String Interpolation vs String.Format

Question

Is there a noticeable performance difference between using string interpolation:

myString += $"{x:x2}";

vs String.Format()?

myString += String.Format("{0:x2}", x);

I am only asking because ReSharper is prompting the fix, and I have been fooled before.

Answer 1

The answer is both yes and no. ReSharper is fooling you by not showing a third variant, which is also the most performant. The two listed variants produce equal IL code, but the following will indeed give a boost:

myString += $"{x.ToString("x2")}";

Full test code

using BenchmarkDotNet.Attributes;
using BenchmarkDotNet.Configs;
using BenchmarkDotNet.Diagnosers;
using BenchmarkDotNet.Diagnostics.Windows;
using BenchmarkDotNet.Running;

namespace StringFormatPerformanceTest
{
    [Config(typeof(Config))]
    public class StringTests
    {
        private class Config : ManualConfig
        {
            public Config() => AddDiagnoser(MemoryDiagnoser.Default, new EtwProfiler());
        }

        [Params(42, 1337)]
        public int Data;

        [Benchmark] public string Format() => string.Format("{0:x2}", Data);
        [Benchmark] public string Interpolate() => $"{Data:x2}";
        [Benchmark] public string InterpolateExplicit() => $"{Data.ToString("x2")}";
    }

    class Program
    {
        static void Main(string[] args)
        {
            var summary = BenchmarkRunner.Run<StringTests>();
        }
    }
}

Test results

|              Method | Data |      Mean |  Gen 0 | Allocated |
|-------------------- |----- |----------:|-------:|----------:|
|              Format |   42 | 118.03 ns | 0.0178 |      56 B |
|         Interpolate |   42 | 118.36 ns | 0.0178 |      56 B |
| InterpolateExplicit |   42 |  37.01 ns | 0.0102 |      32 B |
|              Format | 1337 | 117.46 ns | 0.0176 |      56 B |
|         Interpolate | 1337 | 113.86 ns | 0.0178 |      56 B |
| InterpolateExplicit | 1337 |  38.73 ns | 0.0102 |      32 B |

New test results (.NET 6)

Re-ran the test on .NET 6.0.9.41905, X64 RyuJIT AVX2.

|              Method | Data |      Mean |   Gen0 | Allocated |
|-------------------- |----- |----------:|-------:|----------:|
|              Format |   42 |  37.47 ns | 0.0089 |      56 B |
|         Interpolate |   42 |  57.61 ns | 0.0050 |      32 B |
| InterpolateExplicit |   42 |  11.46 ns | 0.0051 |      32 B |
|              Format | 1337 |  39.49 ns | 0.0089 |      56 B |
|         Interpolate | 1337 |  59.98 ns | 0.0050 |      32 B |
| InterpolateExplicit | 1337 |  12.85 ns | 0.0051 |      32 B |

The InterpolateExplicit() method is faster since we now explicitly tell the compiler to use a string. No need to box the object to be formatted. Boxing is indeed very costly. Also note that NET 6 reduced both CPU and memory allocations - for all methods.

New test results (.NET 7)

Re-ran the test on .NET 7.0.122.56804, X64 RyuJIT AVX2.

|              Method | Data |      Mean |   Gen0 | Allocated |
|-------------------- |----- |----------:|-------:|----------:|
|              Format |   42 |  41.04 ns | 0.0089 |      56 B |
|         Interpolate |   42 |  65.82 ns | 0.0050 |      32 B |
| InterpolateExplicit |   42 |  12.19 ns | 0.0051 |      32 B |
|              Format | 1337 |  41.02 ns | 0.0089 |      56 B |
|         Interpolate | 1337 |  59.61 ns | 0.0050 |      32 B |
| InterpolateExplicit | 1337 |  13.28 ns | 0.0051 |      32 B |

No significant changes since .NET 6.