Span<T>

This chapter will look closely to the Span<T> type. The whole idea behind Span<T> is to not allocate additional memory as it operates on the memory slice behind the given data type.

Substring from a string

Getting a substring of a string via the Substring method will create a new string and therefore a new allocation.

❌ Bad Using Substring to get a part of a string.

var text = "Hello World";
var hello = text.Substring(0, 5);
var hello2 = text[..5]; // Range indexer uses Substring under the hood

✅ Good Using AsSpan to get the underlying memory.

var text = "Hello World";
var hello = text.AsSpan().Slice(0, 5);
var hello2 = text.AsSpan()[..5]; 

Benchmark

[MemoryDiagnoser]
public class SubstringBenchmark
{
    private const string text = "Hello World";

    [Benchmark(Baseline = true)]
    public string Substring()
    {
        return text[..5];
    }

    [Benchmark]
    public ReadOnlySpan<char> SpanSlice()
    {
        return text.AsSpan()[..5];
    }
}

Results:

|    Method |      Mean |     Error |    StdDev | Ratio |  Gen 0 | Allocated | Alloc Ratio |
|---------- |----------:|----------:|----------:|------:|-------:|----------:|------------:|
| Substring | 9.9252 ns | 0.3611 ns | 1.0534 ns |  1.00 | 0.0076 |      32 B |        1.00 |
| SpanSlice | 0.1921 ns | 0.0428 ns | 0.0653 ns |  0.02 |      - |         - |        0.00 |