Filling PDF form fields

Use PDF form filling to set field values from your application data.

Common use cases include:

• Prepopulating onboarding or registration forms
• Filling claim and compliance forms from backend systems
• Running batch form generation from CSV or database records
• Building template-based document assembly pipelines

In this guide, you’ll fill:

• Text fields
• Checkboxes
• Radio button groups
• Combo boxes
• List boxes

How Nutrient helps

Nutrient .NET SDK handles form field access, type-specific operations, and value validation.

The SDK handles:

• PDF form field dictionaries and field type detection
• Low-level value and method validation
• Hierarchy traversal for nested fields
• State synchronization for selection controls

Complete implementation

This example fills multiple form field types in one workflow.

using Nutrient;

Opening a document with form fields

Open the document with a using statement(opens in a new tab), create an editor with PdfEditor.Edit(document), and get all form fields via editor.FormFieldCollection. Use the collection for lookup by fully qualified name. Inspect actual field names first (for example, print FullName for the fields you look up) before applying targeted updates:

try
{
    using Document document = Document.Open("input_forms.pdf");
    using PdfEditor editor = PdfEditor.Edit(document);
    var formFields = editor.FormFieldCollection;

Filling a text field

Find a text field by full name, verify that it exists and has the expected type, and assign the value with the Value property. FindByFullName() returns the concrete field subtype, such as PdfTextField or PdfCheckBoxField, or null when no field has that name. The inherited Value property writes the value for every field type — selection controls take the export value of the choice:

    var textField = formFields.FindByFullName("Text1");
    if (textField != null && textField.FieldType == PdfFormFieldType.Text)
    {
        textField.Value = "John Doe";
    }

Checking a checkbox

Find a checkbox field, verify the type, and assign its on-state export value. Checkbox export values are often "On" or "Yes" — the sample form uses "Yes":

    var checkbox = formFields.FindByFullName("Check1");
    if (checkbox != null && checkbox.FieldType == PdfFormFieldType.CheckBox)
    {
        checkbox.Value = "Yes";
    }

Selecting a radio button option

Find a radio group, verify the type, and assign one of its option export values:

    var radioGroup = formFields.FindByFullName("RadioGroup1");
    if (radioGroup != null && radioGroup.FieldType == PdfFormFieldType.RadioButton)
    {
        radioGroup.Value = "Option2";
    }

Selecting an item in a combo box

Find a combo box field, verify the type, and assign the export value of the choice you want.

Values must match a valid export value exactly:

    var comboBox = formFields.FindByFullName("Dropdown1");
    if (comboBox != null && comboBox.FieldType == PdfFormFieldType.ComboBox)
    {
        comboBox.Value = "Germany";
    }

Selecting an item in a list box

Find a list box field, verify the type, and assign the export value of the choice you want. The generic Value property selects a single value; multi-select isn’t yet supported in the .NET SDK:

    var listBox = formFields.FindByFullName("List1");
    if (listBox != null && listBox.FieldType == PdfFormFieldType.ListBox)
    {
        listBox.Value = "Technology";
    }

Filling several fields in one pass

Apply type-based updates to a list of known field names. Process only terminal, non-read-only fields:

    foreach (var fullName in new[] { "Text1", "Check1" })
    {
        var field = formFields.FindByFullName(fullName);
        if (field != null && field.IsTerminal && !field.IsReadOnly)
        {
            var fieldType = field.FieldType;

            if (fieldType == PdfFormFieldType.Text)
            {
                field.Value = "Sample Value";
            }
            else if (fieldType == PdfFormFieldType.CheckBox)
            {
                field.Value = "Yes";
            }
        }
    }

Saving the filled form

Save the output PDF.

In this sample:

• SaveAs("output.pdf") writes a new file.
• The using statements release the editor and document handles.
• NutrientException handles runtime failures.

    editor.SaveAs("output.pdf");
    Console.WriteLine("Successfully filled form fields");
}
catch (NutrientException e)
{
    Console.Error.WriteLine($"Error: {e.Message}");
    Environment.Exit(1);
}

Conclusion

Use this workflow to fill PDF form fields:

1. Open the document, create a PDF editor, and retrieve the form field collection with the FormFieldCollection property.
2. Use FindByFullName() to locate specific fields by their fully qualified names (including hierarchy).
3. Verify field types with the FieldType property before performing field-specific operations.
4. Fill text fields by setting Value to "...".
5. Fill checkbox, radio, combo, and list controls by setting Value to the control’s export values.
6. For checkbox and radio controls, use valid export values (for example, checkbox values are often "On" or "Yes").
7. For combo/list controls, values must match available option text/export values exactly.
8. Batch-fill by walking a list of field names with conditional logic using IsTerminal and FieldType.
9. Save the document with SaveAs() to persist all filled field values.
10. Handle exceptions with try-catch blocks for robust error recovery in production workflows.
11. The using statements release the editor and document handles.

Field names used in this sample (Text1, Check1, RadioGroup1, Dropdown1, List1) match the sample input form. Inspect actual field names in your PDF first and update lookups accordingly — method calls on a field looked up by a name that doesn’t exist throw a NutrientException.

For related form workflows, refer to the .NET SDK editor guides.