOnly="True"
        IsSynchronizedWithCurrentItem ="False" Width="450">
        


        <DataGrid.Columns>
            <DataGridTextColumn Header="Calendar Year" Binding="{Binding CalendarYear}" Width="Auto" FontWeight="Bold" />
            <DataGridTextColumn Header="Month" Binding="{Binding MonthName}" Width="Auto" FontWeight="Bold"/>
            <DataGridTextColumn Header="Total Gallons" Binding="{Binding SumGal, StringFormat=N0}" Width="Auto" FontWeight="Bold" />
        </DataGrid.Columns>

And below is the code for the click event to export the data. dgEndOfYear.Items[i].Cells is causing issues and compiler states I am missing a reference I have made several attempts to resolve this on my own but have had no luck any help would be appreciated.

private void btnExcel_Click(object sender, EventArgs e)
    {
        // creating Excel Application  
        Microsoft.Office.Interop.Excel._Application app = new Microsoft.Office.Interop.Excel.Application();
        // creating new WorkBook within Excel application  
        Microsoft.Office.Interop.Excel._Workbook workbook = app.Workbooks.Add(Type.Missing);
        // creating new Excelsheet in workbook  
        Microsoft.Office.Interop.Excel._Worksheet worksheet = null;
        // see the excel sheet behind the program  
        app.Visible = true;
        // get the reference of first sheet. By default its name is Sheet1.  
        // store its reference to worksheet  
        worksheet = workbook.Sheets["Sheet1"];
        worksheet = workbook.ActiveSheet;
        // changing the name of active sheet  
        worksheet.Name = "Exported from gridview";
        // storing header part in Excel  
        for (int i = 1; i < dgEndOfYear.Columns.Count + 1; i++)
        {
            worksheet.Cells[1, i] = dgEndOfYear.Columns[i - 1].Header;
        }
        // storing Each row and column value to excel sheet  
        for (int i = 0; i < dgEndOfYear.Columns.Count - 1; i++)
        {
            for (int j = 0; j < dgEndOfYear.Columns.Count; j++)
            {
                worksheet.Cells[i + 2, j + 1] = dgEndOfYear.Items[i].Cells[j].Value.ToString();
            }
        }
        // save the application  
        workbook.SaveAs("c:\\output.xls", Type.Missing, Type.Missing, Type.Missing, Type.Missing, Type.Missing, Microsoft.Office.Interop.Excel.XlSaveAsAccessMode.xlExclusive, Type.Missing, Type.Missing, Type.Missing, Type.Missing);
        // Exit from the application  
        app.Quit();
    }