Spreadsheet control's VisibleRange is null when handling SizeChanged event

Hi Kuan Yong,
Thank you for providing me with a more detailed description of your scenario. I see the issue. The matter is that the SpreadsheetControl.VisibleRange property returns a valid value only when a SpreadsheetControl's layout is completely calculated. In your scenario (maximizing/restoring a main window), the SpreadsheetControl's layout is not entirely calculated when the SpreadsheetControl.SizeChanged event is raised, even if the SpreadsheetControl.VisibleRange property is analyzed within the BeginInvoke method call.
We have plans to provide an additional event which is raised when a visible range in the SpreadsheetControl is changed. However, we have not yet decided in what exact version this event will be implemented.
You can add the following thread to your favorites to be automatically notified of any changes regarding this feature:
Add an event that will be raised in reaction to the visible range change

In the meantime, I can only suggest you re-calculate the SpreadsheetControl's layout by executing the SpreadsheetControl.Refresh method when the the SpreadsheetControl.VisibleRange property value is null. Please refer to the following code:

C#
private void spreadsheetControl1_SizeChanged(object sender, EventArgs e)  
{  
    spreadsheetControl1.BeginInvoke(new MethodInvoker(delegate() {  
        var visibleRange = spreadsheetControl1.VisibleRange;  
        if(visibleRange == null) {  
            spreadsheetControl1.Refresh();  
            visibleRange = spreadsheetControl1.VisibleRange;  
        }  
        System.Diagnostics.Debug.Print("visibleRange: {0}", visibleRange == null ? "null" : visibleRange.ToString());  
        if(visibleRange != null) {  
            System.Diagnostics.Debug.Print("Top row index: {0}", visibleRange.TopRowIndex);  
            System.Diagnostics.Debug.Print("Bottom row index: {0}", visibleRange.BottomRowIndex);  
            System.Diagnostics.Debug.Print("Left column index: {0}", visibleRange.LeftColumnIndex);  
            System.Diagnostics.Debug.Print("Right column index: {0}", visibleRange.RightColumnIndex);  
        }  
    }));  
}  

I hope you will find this information helpful.

Hi Oleg,
Thanks again for your explaination and assistance. I think your suggestion and workaround is useful and will work for now. Please mark the suggestion as solution.
Thank you.

You are welcome!
I am happy to hear that my assistance was helpful to you.
Should you have additional questions regarding our products or need my further assistance, do not hesitate to contact me.

P.S. I have updated my previous answer with information from my last comment. If you wish, you can mark this answer as a solution.