Creating custom column widths in OpenXML (excel)

I think the problem you're running into is creating and appending a NEW columns element to the existing worksheet content. I believe you need to append the new column to an existing columns element.

I created a workbook, saved it, added content in an empty column, then saved the workbook under a new name and closed it.

Using the Open XML SDK 2.5 Productivity Tool's "Compare" feature I selected the worksheet part containing the difference, selected it, then clicked "View Package Code". The code that generates the changed file with the new column from the original file shows me:

Columns columns1=worksheet1.GetFirstChild<Columns>();
//other code here
Column column1 = new Column(){ Min = (UInt32Value)5U, Max = (UInt32Value)5U, Width = 16D, CustomWidth = true };
columns1.Append(column1);

Note that it appears you're also expected to specify the column range of the new column.

The selected answer above didn't fix my issue, but I finally figured it out. The issue for me was when I called the line: Columns columns1=worksheet1.GetFirstChild<Columns>(); there was currently no Columns children in the worksheet so the object returned was null and I got a runtime error when I tried appending a column to the Columns object.

The issue is that Excel is extremely picky. The columns element in the actual sheet.xml file has to be before the sheetdata element. Trying to append my custom columns to the worksheet resulted in a corrupted file due to it placing the columns element after the sheetdata element. Since I knew it had to be before the sheetdata element I had to insert it into the beginning of the worksheet and not append it to the worksheet. Here's the code that worked for me:

// Save the stylesheet formats
stylesPart.Stylesheet.Save();

// Create custom widths for columns
Columns lstColumns = worksheetPart.Worksheet.GetFirstChild<Columns>();
Boolean needToInsertColumns = false;
if (lstColumns == null)
{
    lstColumns = new Columns();
    needToInsertColumns = true;
}
// Min = 1, Max = 1 ==> Apply this to column 1 (A)
// Min = 2, Max = 2 ==> Apply this to column 2 (B)
// Width = 25 ==> Set the width to 25
// CustomWidth = true ==> Tell Excel to use the custom width
lstColumns.Append(new Column() { Min = 1, Max = 1, Width = 25, CustomWidth = true });
lstColumns.Append(new Column() { Min = 2, Max = 2, Width = 9, CustomWidth = true });
lstColumns.Append(new Column() { Min = 3, Max = 3, Width = 9, CustomWidth = true });
lstColumns.Append(new Column() { Min = 4, Max = 4, Width = 9, CustomWidth = true });
lstColumns.Append(new Column() { Min = 5, Max = 5, Width = 13, CustomWidth = true });
lstColumns.Append(new Column() { Min = 6, Max = 6, Width = 17, CustomWidth = true });
lstColumns.Append(new Column() { Min = 7, Max = 7, Width = 12, CustomWidth = true });
// Only insert the columns if we had to create a new columns element
if (needToInsertColumns)
    worksheetPart.Worksheet.InsertAt(lstColumns, 0);

// Get the sheetData cells
SheetData sheetData = worksheetPart.Worksheet.GetFirstChild<SheetData>();

Hope this helps someone!!

I had the same problem that .GetFirstChild<Columns> was null. Creating the Columns object and inserting at index 0 (like the other answer) made Excel complain that the file was invalid. Instead, it seems to want the columns definitions just before the SheetData section:

if (needToInsertColumns)
{
    var sheetData = worksheetPart.Worksheet.GetFirstChild<SheetData>();
    worksheetPart.Worksheet.InsertBefore(columnsList, sheetData);
}