Why is this an issue?

In order to produce a formatted string, both string.Create and either FormattableString.Invariant or FormattableString.CurrentCulture can be used. However, string.Create rents array buffers from ArrayPool<char> making it more performant, as well as preventing unnecessary allocations and future stress on the Garbage Collector.

This applies to .NET versions after .NET 6, when these string.Create overloads were introduced.

What is the potential impact?

We measured a significant improvement both in execution time and memory allocation. For more details see the Benchmarks section from the More info tab.

How to fix it

Replace calls to FormattableString.CurrentCulture or FormattableString.Invariant with calls to string.Create(CultureInfo.CurrentCulture, …​) or string.Create(CultureInfo.InvariantCulture, …​) respectively.

Code examples

Noncompliant code example

string Interpolate(string value) =>
    FormattableString.Invariant($"Value: {value}");

string Interpolate(string value) =>
    FormattableString.CurrentCulture($"Value: {value}");

Compliant solution

string Interpolate(string value) =>
    string.Create(CultureInfo.InvariantCulture, $"Value: {value}");

string Interpolate(string value) =>
    string.Create(CultureInfo.CurrentCulture, $"Value: {value}");

Resources

Documentation

• string.Create
• FormattableString.Invariant
• FormattableString.CurrentCulture

Articles & blog posts

• Compilation of interpolated strings

Benchmarks

The results were generated by running the following snippet with BenchmarkDotNet:

int Value = 42;
DateTime Now = DateTime.UtcNow;

[Params(1_000_000)]
public int N;

[Benchmark]
public void StringCreate()
{
    for (int i = 0; i < N; i++)
    {
        _ = string.Create(CultureInfo.InvariantCulture, $"{Now}: Value is {Value}");
    }
}

[Benchmark]
public void FormattableStringInvariant()
{
    for (int i = 0; i < N; i++)
    {
        _ = FormattableString.Invariant($"{Now}: Value is {Value}");
    }
}

Hardware configuration:

BenchmarkDotNet=v0.13.5, OS=Windows 10 (10.0.19045.2728/22H2/2022Update)
11th Gen Intel Core i7-11850H 2.50GHz, 1 CPU, 16 logical and 8 physical cores
.NET SDK=7.0.203
  [Host]   : .NET 7.0.5 (7.0.523.17405), X64 RyuJIT AVX2
  .NET 7.0 : .NET 7.0.5 (7.0.523.17405), X64 RyuJIT AVX2