Casting and TrimEnd

Question

http://www.dotnetperls.com/string-join
Following program is given in the above website. Please explain the reasons for the followings:
-In order to cast builder why it is not correct to use ( string )builder.
-TrimEnd({ ',' }) is doing the job instead of TrimEnd( new char[] { ',' })
Problem is highlited.
using System;
using System.Text;
class Program
{
static void Main()
{
string[] catSpecies = { "Aegean", "Birman", "Main Coon", "Nebulung" };
Console.WriteLine(CombineA(catSpecies));
Console.WriteLine(CombineB(catSpecies));
Console.Read();
}
///
/// Combine strings with commas.
///
static string CombineA(string[] arr)
{
return string.Join(",", arr);
}
///
/// Combine strings with commas.
///
static string CombineB(string[] arr)
{
StringBuilder builder = new StringBuilder();
foreach (string s in arr)
{
builder.Append(s).Append(",");
}
return builder .ToString(). TrimEnd(new char[] { ',' }) ;
}
}
/*
Aegean,Birman,Main Coon,Nebulung
Aegean,Birman,Main Coon,Nebulung
*/

Answer

Thank you for your explanation.

Vulpes · Answer

For a cast to string to work, the StringBuilder class would need to contain an explicit user-defined conversion (see   http://msdn.microsoft.com/en-us/library/xhbhezf4.aspx ).
However, it doesn't support such an conversion and so you have to use the ToString() method to convert a StringBuilder instance to a string.
The String.TrimEnd method takes a char 'params' array as its argument.
This means that you can either pass it a char array or you can pass one or more individual char arguments. If there's more than one, then the arguments should be separated by commas.