on the child form add a button and on the click event if certain conditions are met use
string something = "some value"; this.DialogResult=DialogResult.OK;
and if the conditions are not met then
this.DialogResult=DialogResult.Cancel;
Now in the parent form set if the child form's DialogResult is DialogResult.OK then set child form's string to textBox field on parent form like this.
ChildForm childform = new ChildForm(); if (cildform.ShowDialog() == DialogResult.Cancel) return; textBox1.Text=childform.something;