Question

In many places in our code we have collections of objects, from which we need to create a comma-separated list. The type of collection varies: it may be a DataTable from which we need a certain column, or a List<Customer>, etc.

Now we loop through the collection and use string concatenation, for example:

string text = ''; string separator = ''; foreach (DataRow row in table.Rows) {     text += separator + row['title'];     separator = ', '; } 

Is there a better pattern for this? Ideally I would like an approach we could reuse by just sending in a function to get the right field/property/column from each object.

Answer 1

// using System.Collections; // using System.Collections.Generic; // using System.Linq  public delegate string Indexer<T>(T obj);  public static string concatenate<T>(IEnumerable<T> collection, Indexer<T> indexer, char separator) {     StringBuilder sb = new StringBuilder();     foreach (T t in collection) sb.Append(indexer(t)).Append(separator);     return sb.Remove(sb.Length - 1, 1).ToString(); }  // version for non-generic collections public static string concatenate<T>(IEnumerable collection, Indexer<T> indexer, char separator) {     StringBuilder sb = new StringBuilder();     foreach (object t in collection) sb.Append(indexer((T)t)).Append(separator);     return sb.Remove(sb.Length - 1, 1).ToString(); }  // example 1: simple int list string getAllInts(IEnumerable<int> listOfInts) {     return concatenate<int>(listOfInts, Convert.ToString, ','); }  // example 2: DataTable.Rows string getTitle(DataRow row) { return row['title'].ToString(); } string getAllTitles(DataTable table) {     return concatenate<DataRow>(table.Rows, getTitle, '\n'); }  // example 3: DataTable.Rows without Indexer function string getAllTitles(DataTable table) {     return concatenate<DataRow>(table.Rows, r => r['title'].ToString(), '\n'); }