## Why is this an issue?

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

    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 `Substring` method. When chaining it with any of the following methods, it creates a new object for one-time use only:

- `IndexOf`
- `IndexOfAny`
- `LastIndexOf`
- `LastIndexOfAny`

### What is the potential impact?

Creating a new object consumes significant 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:

    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

- `String.Substring` Method
- `String.IndexOf` Method
- `String.IndexOfAny` Method
- `String.LastIndexOf` Method
- `String.LastIndexOfAny` Method

### Benchmarks

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 BenchmarkDotNet:

    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:

    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