First check whether ParentForm is null or the casting is wrong.

Hi Betty,

ParentForm is used for when a Control is contained within another  
control.  This is true if the Parent Form is an MdiContainer and you set  
the Child control's MdiParent property to point to this parent.

Try something like this instead:

FSGMaint fsdgm = new FSGMaint (this);
fsdgm.ShowDialog();

And in the constructor of FSGMaint:

private FSDSetup myParent;

public FSGMaint(FSDSetup parent)
{
    myParent = parent;
}

Then in the click event:

string x = myParent.ProductDesc;

====================================
Or using MdiParent:

this.IsMdiContainer = true;
FSGMaint fsdgm = new FSGMaint();
fsdgm.MdiParent = this;
fsdgm.Show();

...

string x = ((FSDSetup)this.ParentForm).ProductDesc;