S6610 — "StartsWith" and "EndsWith" overloads that take a "char" should be used instead of the ones that take a "string"

Why is this an issue?

With string.StartsWith(char) and string.EndsWith(char), only the first character of the string is compared to the provided character, whereas the string versions of those methods have to do checks about the current StringComparison and CultureInfo. Thus, the char overloads are significantly faster for default comparison scenarios.

These overloads were introduced in .NET Core 2.0.

What is the potential impact?

We measured at least 3.5x improvement in execution time. For more details see the Benchmarks section from the More info tab.

How to fix it

If you are targeting a runtime version equal or greater than .NET Core 2.0, the string.StartsWith and string.EndsWith overloads are available, with the argument’s type being char instead of string. Thus, an argument of char type can be provided.

Code examples

Noncompliant code example

Function StartsWithSlash(s As String) As Boolean
    Return s.StartsWith("/")
End Function

Function EndsWithSlash(s As String) As Boolean
    Return s.EndsWith("/")
End Function

Compliant solution

Function StartsWithSlash(s As String) As Boolean
    Return s.StartsWith("/"c)
End Function

Function EndsWithSlash(s As String) As Boolean
    Return s.EndsWith("/"c)
End Function

Resources

Documentation

• string.StartsWith
• string.EndsWith

Benchmarks

Glossary

• Mean
• Standard Deviation

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

private List<string> data;

[Params(1_000_000)]
public int N { get; set; }

[GlobalSetup]
public void Setup() =>
    data = Enumerable.Range(0, N).Select(_ => Guid.NewGuid().ToString()).ToList();

[Benchmark]
public void StartsWith_String()
{
    _ = data.Where(guid => guid.StartsWith("d")).ToList();
}

[Benchmark]
public void StartsWith_Char()
{
    _ = data.Where(guid => guid.StartsWith('d')).ToList();
}

[Benchmark]
public void EndsWith_String()
{
    _ = data.Where(guid => guid.EndsWith("d")).ToList();
}

[Benchmark]
public void EndsWith_Char()
{
    _ = data.Where(guid => guid.EndsWith('d')).ToList();
}

Hardware configuration:

BenchmarkDotNet=v0.13.5, OS=Windows 10 (10.0.19045.2846/22H2/2022Update)
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