Why does using an array in EF Core LINQ queries instead of a list cause performance issues in .NET10 / C#14.0?

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Summary

The recent upgrade to .NET10 from .NET9 has introduced performance issues in existing EF Core LINQ queries. The root cause of this issue is the use of arrays instead of lists in these queries. In this article, we will explore the reasons behind this performance degradation and why switching to lists resolves the issue.

Root Cause

The root cause of this performance issue is due to the differences in how arrays and lists are handled by EF Core. Key factors include:

  • Array size limitations
  • Array element type restrictions
  • EF Core query translation mechanisms

Why This Happens in Real Systems

In real-world systems, this issue arises due to the following reasons:

  • Arrays are often used for simplicity and performance in older systems
  • EF Core query optimization mechanisms may not always handle arrays efficiently
  • Upgrades to newer frameworks like .NET10 can expose existing performance bottlenecks

Real-World Impact

The impact of this issue can be significant, including:

  • Performance degradation: slower query execution times
  • Increased latency: delayed response times for users
  • Resource utilization: increased CPU and memory usage

Example or Code

using Microsoft.EntityFrameworkCore;
using System;
using System.Collections.Generic;
using System.Linq;

public class MyEntity
{
    public int Id { get; set; }
    public string Name { get; set; }
}

public class MyContext : DbContext
{
    public DbSet MyEntities { get; set; }

    protected override void OnConfiguring(DbContextOptionsBuilder optionsBuilder)
    {
        optionsBuilder.UseInMemoryDatabase("MyDatabase");
    }
}

class Program
{
    static void Main()
    {
        using var context = new MyContext();

        // Using an array
        int[] array = { 1, 2, 3 };
        var resultArray = context.MyEntities.Where(e => array.Contains(e.Id)).ToList();

        // Using a list
        List list = new List { 1, 2, 3 };
        var resultList = context.MyEntities.Where(e => list.Contains(e.Id)).ToList();
    }
}

How Senior Engineers Fix It

Senior engineers fix this issue by:

  • Identifying performance bottlenecks: using profiling tools to detect slow queries
  • Optimizing EF Core queries: rewriting queries to use lists instead of arrays
  • Testing and verifying: ensuring the optimized queries resolve the performance issues

Why Juniors Miss It

Junior engineers may miss this issue due to:

  • Lack of experience: limited exposure to EF Core and LINQ query optimization
  • Insufficient knowledge: unfamiliarity with the differences between arrays and lists in EF Core
  • Inadequate testing: failure to thoroughly test and profile queries for performance issues