C# Memory Management and Garbage Collection

Memory Fundamentals

The .NET runtime manages memory automatically through the garbage collector (GC). Understanding how it works helps you write efficient code and avoid memory-related issues.

Stack vs Heap

public void MemoryExample()
{
    // Stack allocation - value types and references
    int count = 42;           // Value stored on stack
    double price = 19.99;     // Value stored on stack

    // Heap allocation - objects
    var customer = new Customer();  // Reference on stack, object on heap
    string name = "Alice";          // Reference on stack, string on heap
    int[] numbers = new int[100];   // Reference on stack, array on heap
}

Stack

Allocation: Very fast (pointer move)
Deallocation: Automatic (scope exit)
Size: Small (~1MB per thread)
Lifetime: Method scope
Content: Value types, references

Heap

Allocation: Slower (GC managed)
Deallocation: GC determines when
Size: Large (limited by RAM)
Lifetime: GC decides
Content: Objects, arrays

The Managed Heap

.NET divides the managed heap into generations based on object lifetime:

Generation 0 (Gen0): Newly allocated objects. Most objects die young.
Generation 1 (Gen1): Survived one GC. Buffer between Gen0 and Gen2.
Generation 2 (Gen2): Long-lived objects. Expensive to collect.
Large Object Heap (LOH): Objects >= 85,000 bytes. Collected with Gen2.
Pinned Object Heap (POH): .NET 5+. Pinned objects to avoid fragmentation.

// Check which generation an object is in
var obj = new byte[1000];
int generation = GC.GetGeneration(obj);  // Usually 0 for new objects

// After surviving collections
GC.Collect(0);  // Gen0 collection
generation = GC.GetGeneration(obj);  // Now in Gen1

How Garbage Collection Works

GC Triggers

Garbage collection runs when:

Gen0 threshold reached (most common)
System memory pressure
GC.Collect() called explicitly
Application is idle (workstation GC)

Collection Process

Mark: Identify live objects by tracing from roots (statics, stack, CPU registers)
Sweep/Compact: Remove dead objects, compact memory (except LOH by default)
Promote: Move surviving objects to next generation

// GC roots include:
// - Static variables
// - Local variables on stack
// - CPU registers
// - Finalization queue
// - GC handles (GCHandle)

public class RootExample
{
    private static Customer? _staticCustomer;  // GC root

    public void Method()
    {
        var local = new Customer();  // GC root while in scope
        _staticCustomer = local;     // Now rooted by static field
    }  // local goes out of scope, but object still rooted by static
}

GC Modes

// Check current GC settings
bool isServer = GCSettings.IsServerGC;
GCLatencyMode latency = GCSettings.LatencyMode;

// Server GC: One heap per CPU core, parallel collection
// Workstation GC: Single heap, concurrent collection

// Configure in project file
// <ServerGarbageCollection>true</ServerGarbageCollection>

Mode	Best For	Characteristics
Workstation	Desktop apps	Lower latency, one heap
Server	Web servers	Higher throughput, parallel
Concurrent	UI apps	Background collection
Background	Most apps	Default, non-blocking Gen2

Latency Modes

// Temporarily suppress GC for latency-critical sections
var oldMode = GCSettings.LatencyMode;
try
{
    GCSettings.LatencyMode = GCLatencyMode.LowLatency;
    PerformLatencyCriticalWork();
}
finally
{
    GCSettings.LatencyMode = oldMode;
}

// Available modes:
// - Batch: Max throughput, full blocking collections
// - Interactive: Default, balanced
// - LowLatency: Minimize pauses (may increase memory)
// - SustainedLowLatency: Long-term low latency
// - NoGCRegion: Prevent GC entirely (limited allocation)

IDisposable and Resource Management

The GC handles memory, but unmanaged resources (files, connections, handles) need explicit cleanup.

The Dispose Pattern

public class ResourceHolder : IDisposable
{
    private FileStream? _fileStream;
    private bool _disposed;

    public ResourceHolder(string path)
    {
        _fileStream = new FileStream(path, FileMode.Open);
    }

    public void DoWork()
    {
        ObjectDisposedException.ThrowIf(_disposed, this);
        // Use _fileStream
    }

    public void Dispose()
    {
        Dispose(disposing: true);
        GC.SuppressFinalize(this);  // No need for finalizer
    }

    protected virtual void Dispose(bool disposing)
    {
        if (_disposed) return;

        if (disposing)
        {
            // Dispose managed resources
            _fileStream?.Dispose();
            _fileStream = null;
        }

        // Free unmanaged resources here (rare)

        _disposed = true;
    }
}

// Usage
using var holder = new ResourceHolder("file.txt");
holder.DoWork();
// Automatically disposed at end of scope

Finalizers (Destructors)

Finalizers are a safety net for unmanaged resources if Dispose isn’t called. They have significant performance cost.

public class UnmanagedWrapper : IDisposable
{
    private IntPtr _handle;  // Unmanaged resource
    private bool _disposed;

    public UnmanagedWrapper()
    {
        _handle = NativeMethods.CreateResource();
    }

    ~UnmanagedWrapper()  // Finalizer
    {
        Dispose(disposing: false);
    }

    public void Dispose()
    {
        Dispose(disposing: true);
        GC.SuppressFinalize(this);  // Don't run finalizer
    }

    protected virtual void Dispose(bool disposing)
    {
        if (_disposed) return;

        if (disposing)
        {
            // Dispose managed resources
        }

        // Always free unmanaged resources
        if (_handle != IntPtr.Zero)
        {
            NativeMethods.CloseResource(_handle);
            _handle = IntPtr.Zero;
        }

        _disposed = true;
    }
}

Finalizer Costs

Objects with finalizers survive Gen0 collection (promoted to Gen1)
Finalizers run on dedicated thread (delays cleanup)
Finalizer exceptions can crash the app
Use only when wrapping unmanaged resources directly

Prefer SafeHandle over manual finalizers whenever possible.

// Prefer SafeHandle over manual finalizers
public class SafeResourceHandle : SafeHandleZeroOrMinusOneIsInvalid
{
    public SafeResourceHandle() : base(true) { }

    protected override bool ReleaseHandle()
    {
        return NativeMethods.CloseResource(handle);
    }
}

Memory Allocation Patterns

Reducing Allocations

// BAD: Allocates new string each call
public string GetGreeting(string name)
{
    return $"Hello, {name}!";  // String allocation
}

// GOOD: Use Span for parsing without allocation
public int ParseNumber(ReadOnlySpan<char> input)
{
    int index = input.IndexOf(':');
    var numberSpan = input[(index + 1)..].Trim();
    return int.Parse(numberSpan);  // No string allocation
}

// BAD: LINQ creates many small allocations
public int SumEven(int[] numbers)
{
    return numbers.Where(n => n % 2 == 0).Sum();  // Allocates enumerator
}

// GOOD: Manual loop avoids allocations
public int SumEvenNoAlloc(int[] numbers)
{
    int sum = 0;
    foreach (var n in numbers)
        if (n % 2 == 0) sum += n;
    return sum;
}

ArrayPool for Temporary Buffers

// BAD: Frequent allocation of temporary arrays
public byte[] ProcessData(Stream source)
{
    var buffer = new byte[4096];  // Allocation
    source.Read(buffer, 0, buffer.Length);
    return Transform(buffer);
}

// GOOD: Rent from pool
public byte[] ProcessDataPooled(Stream source)
{
    byte[] buffer = ArrayPool<byte>.Shared.Rent(4096);
    try
    {
        int read = source.Read(buffer, 0, 4096);
        return Transform(buffer.AsSpan(0, read));
    }
    finally
    {
        ArrayPool<byte>.Shared.Return(buffer);
    }
}

Object Pooling

using Microsoft.Extensions.ObjectPool;

// Configure pool
var policy = new DefaultPooledObjectPolicy<StringBuilder>();
var pool = new DefaultObjectPool<StringBuilder>(policy, maximumRetained: 100);

// Use pooled object
public string BuildReport(IEnumerable<Item> items)
{
    var sb = pool.Get();
    try
    {
        foreach (var item in items)
            sb.AppendLine($"{item.Name}: {item.Value}");
        return sb.ToString();
    }
    finally
    {
        sb.Clear();
        pool.Return(sb);
    }
}

Value Types to Avoid Heap Allocation

// Class - allocated on heap
public class PointClass
{
    public int X { get; set; }
    public int Y { get; set; }
}

// Struct - allocated on stack (when local) or inline
public struct PointStruct
{
    public int X { get; set; }
    public int Y { get; set; }
}

// Record struct combines value semantics with record features
public readonly record struct PointRecord(int X, int Y);

// Array of structs: single allocation, values inline
PointStruct[] structArray = new PointStruct[1000];  // One allocation

// Array of classes: 1001 allocations (array + each object)
PointClass[] classArray = new PointClass[1000];
for (int i = 0; i < 1000; i++)
    classArray[i] = new PointClass();  // 1000 additional allocations

Large Object Heap

Objects >= 85,000 bytes go to the LOH. Different collection rules apply.

// LOH threshold
const int LohThreshold = 85_000;

// This goes to SOH (Small Object Heap)
var smallArray = new byte[84_000];

// This goes to LOH
var largeArray = new byte[86_000];

// LOH considerations:
// - Collected with Gen2 (expensive)
// - Not compacted by default (fragmentation)
// - Survives longer in memory

// Enable LOH compaction (use sparingly)
GCSettings.LargeObjectHeapCompactionMode =
    GCLargeObjectHeapCompactionMode.CompactOnce;
GC.Collect();  // Compact happens on next collection

Avoiding LOH Fragmentation

// Strategy 1: Use ArrayPool for large buffers
var buffer = ArrayPool<byte>.Shared.Rent(100_000);
try
{
    // Use buffer
}
finally
{
    ArrayPool<byte>.Shared.Return(buffer);
}

// Strategy 2: Pre-allocate and reuse
public class LargeBufferPool
{
    private readonly byte[][] _buffers;
    private int _index;

    public LargeBufferPool(int count, int size)
    {
        _buffers = new byte[count][];
        for (int i = 0; i < count; i++)
            _buffers[i] = new byte[size];
    }

    public byte[] Rent() => _buffers[_index++ % _buffers.Length];
}

Memory Diagnostics

Monitoring GC

// GC statistics
int gen0Collections = GC.CollectionCount(0);
int gen1Collections = GC.CollectionCount(1);
int gen2Collections = GC.CollectionCount(2);
long totalMemory = GC.GetTotalMemory(forceFullCollection: false);

// Detailed info
GCMemoryInfo info = GC.GetGCMemoryInfo();
Console.WriteLine($"Heap size: {info.HeapSizeBytes}");
Console.WriteLine($"Fragmented: {info.FragmentedBytes}");
Console.WriteLine($"High memory: {info.HighMemoryLoadThresholdBytes}");

// Generation sizes
foreach (var genInfo in info.GenerationInfo)
{
    Console.WriteLine($"Gen{genInfo.Generation}: {genInfo.SizeAfterBytes}");
}

Finding Memory Leaks

Common leak patterns:

Event handlers not unsubscribed
Static collections growing unbounded
Closures capturing objects unintentionally
Circular references with weak references

// LEAK: Event handler keeps subscriber alive
public class Publisher
{
    public event EventHandler? DataChanged;
}

public class Subscriber
{
    public Subscriber(Publisher pub)
    {
        pub.DataChanged += OnDataChanged;  // Publisher references Subscriber
    }

    private void OnDataChanged(object? sender, EventArgs e) { }
}

// FIX: Unsubscribe or use weak events
public class SafeSubscriber : IDisposable
{
    private readonly Publisher _publisher;

    public SafeSubscriber(Publisher pub)
    {
        _publisher = pub;
        _publisher.DataChanged += OnDataChanged;
    }

    public void Dispose()
    {
        _publisher.DataChanged -= OnDataChanged;
    }
}

WeakReference for Caches

public class WeakCache<TKey, TValue> where TKey : notnull where TValue : class
{
    private readonly Dictionary<TKey, WeakReference<TValue>> _cache = new();

    public void Add(TKey key, TValue value)
    {
        _cache[key] = new WeakReference<TValue>(value);
    }

    public TValue? Get(TKey key)
    {
        if (_cache.TryGetValue(key, out var weakRef))
        {
            if (weakRef.TryGetTarget(out var value))
                return value;

            _cache.Remove(key);  // Clean up dead reference
        }
        return null;
    }
}

GC Control (Use Sparingly)

// Force collection (rarely needed)
GC.Collect();                    // All generations
GC.Collect(0);                   // Gen0 only
GC.Collect(2, GCCollectionMode.Optimized);  // Let GC decide

// Wait for finalizers
GC.WaitForPendingFinalizers();

// No-GC region for real-time scenarios
if (GC.TryStartNoGCRegion(1024 * 1024))  // 1MB allocation budget
{
    try
    {
        PerformRealTimeWork();
    }
    finally
    {
        GC.EndNoGCRegion();
    }
}

// Keep object alive past last use
void ProcessWithHandle(object resource)
{
    var handle = CreateHandle(resource);
    Process(handle);
    GC.KeepAlive(resource);  // Ensure resource not collected during Process
}

Best Practices

Do

Let GC manage memory automatically
Use using statements for IDisposable resources
Pool frequently allocated temporary objects
Prefer value types for small, immutable data
Use Span for slicing without allocation

Don’t

Call GC.Collect() without profiling justification
Implement finalizers unless wrapping unmanaged resources directly
Hold references longer than needed
Allocate large objects frequently
Ignore memory warnings from profilers

Key Takeaways

Trust the GC: It’s highly optimized. Manual intervention rarely helps and often hurts.

Reduce allocations: Fewer allocations means less GC work. Use pooling, Span, and value types strategically.

Dispose deterministically: Use using for unmanaged resources. Don’t rely on finalizers for cleanup.

Profile before optimizing: Use profilers to identify actual memory issues before adding complexity.

Watch for leaks: Event handlers, static collections, and captured closures are common leak sources.

LOH awareness: Large objects have different lifecycle. Pool them or break into smaller chunks when possible.