How to export DataTable to Excel

Question

How can I export a DataTable to Excel in C#? I am using Windows Forms. The DataTable is associated with a DataGridView control. I have

Answer 1

Private tmr As System.Windows.Forms.Timer

Private Sub TestExcel() Handles Button1.Click

    '// Initial data: SQL Server table with 6 columns and 293000 rows.


    '// Data table holding all data
    Dim dt As New DataTable("F161")

    '// Create connection
    Dim conn As New SqlConnection("Server=MYSERVER;Database=Test;Trusted_Connection=Yes;")
    Dim fAdapter As New SqlDataAdapter With
    {
        .SelectCommand = New SqlCommand($"SELECT * FROM dbo.MyTable", conn)
    }

    '// Fill DataTable
    fAdapter.Fill(dt)

    '// Create Excel application
    Dim xlApp As New Excel.Application With {.Visible = True}

    '// Temporarily disable screen updating
    xlApp.ScreenUpdating = False

    '// Create brand new workbook
    Dim xlBook As Excel.Workbook = xlApp.Workbooks.Add()
    Dim xlSheet As Excel.Worksheet = DirectCast(xlBook.Sheets(1), Excel.Worksheet)

    '// Get number of rows
    Dim rows_count = dt.Rows.Count
    '// Get number of columns
    Dim cols_count = dt.Columns.Count

    '// Here 's the core idea: after receiving data
    '// you need to create an array and transfer it to sheet.
    '// Why array?
    '// Because it's the fastest way to transfer data to Excel's sheet.
    '// So, we have two tasks:
    '// 1) Create array
    '// 2) Transfer array to sheet

    '// =========================================================
    '// TASK 1: Create array
    '// =========================================================
    '// In order to create array, we need to know that
    '// Excel's Range object expects 2-D array whose lower bounds
    '// of both dimensions start from 1.
    '// This means you can't use C# array.
    '// You need to manually create such array.
    '// Since we already calculated number of rows and columns,
    '// we can use these numbers in creating array.
    Dim arr = Array.CreateInstance(GetType(Object), {rows_count, cols_count}, {1, 1})

    '// Fill array
    For r = 0 To rows_count - 1
        For c = 0 To cols_count - 1
            arr(r + 1, c + 1) = dt.Rows(r)(c)
        Next
    Next

    '// =========================================================
    '// TASK 2: Transfer array to sheet
    '// =========================================================
    '// Now we need to transfer array to sheet.
    '// So, how transfer array to sheet fast?
    '// 
    '// THE FASTEST WAY TO TRANSFER DATA TO SHEET IS TO ASSIGN ARRAY TO RANGE.
    '// We could, of course, hard-code values, but Resize property
    '// makes this work a breeze:
    xlSheet.Range("A1").Resize.Resize(rows_count, cols_count).Value = arr

    '// If we decide to dump data by iterating over array,
    '// it will take LOTS of time.
    '// For r = 1 To rows_count
    '//     For c = 1 To cols_count
    '//         xlSheet.Cells(r, c) = arr(r, c)
    '//     Next
    '// Next

    '// Here are time results:
    '// 1) Assigning array to Range: 3 seconds
    '// 2) Iterating over array: 45 minutes

    '// Turn updating on
    xlApp.ScreenUpdating = True
    xlApp = Nothing
    xlBook = Nothing
    xlSheet = Nothing

    '// Here we have another problem:
    '// creating array took lots of memory (about 150 MB).
    '// Using 'GC.Collect()', by unknown reason, doesn't help here.
    '// However, if you run GC.Collect() AFTER this procedure is finished
    '// (say, by pressing another button and calling another procedure),
    '// then the memory is cleaned up.
    '// I was wondering how to avoid creating some extra button to just release memory,
    '// so I came up with the idea to use timer to call GC.
    '// After 2 seconds GC collects all generations.
    '// Do not forget to dispose timer since we need it only once.

    tmr = New Timer()
    AddHandler tmr.Tick,
        Sub()
            GC.Collect()
            GC.WaitForPendingFinalizers()
            GC.WaitForFullGCComplete()
            tmr.Dispose()
        End Sub
    tmr.Interval = TimeSpan.FromSeconds(2).TotalMilliseconds()
    tmr.Start()

End Sub