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ToListForEach

Diagnostic Rule Overview​

FieldValue
IDSHIMMER1010
Analyzer title.ToList().ForEach() causes unnecessary memory allocation
Analyzer messageReplace .ToList().ForEach() with a foreach loop to reduce memory usage
Code fix titleReplace with a foreach loop
Default severityWarning
Minimum framework/language versionN/A
CategoryShimmeringUsage
Link to codeToListForEachAnalyzer.cs
Code fix exists?Yes

Detailed Explanation​

Avoid converting an IEnumerable<T> to a List<T> just to use the List<T>.ForEach() method. The unnecessary .ToList() call causes extra memory allocation, while a simple foreach loop achieves the same result without this overhead. If you do want to a list, you can work around this diagnostic by using a plain foreach loop.

Note that this diagnostic is not triggered if the preceding enumerable implements IQueryable<T>, as using .ToList() for materialization is a valid use case.

Examples​

Flagged code:

using System;
using System.Linq;

namespace Tests;
class Test
{
    void Do(int[] numbers)
    {
        numbers.ToList().ForEach(n => Console.WriteLine(n));
    }
}

Fixed code:

using System;
using System.Linq;

namespace Tests;
class Test
{
    void Do(int[] numbers)
    {
        foreach (var n in numbers)
        {
            Console.WriteLine(n);
        }
    }
}

Justification of the Severity​

While this is not a bug, this will slow down your code and increase memory usage with no benefits. As a reminder, the diagnostic doesn't flag if the preceding enumerable implements IQueryable<T>, which is likely the main case where you'd actually want to allocate a list.

Inspiration​

This was inspired by @Treit's blog post https://mtreit.com/programming,/.net/2024/07/30/ToList.html. The same material was also covered in this YouTube video.