Aspose.Words: suppling default values from non-existent data source fields during merge

Aspose.Words Product Family
1

Aspose version: Aspose Words.NET version 23.6.0
Environment: Windows and Linux

Query on merging from a XML data source and suppling a default merge value when the XML does not contain the data field for the corresponding Word merge field.

As an example:

On merging the merge field {MERGEFIELD FIELD1}, when the data field does not exist:

<root>
</root>

… results in an unmerged merge field - which is to be expected.

What we would like is for the merge to react ‘as if’ the XML had defined the field, but the field is an empty value, like below:

<root>
	<FIELD1></FIELD1>
</root>

We do not want to (can’t) remove the fields using Document.MailMerge.DeleteFields() or by specifying MailMergeCleanupOptions.RemoveUnusedFields - we need to supply the default value.

The documentation suggests we can ‘intercept’ the data source field value in the FieldMergingArgs object raised when implementing IFieldMergingCallback and change it.

I experimented with:

public void FieldMerging(FieldMergingArgs args)
{
	if (args.FieldValue == null)
	{
		args.FieldValue = string.Empty;
	}
}

Question 1: is args.FieldValue == null the correct way to check for a missing field declaration in the XML?
Question 2: the above code changes the args.FieldValue value in debug, but the merge field in the document remains unmerged, implying the added field value is not coming through?
Question 3: is there a way of doing this please? The ‘missing’ field data could be at the top level or within regions.

Thanks in advance,

James

2

@JamesMurdoch

To supply default values for non-existent data source fields during a mail merge in Aspose.Words, you can utilize the IFieldMergingCallback interface. This allows you to intercept the merging process and provide a default value when the corresponding field is not found in the data source.

Answer to Your Questions:

Question 1:
To check if a field declaration is missing in the XML, you can indeed use args.FieldValue == null. This condition will help you identify when the field is not present in the data source.

Question 2:
If you are setting args.FieldValue to an empty string but the merge field remains unmerged, ensure that you are also setting args.Text to an empty string. This is crucial because if you do not suppress the default behavior of inserting the original field value, the merge engine will still attempt to insert the unmodified field value, which may lead to the field remaining unmerged.

Question 3:
To implement this correctly, you can modify your FieldMerging method as follows:

public void FieldMerging(FieldMergingArgs args)
{
    if (args.FieldValue == null)
    {
        args.FieldValue = string.Empty; // Set default value
        args.Text = string.Empty; // Suppress default text insertion
    }
}

This way, when the field is not found in the XML, it will behave as if the field exists but is empty, thus allowing the merge to proceed without leaving unmerged fields.

Additional Considerations:

  • Ensure that your XML data source is structured correctly, even if it does not contain values for certain fields. For example, having an empty tag like <FIELD1></FIELD1> can help in testing the behavior.
  • If you are working with regions, make sure that the logic accounts for fields that may be nested within those regions.

By following these steps, you should be able to achieve the desired behavior during the mail merge process.

Sources:

3

@JamesMurdoch

  1. Yes, it is correct way to detect missed values.

  2. Use FieldMergingArgs.Text instead:

public void FieldMerging(FieldMergingArgs args)
{
    if (args.FieldValue == null)
    {
        args.Text = "default value";
    }
}
  1. Alternatively you can loop through the non-merged fields and inert the default value.
DataSet ds = new DataSet();
ds.ReadXml(@"C:\Temp\data.xml");
        
Document doc = new Document(@"C:\Temp\in.docx");
doc.MailMerge.Execute(ds.Tables[0]);
            
DocumentBuilder builder = new DocumentBuilder(doc);
foreach (string name in doc.MailMerge.GetFieldNames())
{
    builder.MoveToMergeField(name);
    builder.Write("missed");
}

doc.Save(@"C:\Temp\out.docx");
4

Thank you Alexey,

As a follow-up point / question - which might be relevant. We are using MailMergeCleanupOptions.RemoveContainingFields as part of the merge to also remove IF statements from the merged document.

The ‘missing’ merge field could also be part of the IF the condition (hence why we cannot handle the missing merge fields after the merge as per your second kind alternative): i.e.

{IF {MERGEFIELD FIELD1} = "a value" "TRUE" " FALSE"}

Question: With that in mind, should we simply set args.Text as per your example:

public void FieldMerging(FieldMergingArgs args)
{
    if (args.FieldValue == null)
    {
        args.Text = "default value (FOR US: string.Empty )";
    }
}

Or, use the example offered (by what I presume to be) the automated response above yours on this call?

public void FieldMerging(FieldMergingArgs args)
{
    if (args.FieldValue == null)
    {
        args.FieldValue = string.Empty; // Set default value
        args.Text = string.Empty; // Suppress default text insertion
    }
}

I ask the question because we don’t really have sight of any subtle nuances which might be going on ‘under-the-hood’ in the Aspose merge process between the two suggestions.

One of the potential nuances I was considering was the column data type. args.FieldValue can contain an object of any data type, but args.Text can only contain a string. We are loading up the XML straight into a DataSet like the Aspose documentation example:

DataSet customersDs = new DataSet();
customersDs.ReadXml(MyDir + "Mail merge data - Customers.xml");

So, I am assuming the column data types (in the DataSet) will always be treated as strings anyway and therefore args.FieldValue would always have a string object when we have data?

Unless that assumption is incorrect or the consideration is relevant, then the question is which of the examples kindly supplied should we follow?

Thank you again,

James

5

@JamesMurdoch When you set FieldMergingArgs.Text, the field is replaced with this text regardless formatting switches or text before/text after properties of the field.
When FieldMergingArgs.FieldValue is set, the value will be formatted according to the field format switches and properties. FieldMergingArgs.FieldValue will not work in your case, since the field is missed, so it is not actually filed with data and setting this property will ne have any effect.

FieldValue is not always treated as string, since it is required for properly format the value. For example date or double value.