Consider the following comma separated list that is created dynamically using a merge block:

Pay attention to the last circle where the comma is intentionally missing. In you define the comma as part of the TextAfter property of a MergeField, the comma would be added to the last entry in the merge block as well.

As a result, an unwanted comma is added at the end of the list.

In order to solve that problem, the flexible events of MailMerge are used:

• BlockMerging
• FieldMerged
• BlockRowMerged

For merge blocks, these events are fired in the following order:

To define conditional text after processing, a unique keyword can be defined that is part of the TextAfter property:

The TextBefore property value starts with %REMOVELAST%: followed by the actual string that should be rendered. In our case the comma (",").

When merging the template with data, the above events are attached before calling one of the merge methods:

using (MailMerge mm = new MailMerge()) {
  mm.TextComponent = textControl1;

  // attach events
  mm.BlockMerging += Mm_BlockMerging;
  mm.FieldMerged += Mm_FieldMerged;
  mm.BlockRowMerged += Mm_BlockRowMerged;

  // merge template
  mm.MergeJsonData(jsonData);
}

The flag bLastBlockRow that indicates that the last row of a block is processed is reset for each new merge block:

bool bLastBlockRow = false;

private void Mm_BlockMerging(object sender, MailMerge.BlockMergingEventArgs e) {
  // reset counter for new block
  bLastBlockRow = false;
}

bLastBlockRow is updated in the BlockRowMerged event:

private void Mm_BlockRowMerged(object sender, MailMerge.BlockRowMergedEventArgs e) {
  // before last row gets merged
  if (e.DataRowNumber == e.DataRowCount - 2) {
    bLastBlockRow = true;
  }
}

The actual processing is done in the FieldMerged event. If the field contains the unique keyword and the last row of the merge block is being processed, the complete text after is removed. Otherwise, it is rendered.

private void Mm_FieldMerged(object sender, MailMerge.FieldMergedEventArgs e) {

  var keyword = "%REMOVELAST%:";

  // return if field is outside block
  if (e.MergeBlockName == "" || e.MailMergeFieldAdapter.TypeName != "MERGEFIELD")
    return;

  // convert to the MergeField
  MergeField field = ((MergeField)e.MailMergeFieldAdapter);

  // if "text after" contains keyword
  if (field.TextAfter.StartsWith(keyword)) {
    if (bLastBlockRow == true) { // remove "text after" when last row
      field.TextAfter = "";
    }
    else { // else keep "text after" string
      field.TextAfter = field.TextAfter.Substring(
        keyword.Length, 
        field.TextAfter.Length - keyword.Length);
    }
  }

}