test.cs

string jsonData = System.IO.File.ReadAllText("data.json");

using (MailMerge mm = new MailMerge() { TextComponent = textControl1 }) {
  mm.FieldMerged += Mm_FieldMerged;
  mm.RemoveEmptyFields = true;
  mm.RemoveEmptyLines = true;
  mm.MergeJsonData(jsonData);
}