C＃one表单即时访问控件

Question

I have the following code that checks if an instant of the form is open. However, I cant access the controls (DataGridView dgvPDF), if the form is already opened:

            Form frmPDF = Application.OpenForms["frmPDFs"];
            if (frmPDF != null)
            {
                frmPDF.Focus();
                frmPDF.dgvPDF.DataSource = PDF.CheckPDFs(Files, frmPDF.tblPDFs); <--- red squiggly lines

            }
            else
            {
                frmPDFs frmPDF1 = new frmPDFs();
                frmPDF1.Show();
                frmPDF1.dgvPDF.DataSource = PDF.CheckPDFs(Files, frmPDF1.tblPDFs);
            }

The error message was:

Error 1 'System.Windows.Forms.Form' does not contain a definition for 'dgvPDF' and no extension method 'dgvPDF' accepting a first argument of type 'System.Windows.Forms.Form' could be found (are you missing a using directive or an assembly reference?)

Answer 1

This is a strongly typed variable:

Form frmPDF = Application.OpenForms["frmPDFs"];

So frmPDF is now an instance of type Form , which is the base class for your form. It's not the type of your actual form. (It's an instance of your specific form type, but that instance is known to the compiler as its base type in this case.)

What type is your actual form? Let's assume for a moment it's Form1 . Then make it that type:

Form1 frmPDF = (Form1)Application.OpenForms["frmPDFs"];

Then you can access public members of the type Form1 on frmPDF .

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Edit: According to your comment below, the type is frmPDFs . So use that:

frmPDFs frmPDF = (frmPDFs)Application.OpenForms["frmPDFs"];

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Edit: Casting might be dangerous, by the way. You could include some type checking and handle errors more gracefully:

frmPDFs frmPDF = Application.OpenForms["frmPDFs"] as frmPDFs
if (frmPDF == null)
{
    // the form was the wrong type, something isn't right
}