Joe Duffy's Blog

C# 1.0 shipped with the ability to stack allocate data with the stackalloc keyword, much like C++’s
alloca. There are restrictions, however, around what you can allocate on the stack: Inline arrays of primitive types or structs that themselves have fields of primitive types (or structs that etc…). That’s it. C# 2.0 now allows you to embed similar inline arrays inside other value types, even for those that are allocated inside of a reference type on the heap, by using the fixed keyword.

And of course, you can allocate arrays of those value types on the stack too:

using System;

unsafe class Program {
    struct A {
        internal int x;
        internal fixed byte y[1024];
    }

    public static void Main() {
        byte * bb = stackalloc byte[2048];
        Console.WriteLine("&bb         : {0:X}", (uint)&bb);
        Console.WriteLine("&bb[1]      : {0:X}", (uint)&bb[1]);
        Console.WriteLine("&bb[2048]   : {0:X}", (uint)&bb[2048]);

        A * a = stackalloc A[2048];
        Console.WriteLine("&a          : {0:X}", (uint)&a);
        Console.WriteLine("&a->x       : {0:X}", (uint)&a->x);
        Console.WriteLine("&a->y[0]    : {0:X}", (uint)&a->y[0]);
        Console.WriteLine("&a->y[2048] : {0:X}", (uint)&a->y[2048]);
        Console.WriteLine("&a[1]       : {0:X}", (uint)&a[1]);

        Console.WriteLine("&a[2048]    : {0:X}", (uint)&a[2048]);
    }
}

The use of this is of course almost always limited to unmanaged interop scenarios. For example, there’s at least one place in the BCL where we use this to stack allocate the binary layout of a security descriptor that we then pass into the Win32 CreateMutex API, which avoids having to create a new interop struct. (Whether such hacks are a good thing to put in our code-base is another topic altogether…)

The stack allocated data doesn’t outlive the stack frame, so as soon as you return from the function in which the stackalloc occurs, the data is gone. If you pass a pointer to it and somebody stores it, they could later try to dereference a pointer into dead (and possibly since reused) stack space. And reading too far can lead to buffer over- or underflows which bash other data on the stack. Using this requires compilation with /unsafe, and needless to say, you need to be careful with it (if not avoid it altogether).