Thanks for your inquiry. Implement IFieldMergingCallback interface if you want to control how data is inserted into merge fields during a mail merge operation. In your case, the FirstCap switch does not work. I suggest you following solution for your scenario.
Thanks for your inquiry. Generally, Aspose.Words' mail merge engine supports formatting switches specified in merge fields. However, please note that the FieldMergingArgs.Text property inserts the text in document as it is without applying the switches. Switches only work when constructing field's result during normal mail merge process. So, to get the desired result, you will need to build the same "string" manually and pass it to FieldMergingArgs.Text property. Please let us know if we can be of any further assistance.