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Showing content from https://docs.microsoft.com/en-us/dotnet/fundamentals/code-analysis/quality-rules/ca1846 below:

CA1846: Prefer AsSpan over Substring - .NET

Property Value Rule ID CA1846 Title Prefer AsSpan over Substring Category Performance Fix is breaking or non-breaking Non-breaking Enabled by default in .NET 9 As suggestion Cause

The result of a call to one of the String.Substring overloads is passed to a method with an available overload that accepts ReadOnlySpan<Char>.

Rule description

Substring allocates a new string object on the heap and performs a full copy of the extracted text. String manipulation is a performance bottleneck for many programs. Allocating many small, short-lived strings on a hot path can create enough collection pressure to impact performance. The O(n) copies created by Substring become relevant when the substrings get large. The Span<T> and ReadOnlySpan<T> types were created to solve these performance problems.

Many APIs that accept strings also have overloads that accept a ReadOnlySpan<System.Char> argument. When such overloads are available, you can improve performance by calling AsSpan instead of Substring.

How to fix violations

To fix a violation of this rule, replace the call to string.Substring with a call to one of the MemoryExtensions.AsSpan extension methods.

using System;

public void MyMethod(string iniFileLine)
{
    // Violation
    int.TryParse(iniFileLine.Substring(7), out int x);
    int.TryParse(iniFileLine.Substring(2, 5), out int y);

    // Fix
    int.TryParse(iniFileLine.AsSpan(7), out int x);
    int.TryParse(iniFileLine.AsSpan(2, 5), out int y);
}

Imports System

Public Sub MyMethod(iniFileLine As String)
    Dim x As Integer
    Dim y As Integer

    ' Violation
    Integer.TryParse(iniFileLine.Substring(7), x)
    Integer.TryParse(iniFileLine.Substring(2, 5), y)

    ' Fix
    Integer.TryParse(iniFileLine.AsSpan(7), x)
    Integer.TryParse(iniFileLine.AsSpan(2, 5), y)
End Sub

When to suppress warnings

It is safe to suppress warnings from this rule if performance is not a concern.