ASP.NET Core  

Anti-Patterns to Avoid When Using Memory Pools in ASP.NET Core

Memory pooling is one of the most powerful performance optimizations in ASP.NET Core, but misuse can negate its benefits and even make your application worse than if you didn’t pool at all.

Even with .NET 10’s automatic pool trimming, these anti-patterns still cause runaway memory usage, GC pressure, and false memory-leak reports in real systems. As an ASP.NET Core architect, I’ve seen these mistakes repeatedly in production, so here’s a practical guide to avoiding them.

1. Holding Pooled Buffers in Static Fields

The Anti-Pattern

public static class BufferCache
{
    public static byte[] SharedBuffer =
        ArrayPool<byte>.Shared.Rent(1024 * 1024);
}

Why It’s Dangerous

  • Buffer is never returned

  • Pool trimming cannot reclaim it

  • Memory becomes permanently pinned

  • Mimics a memory leak in production

Correct Approach

byte[] buffer = ArrayPool<byte>.Shared.Rent(1024 * 1024);
try
{
    // Use buffer
}
finally
{
    ArrayPool<byte>.Shared.Return(buffer);
}

Rule: If you didn’t rent it inside the method, you probably shouldn’t hold it elsewhere.

2. Treating Pooled Objects as Reusable State

The Anti-Pattern

var buffer = ArrayPool<byte>.Shared.Rent(4096);
_myService.CurrentBuffer = buffer; // Storing business data

Why It Breaks Things

  • Pooled memory is not owned

  • Contents are undefined after return

  • Can cause data corruption

  • Breaks concurrency safety

Correct Approach

  • Copy persistent data out of the buffer

  • Treat pooled memory as temporary scratch space only

3. Forgetting to Return Buffers on Exception Paths

The Anti-Pattern

var buffer = ArrayPool<byte>.Shared.Rent(8192);
Process(buffer); // Exception may occur
ArrayPool<byte>.Shared.Return(buffer);

Why This Is Costly

  • Leaks buffers under load

  • Pool grows unnecessarily

  • GC pressure increases

  • Long-running services degrade over time

Correct Pattern

var buffer = ArrayPool<byte>.Shared.Rent(8192);
try
{
    Process(buffer);
}
finally
{
    ArrayPool<byte>.Shared.Return(buffer);
}

Rule: If it’s rented, it belongs in a try/finally.

4. Over-Pooling Small, Short-Lived Objects

The Anti-Pattern

byte[] buffer = ArrayPool<byte>.Shared.Rent(32);

Why This Is a Mistake

  • Small allocations are cheap

  • Pooling adds CPU and complexity overhead

  • Makes code harder to maintain

Better Approach

Span<byte> buffer = stackalloc byte[32];
// or
byte[] buffer = new byte[32];

Rule: Pool large or frequently reused buffers—not tiny ones.

5. Assuming Pools Eliminate the Need for Disposal

The Anti-Pattern

var stream = new MemoryStream(); // No disposal

Why This Is Wrong

  • Streams own internal buffers and native handles

  • GC finalization is expensive

  • Pooled memory may not be returned promptly

Correct Pattern

using var stream = new MemoryStream();
// Work with stream

Rule: Pooling does not replace Dispose().

6. Holding Request Buffers Beyond Request Lifetime

The Anti-Pattern

app.MapPost("/upload", async (HttpRequest request) =>
{
    _cachedBody = request.Body; // BAD
    return Results.Ok();
});

Why This Is Dangerous

  • Request buffers belong to ASP.NET Core

  • Invalid after request completes

  • Leads to undefined behavior

  • Prevents pool trimming

Correct Approach

using var ms = new MemoryStream();
await request.Body.CopyToAsync(ms);
var data = ms.ToArray(); // Copy if needed

7. Creating Custom Pools Without Strong Justification

The Anti-Pattern

private static readonly ConcurrentBag<byte[]> CustomPool = new();

Why This Is Risky

  • No trimming logic

  • No GC cooperation

  • Hard to maintain and debug

Better Alternative

  • Use ArrayPool or ASP.NET Core built-in pooling

  • Let .NET 10 handle trimming automatically

8. Clearing Buffers Unnecessarily

The Anti-Pattern

ArrayPool<byte>.Shared.Return(buffer, clearArray: true);

Why This Hurts Performance

  • Eagerly clears memory

  • Increases CPU usage

  • Usually unnecessary for transient data

When to Clear

  • Security-sensitive information (PII, secrets)

  • Cryptographic buffers

Rule: Clear only when security requires it.

9. Using Pooling to Mask Real Memory Leaks

The Anti-Pattern

“Let’s pool it so GC doesn’t run as much.”

Why This Fails

  • Pools hide leaks but do not fix them

  • Memory still grows under load

  • Production failures are delayed, not prevented

Correct Approach

  • Fix object lifetimes

  • Use diagnostic tools:

    • dotnet-counters

    • dotnet-dump

    • dotnet-trace

  • Verify Gen 2 heap behavior over time

10. Ignoring Idle Memory in Long-Running Services

The Anti-Pattern

“Memory usage is fine during peak traffic.”

Why This Is Shortsighted

  • Apps spend most time idle

  • Idle memory affects:

    • Container limits

    • Operational costs

    • Stability

.NET 10 Advantage

  • Idle pooled memory is now automatically reclaimed

  • But only if you follow good pooling practices

Rules of Thumb

  • Rent late, return early

  • Never store pooled memory outside its scope

  • Always use try/finally

  • Dispose everything

  • Let the runtime manage pools

  • Trust .NET 10—but don’t abuse it

Key Takeaway

Memory pooling is a performance optimization, not a memory ownership model.

.NET 10 makes pooling safer and more forgiving, but discipline still matters. Follow these rules, and your ASP.NET Core services will remain fast, stable, and predictable—exactly what you want in production.

I write about modern C#, .NET, and real-world development practices. Follow me on C# Corner for regular insights, tips, and deep dives.

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