How to Resolve Error "CS0255: stackalloc may not be used in a catch or finally block" in C#

The Compiler Error CS0255 is a memory safety restriction. The message reads: "stackalloc may not be used in a catch or finally block".

In C#, stackalloc allocates a block of memory on the execution stack. This is a high-performance operation typically used with Span<T> or pointers. However, the stack is heavily involved during Exception Handling. When an exception is thrown, the runtime "unwinds" the stack to find a handler. Allocating new stack memory inside a catch or finally block disrupts the stability of the stack frame required for this unwinding and cleanup process. Therefore, the compiler forbids it.

This guide explains how to restructure your code to use stack memory safely around exception handlers.

Understanding Stack Unwinding

When you enter a try block, the runtime marks the current stack state.

• stackalloc: Pushes the stack pointer to reserve space.
• catch / finally: These run when the stack is being inspected or cleaned up (unwound) due to an error.

If you were allowed to stackalloc inside a catch block, you would be modifying the stack pointer while the runtime is trying to stabilize or traverse previous stack frames. This creates undefined behavior and potential stack corruption.

Scenario: Allocating Inside Exception Blocks

This error occurs if you attempt to create a temporary buffer for error handling or cleanup using stack memory inside the protected blocks.

Example of error:

using System;

public class MemoryHandler
{
    public void ProcessData()
    {
        try
        {
            // Normal processing
            throw new Exception("Something went wrong");
        }
        catch (Exception ex)
        {
            // ⛔️ Error CS0255: stackalloc may not be used in a catch or finally block.
            // We are trying to allocate a buffer to format an error message.
            Span<char> errorBuffer = stackalloc char[128];
            
            Console.WriteLine("Error processed.");
        }
        finally
        {
            // ⛔️ Error CS0255: Also illegal in finally blocks.
            Span<byte> cleanupBuffer = stackalloc byte[64];
        }
    }
}

Solution 1: Allocate Before the Try Block

The scope of a variable declared before the try block includes the try, catch, and finally blocks. Since the allocation happens during normal execution flow (before the exception mechanism engages), it is perfectly safe.

Solution: move the stackalloc line up.

using System;

public class MemoryHandler
{
    public void ProcessData()
    {
        // ✅ Correct: Allocating outside the protected region.
        // This memory remains valid until the method returns.
        Span<char> errorBuffer = stackalloc char[128];

        try
        {
            throw new Exception("Failure");
        }
        catch (Exception ex)
        {
            // We can safely USE the buffer here
            bool success = ex.Message.TryCopyTo(errorBuffer);
            Console.WriteLine($"Error captured in stack buffer.");
        }
    }
}

note

Performance Impact: Since stack allocation is extremely fast (just moving a pointer), moving it outside the try block has negligible performance cost, even if an exception is never thrown.

Solution 2: Use Heap Allocation (Standard Array)

If you specifically need a fresh buffer only when an exception occurs (to save stack space in the success path), you should use a standard managed array (Heap allocation).

While slower than stackalloc, the Heap is safe to access and allocate anywhere, including inside exception handlers.

Solution: replace stackalloc with new.

public void ProcessData()
{
    try
    {
        // ... code ...
    }
    catch (Exception ex)
    {
        // ✅ Correct: 'new char[]' allocates on the Heap. 
        // This is valid inside catch/finally.
        Span<char> errorBuffer = new char[128];
        
        // Or simply:
        // char[] heapArray = new char[128];
    }
}

Conclusion

CS0255 prevents you from corrupting the stack during error recovery.

1. Move Up: If you need stack performance, define the stackalloc variable before the try statement.
2. Use Heap: If the allocation is optional or large, use new Type[] inside the catch block instead.