== Why is this an issue?

It is important to be careful when searching for characters within a substring. Let's consider the following example:

[source,csharp,diff-id=1,diff-type=noncompliant]
----
if (str.SubString(startIndex).IndexOf(char1) == -1) // Noncompliant: a new string is going to be created by "Substring"
{
   // ...
}
----

A new `string` object is created with every call to the https://learn.microsoft.com/en-us/dotnet/api/system.string.substring[`Substring`] method. When chaining it with any of the following methods, it creates a new object for one-time use only:

* https://learn.microsoft.com/en-us/dotnet/api/system.string.indexof[`IndexOf`]
* https://learn.microsoft.com/en-us/dotnet/api/system.string.indexofany[`IndexOfAny`]
* https://learn.microsoft.com/en-us/dotnet/api/system.string.lastindexof[`LastIndexOf`]
* https://learn.microsoft.com/en-us/dotnet/api/system.string.lastindexofany[`LastIndexOfAny`]

=== What is the potential impact?

Creating a new object consumes significant https://en.wikipedia.org/wiki/System_resource[system resources], especially CPU and memory. When using `Substring` repeatedly, such as in a loop, it can greatly impact the overall performance of the application or program.

To mitigate the creation of new objects while searching for characters within a substring, it is possible to use an overload of the mentioned methods with a `startIndex` parameter:

[source,csharp,diff-id=1,diff-type=compliant]
----
if (str.IndexOf(char1, startIndex) == -1)           // Compliant: no new instance of string is created
{
   // ...
}
----

Using these methods gives the same result while avoiding the creation of additional `string` objects.

== Resources

=== Documentation

* https://learn.microsoft.com/en-us/dotnet/api/system.string.substring[`String.Substring` Method]
* https://learn.microsoft.com/en-us/dotnet/api/system.string.indexof[`String.IndexOf` Method]
* https://learn.microsoft.com/en-us/dotnet/api/system.string.indexofany[`String.IndexOfAny` Method]
* https://learn.microsoft.com/en-us/dotnet/api/system.string.lastindexof[`String.LastIndexOf` Method]
* https://learn.microsoft.com/en-us/dotnet/api/system.string.lastindexofany[`String.LastIndexOfAny` Method]

=== Benchmarks

[options="header"]
|========================================================================================================
| Method               | Runtime              | StringSize | Mean      | StdDev    | Ratio | Allocated
| SubstringThenIndexOf | .NET 7.0             | 10         | 11.234 ms | 0.1319 ms | 1.00  | 40000008 B
| IndexOfOnly          | .NET 7.0             | 10         | 2.477 ms  | 0.0473 ms | 0.22  | 2 B
| SubstringThenIndexOf | .NET Framework 4.6.2 | 10         | 8.634 ms  | 0.4195 ms | 1.00  | 48141349 B
| IndexOfOnly          | .NET Framework 4.6.2 | 10         | 1.724 ms  | 0.0346 ms | 0.19  | -
| SubstringThenIndexOf | .NET 7.0             | 100        | 14.651 ms | 0.2977 ms | 1.00  | 176000008 B
| IndexOfOnly          | .NET 7.0             | 100        | 2.464 ms  | 0.0501 ms | 0.17  | 2 B
| SubstringThenIndexOf | .NET Framework 4.6.2 | 100        | 13.363 ms | 0.2044 ms | 1.00  | 176518761 B
| IndexOfOnly          | .NET Framework 4.6.2 | 100        | 1.689 ms  | 0.0290 ms | 0.13  | -
| SubstringThenIndexOf | .NET 7.0             | 1000       | 78.688 ms | 2.4727 ms | 1.00  | 1528000072 B
| IndexOfOnly          | .NET 7.0             | 1000       | 2.456 ms  | 0.0397 ms | 0.03  | 2 B
| SubstringThenIndexOf | .NET Framework 4.6.2 | 1000       | 75.994 ms | 1.2650 ms | 1.00  | 1532637240 B
| IndexOfOnly          | .NET Framework 4.6.2 | 1000       | 1.699 ms  | 0.0216 ms | 0.02  | -
|========================================================================================================

The results were generated by running the following snippet with https://github.com/dotnet/BenchmarkDotNet[BenchmarkDotNet]:

[source,csharp]
----
private string theString;
private int iteration = 1_000_000;

[Params(10, 100, 1_000)]
public int StringSize { get; set; }

[GlobalSetup]
public void Setup() =>
    theString = new string('a', StringSize);

[Benchmark(Baseline = true)]
public void SubstringThenIndexOf()
{
    for (var i = 0; i < iteration; i++)
    {
        _ = theString.Substring(StringSize / 4).IndexOf('a');
    }
}

[Benchmark]
public void IndexOfOnly()
{
    for (var i = 0; i < iteration; i++)
    {
        _ = theString.IndexOf('a', StringSize / 4) - StringSize / 4;
    }
}
----

Hardware configuration:

[source]
----
BenchmarkDotNet=v0.13.5, OS=Windows 10 (10.0.19045.2965/22H2/2022Update)
12th Gen Intel Core i7-12800H, 1 CPU, 20 logical and 14 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
  .NET Framework 4.6.2 : .NET Framework 4.8 (4.8.4614.0), X64 RyuJIT VectorSize=256
----

include::../rspecator-dotnet.adoc[]
