title: PDF Checkbox Error : Value cannot be null. Parameter name: value
body: <p>I am trying to check a checkbox in PDFform using PDFsharp. I am using below code </p>
  
  <pre><code>PdfCheckBoxField chkbox = (PdfCheckBoxField)(pdf.AcroForm.Fields["chkbox"]);
  chk.ReadOnly = false;                   
  chk.Checked = true;
  chk.ReadOnly = true;
  </code></pre>
  
  <p>I am getting below error on line chk.Checked = true;</p>
  
  <p>ArgumentNullException was unhandled
  Value cannot be null.
  Parameter name: value</p>