Function pointers are pointers that point to functions instead of data types. They can be used to allow variability in the function that is to be called, at run-time.
Data validation allows user to create a drop-down list and restrict values in the cell to these entries. Due to limitation Excel can't bind more than 256 characters programmatically. To bind more than 256 characters one can follow explained approach.
Creating the data validation constraint can be tricky and time taking in NPOI. I have shared some of my workable approach. These approach will give good idea to customize your own constraint types.