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Custom Fields

Fields are a convenient way to show non-static data in the document. In this way, you could present different data to the end-user without actually changing the document content.

For a description of the supported out of the box field types, please refer to this article.

This topic will explain how custom fields can be created in order to suit application-specific purposes. It contains the following sections:

Create Custom Field

In some cases, it is convenient to extend the currently available fields to suit application-specific scenarios. This can be done by inheriting from CodeBasedField or (if the functionality of the new field will be closely connected to Mail Merge) from MergeField.

This topic will list the steps for creating such a custom field.

1. Have your class inherit from MergeField or CodeBasedField:

public class CustomField : CodeBasedField

Public Class CustomField
Inherits CodeBasedField

2. Give a FieldTypeName to instances of your field:

private static readonly string FieldType = "CUSTOMFIELD";

public override string FieldTypeName
        return CustomField.FieldType;

Private Shared ReadOnly FieldType As String = "CUSTOMFIELD"
Public Overrides ReadOnly Property FieldTypeName() As String
        Return CustomField.FieldType
    End Get
End Property

The field type is shown when the field is in code mode, just after the opening curly bracket. If you toggle the field to Code mode and modify this first word, the type of the field will also change. That is why it is important to give a meaningful name to the field.

3. Register your custom field type in the factory that RadDocument uses. It is convenient to use the static constructor of the class for that purpose:

static CustomField()
    CodeBasedFieldFactory.RegisterFieldType(CustomField.FieldType, () => new CustomField());

Shared Sub New()
    CodeBasedFieldFactory.RegisterFieldType(CustomField.FieldType, Function() New CustomField())
End Sub

The function passed as a second parameter essentially tells the document how it could create a new instance of the field. This is required because fields have to be created internally when you copy/paste or even when you toggle field modes.

4. Override the CreateInstance() method to return a new instance of your custom field:

public override Field CreateInstance()
    return new CustomField();

Public Overrides Function CreateInstance() As Field
    Return New CustomField()
End Function

5. If you have inherited from CodeBasedField or you would like to customize the way the field appears in the document in ResultMode, override the GetResultFragment method. In short, it will return a DocumentFragment with the content that should be shown when the field is updated and its mode is changed to Result.

That is all that needs to be done in order to be able to get your custom field working, insert it in documents, serialize and deserialize it.

Adding Properties

If you would like to add some properties to this field, similar to the PropertyPath field of MergeFields, you can do so following the steps bellow:

1. Create a read-only FieldProperty:

private readonly FieldProperty myProperty;

Private ReadOnly _myProperty As FieldProperty

2. Add a public CLR property. Properties of type string can be set and retrieved from the FieldProperty directly. No other types are accepted, as it would not be possible to convert them correctly to text for the purpose of field evaluation when inserted in the document. Therefore, if you would like to have non-string properties, you would have to make the conversion to/from string yourself:

public string MyProperty
        return this.myProperty.GetValue();
        if (!this.myProperty.IsNestedField && this.myProperty.GetValue() == value)


Public Property MyProperty() As String
        Return Me._myProperty.GetValue()
    End Get
    Set(ByVal value As String)
        If (Not Me._myProperty.IsNestedField) AndAlso Me._myProperty.GetValue() = value Then
        End If
    End Set
End Property

The XamlSerializable attribute ensures that this property will be exported and imported to/from XAML when XamlFormatProvider is used. The other part of the code ensures that the field will be reset only if the new value is different and that the code will be invalidated, so as to be correctly updated on the next pass.

3. Declare a static FieldPropertyDefinition wired to your property:

public static readonly FieldPropertyDefinition MyPropertyProperty = new FieldPropertyDefinition("MyProperty");

Public Shared ReadOnly MyPropertyProperty As New FieldPropertyDefinition("MyProperty")

4. Make sure to initialize the FieldProperty in the constructor of your field:

public CustomField()
    this.myProperty = new FieldProperty(this, CustomField.MyPropertyProperty);

Public Sub New()
    Me._myProperty = New FieldProperty(Me, CustomField.MyPropertyProperty)
End Sub

5. Ensure that your custom property is copied along with the other field properties:

protected override void CopyPropertiesFromCodeExpression(FieldCodeExpression fieldCodeExpression)

Protected Overrides Sub CopyPropertiesFromCodeExpression(ByVal fieldCodeExpression As FieldCodeExpression)
End Sub

This method is used when a field is created from field fragment. This happens when you update a field and is what enables changing the property path of a merge field by typing in the editor or even changing a PageField to a DateField. As the type of the new field is inferred from the text in the document, it is required that the field be created anew and the respective properties must be copied as well.

protected override void BuildCodeOverride()

Protected Overrides Sub BuildCodeOverride()
End Sub

This method is invoked when the CodeFragment of the field is requested. In it, the field arguments and switches must be added, so that they are properly included in the field.

public override void CopyPropertiesFrom(Field fromField)
    CustomField customField = (CustomField)fromField;

Public Overrides Sub CopyPropertiesFrom(ByVal fromField As Field)
    Dim _customField As CustomField = CType(fromField, CustomField)
End Sub

As field ranges are copyable, this method must be overridden in order to ensure that the added properties will also be copied.

6. If you like, you can also customize the fragment that is returned when the ResultFragment is requested. This is done by overriding the GetResultFragment method. Here is an example:

protected override DocumentFragment GetResultFragment()
    return DocumentFragment.CreateFromInline(new Span(this.MyProperty));

Protected Overrides Function GetResultFragment() As DocumentFragment
    Return DocumentFragment.CreateFromInline(New Span(Me.MyProperty))
End Function

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