Join list and single string

Question

Why this input:

var list = new List<string>()
{
    "a","b","c"
};

var out1 = string.Join(", ", list.Select(t => t));
var out2 = string.Join(", ", list.Select(t => t), "d");

Console.WriteLine("out1 = " + out1);
Console.WriteLine("out2 = " + out2);

gives that output:

out1 = a, b, c
out2 = System.Linq.Enumerable+WhereSelectListIterator`2[System.String,System.String], d

I don't understand why I got:

System.Linq.Enumerable+WhereSelectListIterator`2[System.String,System.String]

in out2. Any ideas?

Answer 1

Your second example is calling this overload:

public static string Join(string? separator, params object?[] values);

So your second and third argument are being passed as an object[] here:

var out2 = string.Join(", ", list.Select(t => t), "d");

string.Join is internally calling ToString() on each item in the array, hence the output.

---

If you want to append a single string to the list, you could use Enumerable.Append:

var out2 = string.Join(", ", list.Append("d"));

Which would call this overload:

public static string Join(
    string? separator, System.Collections.Generic.IEnumerable<string?> values);

Answer 2

it's because you're using two different overloads of string.Join() in the first one you're using the one which accepts an IEnumerable such as the one you obtain from list.Select(t => t) , the second one the compiler tries to use the one that wants an array of objects as the extra parameters, then it joins the two objects you are passing: the list.Select(t => t) and "d", i guess it uses .ToString() on both, and you get the Type name for the first