Using LINQ to concatenate strings

This answer shows usage of LINQ (Aggregate) as requested in the question and is not intended for everyday use. Because this does not use a StringBuilder it will have horrible performance for very long sequences. For regular code use String.Join as shown in the other answer

Use aggregate queries like this:

string[] words = { "one", "two", "three" };
var res = words.Aggregate(
   "", // start with empty string to handle empty list case.
   (current, next) => current + ", " + next);
Console.WriteLine(res);

This outputs:

, one, two, three

An aggregate is a function that takes a collection of values and returns a scalar value. Examples from T-SQL include min, max, and sum. Both VB and C# have support for aggregates. Both VB and C# support aggregates as extension methods. Using the dot-notation, one simply calls a method on an IEnumerable object.

Remember that aggregate queries are executed immediately.

More information - MSDN: Aggregate Queries

If you really want to use Aggregate use variant using StringBuilder proposed in comment by CodeMonkeyKing which would be about the same code as regular String.Join including good performance for large number of objects:

 var res = words.Aggregate(
     new StringBuilder(), 
     (current, next) => current.Append(current.Length == 0? "" : ", ").Append(next))
     .ToString();

return string.Join(", ", strings.ToArray());

In .Net 4, there's a new overload for string.Join that accepts IEnumerable<string>. The code would then look like:

return string.Join(", ", strings);

Why use Linq?

string[] s = {"foo", "bar", "baz"};
Console.WriteLine(String.Join(", ", s));

That works perfectly and accepts any IEnumerable<string> as far as I remember. No need Aggregate anything here which is a lot slower.

Have you looked at the Aggregate extension method?

var sa = (new[] { "yabba", "dabba", "doo" }).Aggregate((a,b) => a + "," + b);

Real example from my code:

return selected.Select(query => query.Name).Aggregate((a, b) => a + ", " + b);

A query is an object that has a Name property which is a string, and I want the names of all the queries on the selected list, separated by commas.

I always use the extension method:

public static string JoinAsString<T>(this IEnumerable<T> input, string seperator)
{
    var ar = input.Select(i => i.ToString());
    return string.Join(seperator, ar);
}

There are various alternative answers at this previous question - which admittedly was targeting an integer array as the source, but received generalised answers.

You can combine LINQ and string.join() quite effectively. Here I am removing an item from a string. There are better ways of doing this too but here it is:

filterset = String.Join(",",
                        filterset.Split(',')
                                 .Where(f => mycomplicatedMatch(f,paramToMatch))
                       );