Concat all strings inside a List<string> using LINQ

String.Join(delimiter, list);

is sufficient.

Warning - Serious Performance Issues

Though this answer does produce the desired result, it suffers from poor performance compared to other answers here. Be very careful about deciding to use it

By using LINQ, this should work;

string delimiter = ",";
List<string> items = new List<string>() { "foo", "boo", "john", "doe" };
Console.WriteLine(items.Aggregate((i, j) => i + delimiter + j));

class description:

public class Foo
{
    public string Boo { get; set; }
}

Usage:

class Program
{
    static void Main(string[] args)
    {
        string delimiter = ",";
        List<Foo> items = new List<Foo>() { new Foo { Boo = "ABC" }, new Foo { Boo = "DEF" },
            new Foo { Boo = "GHI" }, new Foo { Boo = "JKL" } };

        Console.WriteLine(items.Aggregate((i, j) => new Foo{Boo = (i.Boo + delimiter + j.Boo)}).Boo);
        Console.ReadKey();

    }
}

And here is my best :)

items.Select(i => i.Boo).Aggregate((i, j) => i + delimiter + j)

^{Note: This answer does not use LINQ to generate the concatenated string. Using LINQ to turn enumerables into delimited strings can cause serious performance problems}

Modern .NET (since .NET 4)

This is for an array, list or any type that implements IEnumerable:

string.Join(delimiter, enumerable);

And this is for an enumerable of custom objects:

string.Join(delimiter, enumerable.Select(i => i.Boo));

Old .NET (before .NET 4)

This is for a string array:

string.Join(delimiter, array);

This is for a List<string>:

string.Join(delimiter, list.ToArray());

And this is for a list of custom objects:

string.Join(delimiter, list.Select(i => i.Boo).ToArray());

using System.Linq;

public class Person
{
  string FirstName { get; set; }
  string LastName { get; set; }
}

List<Person> persons = new List<Person>();

string listOfPersons = string.Join(",", persons.Select(p => p.FirstName));