Clearing all controls
I think you're rather over-complicated this for yourself! A much better solution is to use the pattern matched results (which you are already creating (t)) (this requires C# 7.0 still, I believe).



foreach (Control c in form.Controls)
{
    switch (c)
    {
        case CheckBox cb:
            cb.Checked = false;
            break;
        case ComboBox cb:
            cb.Items.Clear();
            break;
        case CheckEdit ce:
            ce.Checked = false;
            break;
        default:
            c.Text = "";
            break;
    }
}



No dynamic here: Text is already a member of Control.
Foreach loops are easier to read than .Foreach (in my opinion, at least; even if they are a bit funny underneath)
Your original ToList was redundant: no need to cache a result you are going to stream immediately in single-threaded logic
I've renamed t to something slightly less terrible; since they are very local variables indeed, I personally wouldn't worry to much about the names.
Note that this code will also clear the text on labels, and buttons, and everything else... so it seems an odd thing to provide as a form-wide operation.