CsvHelper - How to Create a CsvClassMap for a Column Not in the Class

Question

How do I add a column to my CsvClassMap for a property that doesn't exist in my class?

Class to Write to a File

public class Customer {
   public int ID { get; set; }
   public string Name { get; set; }
}

CsvClassMap

public sealed class CustomerMap : CsvClassMap<Customer>
{
    public CustomerMap() {
        Map(r => r.ID).Index(1)
        Map(r => r.Name).Index(2)
        //need a column called Rec ID that is simple concatenation of ID and Name
        Map(r => ???).Index(0).Name("Rec ID")
    }
}

Desired CSV Output

Rec ID,ID,Name

1Bob,1,Bob

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GitHub Issue Reference

Answer 1

When writing to CSV, I prefer to manually map objects to a class that perfectly matches my expected columns. It's very flexible, explicit, and no more code than needed to write a CSV class map. In your case it would look something like this

var customerRows = customers.Select(c => new {RecID = c.ID + c.Name, c.ID, c.Name });

Then you would write customerRows to the CSV.