It hit us once while debugging a nasty crash—we’d leaned too hard on .NET’s automatic memory management. Assumed the garbage collector would save us. Thought Span would catch everything. But we were wrong. Deep in one of our async calls, unmanaged code had quietly slipped past our guardrails.
A silent buffer overflow, threading its way through the stack. That was when we realized: .NET’s safety is strong, sure, but not perfect. We’ve got to build fences, not just rely on the guard dog.
Key Takeaway
- Leverage .NET’s managed memory features like array bounds checking and Span types to prevent overruns.
- Minimize unsafe code and always validate buffer sizes when working with pointers or native interop.
- Use diagnostic tools and static analyzers regularly to catch memory issues early and enforce best practices.
Core Language Features for Memory Safety
Managed Code Memory Protection
We trust managed code, and most of the time, it deserves that trust. The Common Language Runtime (CLR) steps in, watches our allocations, collects garbage, and guards the edges of our arrays like a hawk. We’ve seen it catch out-of-bounds errors—like when a loop runs wild and pokes past the array’s edge. (1)
Our code might look like this:
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int[] scores = new int[4];
int danger = scores[6]; // CLR says no
That kind of mistake used to crash entire programs in unmanaged environments. In .NET, it throws IndexOutOfRangeException. It’s a lifesaver. And it’s not just arrays. Lists, spans, dictionaries—they all benefit from this silent guardian running beneath our code.
Still, if we drop into unsafe code (and sometimes we do), we strip those protections away. One missed check, one pointer misstep, and we’re back in memory corruption territory.
Span and Memory Types for Safe Buffer Access
We’ve leaned heavily on Span<T> and Memory<T>. These types are like tight-fitting gloves. They let us touch memory directly, but they make sure we don’t grab more than we should. Every access gets bounds-checked. Every loop has a line it can’t cross.
Here’s what we use when reading fast but safe:
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void Inspect(ReadOnlySpan<byte> payload)
{
for (int i = 0; i < payload.Length; i++)
{
var current = payload[i]; // Can’t run past the edge
}
}
We like ReadOnlySpan<T> for inspection, especially with incoming data. For write access, Span<T> works great. If we need to cross an async boundary, we switch to Memory<T>, since Span can’t live on the heap. That rule trips folks up, but it’s worth memorizing.
Our practice:
- Span<T> stays inside the method.
- Memory<T> goes where it’s needed, but not after await without a copy.
- ArrayPool<T>.Shared gets used when memory’s tight and performance counts.
Secure Coding Practices
Input Validation Frameworks
We’ve learned that buffer overflows often start with unchecked input. A form field. An uploaded file. An external API. If we don’t validate sizes, we invite problems. So, we check early and often.
Before copying data, we always make sure the destination can hold it:
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if (source.Length > target.Length)
{
throw new ArgumentException(“Input too large.”);
}
Buffer.BlockCopy(source, 0, target, 0, source.Length);
We use Buffer.BlockCopy to sidestep off-by-one errors. It’s fast and safe. No hand-rolled loops unless we absolutely have to. We treat every copy as a risk.
Some rules we follow like second nature now:
- Reject large payloads up front (usually > 1MB, unless expected)
- Always check .Length before .CopyTo()
- Favor Span<T> slicing for sub-arrays instead of manual index math
Unsafe Code Restrictions and SafeBuffer Usage
We don’t ban unsafe, but we keep it caged. If we use pointers, it’s for performance-critical paths—crypto, compression, interop—and it goes through code review every time. We also wrap all pointer logic in tiny, focused methods that get audited line-by-line.
We’ve started using SafeBuffer more. It’s slow to write at first, but once we’ve got wrappers in place, it makes pinning feel…well, less dangerous. Safer.
Here’s a stripped-down pattern:
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class PinnedBuffer : SafeBuffer
{
public PinnedBuffer(int size) : base(true)
{
Initialize((ulong)size);
}
// Access methods here…
}
We never read from a pointer until the size is double-checked. Every time.
Memory Ownership Patterns
Passing around Span<T> feels natural now, but we remind ourselves: someone owns that memory. It might disappear. It might get reused. So, we either consume it fast or copy what we need.
We avoid holding on to Memory<T> across await unless we’ve cloned it:
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async Task Upload(Memory<byte> buffer)
{
var copy = buffer.ToArray(); // Local copy for safety
await StoreAsync(copy);
}
Ownership is sacred. If we rent from a pool, we return it. If we slice from a buffer, we don’t assume it’ll last forever.
Disposable Pattern Enforcement
We enforce disposal like our lives depend on it—because sometimes they do. File handles. Allocated memory. Crypto keys. If it implements IDisposable, we use using.
Always.
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using (var stream = new FileStream(…))
{
// stream used here
}
We’ve also written analyzers that bark when IDisposable isn’t cleaned up. It’s noisy, but useful. We’d rather be annoyed than exploited.
Secure String Handling in C#
Strings in .NET hang around. GC can’t collect them fast enough, especially if they’re interned. If a password leaks into a string, it might stay in RAM for minutes. We learned that the hard way once. Never again.
We use SecureString for secrets. When we can’t, we fall back to char[], and wipe them like this:
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Array.Clear(passwordBuffer, 0, passwordBuffer.Length);
It’s not elegant. But it keeps things clean.
Cryptographic Buffer Clearing
With cryptographic operations, we treat memory like evidence—burn it after use.
We always clear key material:
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Array.Clear(keyData, 0, keyData.Length);
And in high-security zones, we use OS-specific APIs to zero memory blocks, bypassing the GC.
We do not let secrets linger.
Diagnostic Tooling
Memory Profiling Tools
Profilers tell us things code reviews miss. We’ve used Visual Studio’s diagnostic tools and others like dotMemory to sniff out memory leaks, large allocations, and LOH spikes. (2)
The Large Object Heap causes real headaches. It doesn’t get collected as often, so if we throw a 90KB array around too often, it sits there—cold, unused, wasting memory. We watch the LOH closely. Always.
We look for:
- Large arrays getting created repeatedly
- Pinned objects causing fragmentation
- Objects living longer than they should
A few minutes in the memory profiler can save days of bug-hunting.
Static Analysis and Code Metrics
We run Roslyn analyzers with security rules turned up high. CA2000. CA2015. CA1831. If you don’t know them, you should.
These rules catch mistakes early:
- Buffer methods without length checks
- Async calls that retain spans
- Unchecked unsafe operations
We also watch cyclomatic complexity. If a method gets too clever, it probably needs to be broken up—or burned down.
Here’s a habit we’ve picked up:
- Methods over 15 lines get re-evaluated
- Unsafe code gets manually tested
- Analyzers run on every commit
Unsafe Code Review Checklist
If we absolutely must use unsafe code, we go down this list:
- Validate buffer lengths
- Don’t use stale pointers
- Use SafeBuffer where possible
- Wrap native interop in helper classes
- Document every pointer offset or size assumption
And we comment liberally. Unsafe code should read like a warning label.
Architectural Safeguards
Generational Garbage Collection and LOH Management
.NET’s GC has generations—0, 1, and 2. If we allocate carelessly, we move objects into Gen2 too fast, and GC slows down. The LOH starts at 85KB. It’s a trap. We try not to cross it unless we have to.
Instead, we rent buffers:
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var buffer = ArrayPool<byte>.Shared.Rent(8192);
// use it
ArrayPool<byte>.Shared.Return(buffer);
This helps us reduce GC pressure. Especially helpful in services that run forever and handle thousands of requests an hour.
Async Memory Sharing
Async makes buffer safety trickier. We’ve had bugs where Memory<T> was passed into an async method and outlived its source. That’s a use-after-free bug in managed code. It’s rare, but real.
So, our pattern is strict:
- Never retain Span<T> across await
- Copy Memory<T> if unsure
- Wrap every async buffer access in a safety check
Here’s what we do instead:
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async Task ProcessAsync(Memory<byte> incoming)
{
var safe = incoming.ToArray(); // Safety first
await HandleAsync(safe);
}
Practical Advice
We’ve built our fences over time. Made mistakes, fixed them. Learned to distrust any memory that lives too long, moves too fast, or crosses too many hands.
So, here’s how we keep our .NET apps memory-safe:
- Trust the runtime, but test the edges
- Avoid unsafe unless you’ve got no choice—and even then, doubt yourself
- Clear secrets from memory like someone’s watching
- Profile your heap before it becomes a mountain
- Write analyzers or use them, but always listen to what they say
We don’t get cocky with memory. We stay curious, a little paranoid, and always ready to rewrite the code that looks “safe enough.” Because safe enough is never safe enough. Not here. Not now.
FAQ
What is a buffer overflow and why should dotnet developers care about it?
A buffer overflow happens when your program tries to put more data into a memory space than it can hold. Think of it like trying to pour a gallon of water into a coffee cup – the extra water spills everywhere. In dotnet applications, this can crash your program or create security holes that bad actors can exploit.
How does dotnet’s garbage collector help prevent memory safety issues?
The dotnet garbage collector automatically cleans up memory that your program no longer needs. It works like a helpful janitor who throws away trash without you asking. This automatic cleanup prevents many common memory problems, but you still need to write careful code to avoid creating memory safety issues in the first place.
What are the most dangerous dotnet operations that can cause buffer overflows?
Unsafe code blocks, pointer arithmetic, and direct memory manipulation are the biggest troublemakers. Working with unmanaged libraries through platform invoke calls can also create problems. Array bounds checking failures and string buffer operations without proper validation round out the list of common culprits that cause memory safety headaches.
Should dotnet developers use unsafe code blocks and when are they necessary?
Avoid unsafe code unless you absolutely need it for performance-critical operations or when working with external libraries. When you do use unsafe code, treat it like handling dangerous chemicals – wear protective gear by adding extra validation checks, use fixed statements properly, and test everything thoroughly before shipping your code.
How can array bounds checking protect against buffer overflow attacks in dotnet?
Dotnet automatically checks if you’re trying to access array elements that don’t exist, which prevents many buffer overflows. However, you can turn off these safety checks in unsafe code or when using certain performance optimizations. Always validate array indices before accessing elements, especially when processing user input or external data.
What string handling practices help prevent memory safety problems in dotnet applications?
Use StringBuilder for building long strings instead of concatenating many small strings together. Always validate string lengths before copying data, and use safe string methods that check boundaries automatically. When working with character arrays or buffers, double-check that your destination has enough space for the source data.
How do span and memory types improve dotnet memory safety compared to arrays?
Span and Memory types give you better control over memory without sacrificing safety. They include built-in bounds checking and can’t be misused as easily as raw pointers or arrays. These types help you work with memory slices efficiently while the runtime keeps you from accidentally accessing memory you shouldn’t touch.
What testing strategies help catch memory safety bugs before they reach production?
Run static code analysis tools that scan for unsafe patterns and potential buffer overflows. Use fuzzing techniques to feed random data into your application and see if it breaks. Enable all compiler warnings related to memory safety, and write unit tests that specifically try to break your boundary checking code.
Conclusion
We treat memory safety as a core part of how we build software, not just a technical checkbox. The tools we use help, but it’s on us to follow the patterns and boundaries that keep our code solid. We stay alert, use what’s available, and keep our habits sharp so buffer overflows don’t sneak in. For us, memory safety isn’t just about preventing crashes—it’s about earning trust in every line we write.
Ready to build secure software from day one? Join the Secure Coding Practices Bootcamp and take the next step toward safer code.
Related Articles
- https://securecodingpractices.com/secure-coding-in-c-net/
- https://securecodingpractices.com/managing-secrets-in-python-applications-securely/
- https://securecodingpractices.com/asp-net-core-security-best-practices-checklist/
References
- https://en.wikipedia.org/wiki/Memory_protection
- https://en.wikipedia.org/wiki/List_of_performance_analysis_tools
