Question

I am trying to get all the rows that exist in allData but not in removeData

public static DataTable RemoveDuplicateRows(DataTable allData, 
    DataTable removeData) 
{
    removeData.Merge(allData);
    DataTable newData = removeData.GetChanges(); 
    removeData.RejectChanges();
    return newData;
}

removeData is empty prior to the call in this case (just a new DataTable();)

But newData always has a value of null after the DataTable newData = removeData.GetChanges(); line

Final Solution:

    public static DataTable RemoveDuplicateRows(DataTable allData, DataTable removeData) 
    {
        DataTable duplicate = allData.Clone();
        foreach (DataRow row in allData.Rows)
        {
            duplicate.ImportRow(row);
        }
        foreach (DataRow row in duplicate.Rows)
        {
            row.SetAdded();
        } 

        removeData.Merge(duplicate);
        DataTable newData = removeData.GetChanges(DataRowState.Added);
        removeData.RejectChanges();
        allData.RejectChanges();
        return newData;
    }

Answer 1

Your removeData DataTable needs to have the same columns/fields as allData. In other words, it can’t just be a new DataTable().