System.AccessViolationException: Attempted to read or write protected memory

System.AccessViolationException in C# is a serious runtime error that means your program tried to access memory it doesn’t own or isn’t allowed to touch.

What System.AccessViolationException means?

The runtime (CLR in .NET) protects memory to prevent corruption. This exception happens when:

• You try to read/write invalid memory
• Or memory has already been freed or corrupted

It’s not a typical C# error—it usually points to unsafe or external code issues.

Why System.AccessViolationException happens?

1) Unmanaged code (most common cause)

When using:

• P/Invoke (DllImport)
• COM objects
• Native libraries (C/C++)

Example problems:

• Wrong function signatures
• Passing invalid pointers
• Mismatched data types

2) Unsafe code in C#

Using unsafe blocks:

unsafe
{
    int* ptr = null;
    *ptr = 10; // crash
}

3) Memory already freed

• Accessing objects after disposal
• Native library frees memory but C# still references it

4) Incorrect marshaling

Passing data between managed and unmanaged code incorrectly:

• Wrong struct layout
• Wrong string encoding
• Incorrect array sizes

5) Third-party library bugs

• If a native dependency is buggy, it can corrupt memory and cause this exception.

6) Hardware / environment issues (rare)

• Faulty RAM
• Driver issues

When does System.AccessViolationException occur?

Typically:

• During interop calls
• While working with:
• Image processing libraries
• Device drivers
• Game engines
• Low-level system APIs

Rare in pure managed C# code.

Example scenario (real-world)

[DllImport("native.dll")]
public static extern void ProcessData(byte[] data);

If the native function expects a different structure → boom → AccessViolationException.

Key characteristics

• Often random or intermittent
• Hard to reproduce
• Can crash the entire application
• Usually not catchable safely

Can you catch System.AccessViolationException?

Technically:

try
{
    // risky code
}
catch (AccessViolationException)
{
}

But:

• In modern .NET, it’s considered a corrupted state exception
• Catching it is not recommended
• App may already be unstable

How to fix System.AccessViolationException?

1) Check interop signatures

Ensure:

• Correct data types
• Correct calling conventions
• Correct struct layouts

2) Avoid unsafe code

• Unless absolutely necessary

3) Validate pointers and buffers

• Ensure arrays are large enough
• Avoid null pointers

4) Use safe wrappers

• Prefer managed libraries instead of native ones

5) Update dependencies

• Native DLLs
• Drivers
• Libraries

6) Enable debugging tools

In Visual Studio

Use Microsoft Visual Studio:

• Enable Native Code Debugging
• Check Call Stack
• Use Mixed Mode Debugging

7) Use memory diagnostic tools

• WinDbg
• Application Verifier

Prevention tips

• Stick to managed code when possible
• Double-check all P/Invoke declarations
• Avoid manual memory handling
• Use Span / Memory instead of pointers when possible

More about System.AccessViolationException

After 

• migrating to another platform, or
• build your code in DEBUG mode, or 
• while debugging your code line by line

system may throw the following exception:

Unhandled Exception: System.AccessViolationException: Attempted to read or write protected memory.
This is often an indication that other memory is corrupt

This issue shouldn't happen in managed code. You can try setting Visual Studio Debugger to bypass this exception through:

Tools menu -> Options -> Debugging -> General
Uncheck "Suppress JIT optimization on module load"