Question

I am inserting a column in a DataGridView programmatically (i.e., not bound to any data tables/databases) as follows:

int lastIndex = m_DGV.Columns.Count - 1;  // Count = 4 in this case DataGridViewTextBoxColumn col = (DataGridViewTextBoxColumn)m_DGV.Columns[lastIndex]; m_DGV.Columns.RemoveAt(lastIndex); m_DGV.Columns.Insert(insertIndex, col);  // insertIndex = 2 

I have found that my columns are visually out of order sometimes using this method. A workaround is to manually set the DisplayIndex property of the column afterwards. Adding this code ‘fixes it’, but I don’t understand why it behaves this way.

Console.Write(m_DGV.Columns[0].DisplayIndex); // Has value of 0 Console.Write(m_DGV.Columns[1].DisplayIndex); // Has value of 1 Console.Write(m_DGV.Columns[2].DisplayIndex); // Has value of 3 Console.Write(m_DGV.Columns[3].DisplayIndex); // Has value of 2 col.DisplayIndex = insertIndex; Console.Write(m_DGV.Columns[0].DisplayIndex); // Has value of 0 Console.Write(m_DGV.Columns[1].DisplayIndex); // Has value of 1 Console.Write(m_DGV.Columns[2].DisplayIndex); // Has value of 2 Console.Write(m_DGV.Columns[3].DisplayIndex); // Has value of 3 

As an aside, my grid can grow its column count dynamically. I wanted to grow it in chunks, so each insert didn’t require a column allocation (and associated initialization). Each ‘new’ column would then be added by grabbing an unused column from the end, inserting it into the desired position, and making it visible.

Answer 1

I suspect this is because the order of the columns in the DataGridView do not necessarily dictate the display order, though without explicitly being assigned by default the order of the columns dictate the DisplayIndex property values. That is why there is a DisplayIndex property, so you may add columns to the collection without performing Inserts – you just need to specify the DisplayIndex value and a cascade update occurs for everything with an equal or greater DisplayIndex. It appears from your example the inserted column is also receiving the first skipped DisplayIndex value.

From a question/answer I found:

Changing the DisplayIndex will cause all the columns between the old DisplayIndex and the new DisplayIndex to be shifted.

As with nearly all collections (other than LinkedLists) its always better to add to a collection than insert into a collection. The behavior you are seeing is a reflection of that rule.