How to trim a string without additional memory allocations in C#?

Question

I have a string input. I want to remove the last symbol from it. I can do it in this way:

static void Main(string[] args)
{
    string input = "Hello World-";
    string result = input.TrimEnd('-');
    Console.WriteLine(result); // Hello World
}

It works, but TrimEnd() calls System.String.FastAllocateString() internally. It makes sense, because string is immutable data structure, and in common case we cannot do anything else. However, in my case I don't need input anymore, so I would like to reuse its internal buffer, rather than allocate additional buffer, and "ask" GC to clean input buffer eventually. It could reduce overall allocations, and reduce GC work.

Answer 1

Console.Out.WriteLine does not accept a ReadOnlySpan<char> as input, but Console.Out, which is a TextWriter, does expose such a method. Therefore, the correct way to avoid allocations is:

ReadOnlySpan<char> input = "Hello World-";
Console.Out.WriteLine(input.TrimEnd('-'));

// or

// Uses an indexer to always remove the last character:
Console.Out.WriteLine(input[..^1]);

---

If you need a string as result and you have to apply several string operations on the input, then using a ReadOnlySpan<char> will save you repeated allocations on the intermediate results, but in the end you will have to allocate a new string and the possible string operations are very limited. Unless you are willing to use unsafe code or to make a detour via COM code, there is no way to break this rule.

Alternatively, you can use a StringBuilder for string manipulations. But this will allocate a char[] buffer and ToString calls FastAllocateString as well. See Reference Source for StringBuilder.

Answer 2

This is what the ReadOnlySpan struct & System.Memory namespace are for

ReadOnlySpan<char> input = "Hello World-";
string result = new string(input.TrimEnd('-'));
Console.WriteLine(result);