DataGridView: Excessive CPU+memory usage when closing form

[dpmm99]

The root cause seems to be that this enumeration clones every row and cell in the DataGridView.

The result is it takes ~12 minutes and ~8 GB extra memory just to dispose a DataGridView with ~700k rows.

Abbreviated stack trace:
DataGridViewRowCollection.this[int].get(int index) Line 137
DataGridView.ReleaseUiaProvider(HWND handle) Line 25605
Control.WmDestroy(ref Message m) Line 11463

Workaround: setting every DataGridView's DataSource = null in the FormClosing event eliminates the issue. That seems to imply there's another problem with consistency--if ReleaseUiaProvider needs to be called for every single cell during WmDestroy, why wouldn't it be needed when clearing the DataSource?

[merriemcgaw]

@LeafShi1 can you take a first look at this?

[LeafShi1]

The direct cause of this problem is the use of foreach (DataGridViewRow row in Rows) in the DataGridView.ReleaseUiaProvider() method to iterate through all rows and release the UIA provider.

This iteration uses the collection's UnsharingRowEnumerator, which forces each row to be materialized from a shared row to a non-shared row (via the Rows[i] indexer), even if these rows remain shared during runtime.

// Original code
foreach (DataGridViewRow row in Rows) // Here, Rows returns an UnsharingRowEnumerator object with 700,000 rows
{
// Each iteration triggers the following chain:
// 1. IEnumerator.Current is accessed
// 2. UnsharingRowEnumerator.Current → return _owner[_current]
// 3. DataGridViewRowCollection[index] indexer is called
// 4. row.Index == -1 is detected (shared row flag)
// 5. The entire row + all cells are cloned
// 6. The DataGridView is re-associated, and all relationships are set
}

Call stack:

System.Windows.Forms.dll!System.Windows.Forms.DataGridViewRowCollection.this[int].get(int index) Line 137 C#
System.Windows.Forms.dll!System.Windows.Forms.DataGridViewRowCollection.UnsharingRowEnumerator.System.Collections.IEnumerator.Current.get() Line 66 C#
System.Windows.Forms.dll!System.Windows.Forms.DataGridView.ReleaseUiaProvider(Windows.Win32.Foundation.HWND handle) Line 25605 C#

Demo project:

WinFormsApp7.zip

[kirsan31]

The regression was introduced in this commit from this PR.

Iterating over rows/cells of DGW is forbidden, except cases with small data and if you absolutely sure what you are doing. This is the law :)

I don't know the mechanics of AccessibilityObjects here - can we clear them through SharedRows? Or may be clearing needed for unshared rows only? If not, then we need to track each created object...

[kirsan31]

@dpmm99

Workaround: setting every DataGridView's DataSource = null in the FormClosing event eliminates the issue. That seems to imply there's another problem with consistency--if ReleaseUiaProvider needs to be called for every single cell during WmDestroy, why wouldn't it be needed when clearing the DataSource?

Welcome to #6859 😢
And after this workaround (when using accessibility objects of course) you should get a memory leak like in #7349?