Unable to select Object from the form

[Arciviz]

Unable to select Object from "Click Button" in the form.

private void DrawBySelect_Click(object sender, EventArgs e)
    {
      IsolatedFootingDGVData = IsolatedFootingDGV;
      var sel = uiDoc.Selection;
      this.Hide();

      var selectionFilter = new KJF.FamilySelectionFilter(
        doc, BuiltInCategory.OST_StructuralFoundation, "Isolated RCC Footing 2022_BT");

      var elem = doc.GetElement(
       sel.PickObject(ObjectType.Element, selectionFilter)) as FamilyInstance;

      var design_ID = elem.get_Parameter(BuiltInParameter.ELEM_TYPE_PARAM).AsValueString();

      var row = IsolatedFootingDGV.Rows.Cast<DataGridViewRow>()
        .Where(r => r.Cells["DesignID"].Value.ToString().Equals(design_ID)).First();
      this.Show();
      
    }

[Arciviz · Accepted solution]

Resolved 

https://forums.autodesk.com/t5/revit-api-forum/call-revit-s-api-from-external-thread/td-p/5941308 

[RPTHOMAS108]

The alternative depending on the complexity of your form would be to close the form after saving it's state carry out the selection operation and then reshow the form with values reinstated.

1. Save values from form
2. Call Form.Close
3. Carry out Revit selection operation
4. Create new Form adding values back from (1)
5. ShowDialog

For a simple modal dialogue this is often more straightforward than external event.

If your form is modeless then external event is the way to do.