Form cannot be both an MDI child and MDI parent. c#

IN  MDI Parent Queery is :-

 

Monthly_sales f1 = new Monthly_sales();
f1.MdiParent = this;
this.WindowState = FormWindowState.Maximized;
// f1.StartPosition = FormStartPosition.CenterScreen;
this.Location = new Point(0, 0);
this.Size = Screen.PrimaryScreen.WorkingArea.Size;
f1.Show(); 

 

And In Monthly_sales form Double Click Queery is :-

 

Daily_Sales_Report abc = new Daily_Sales_Report;
abc.StartPosition = FormStartPosition.CenterScreen;
abc.Show();
 

https://stackoverflow.com/questions/4418604/mdi-parent-form-problem-setting-parent

 

http://www.thecodekey.com/Articles/C_VB/Form_cannot_be_both_MDI_child_and_MDI_parent.aspx

 

https://social.msdn.microsoft.com/Forums/vstudio/en-US/53e70be4-303a-4104-ac21-a5277fb00c22/right-way-to-show-mdi-child-in-mdi-containercant-get-it-to-work-properly?forum=csharpgeneral

 

https://www.codeproject.com/Questions/1100148/Windows-application-mdi-parent-form-error

 

https://www.codeproject.com/Questions/993893/How-Can-fix-this-problem 

Hi Jyoti,

Thanks for posting the code, but unfortunately you didn't mention what is your query. Which is your child and which is your parent form?

my Queery is

 

Daily_Sales_Report abc = new Daily_Sales_Report;
abc.StartPosition = FormStartPosition.CenterScreen;
abc.Show();