C Sharp Programming/Keywords/stackalloc

The keyword is used in an unsafe code context to allocate a block of memory on the stack.

In the example above, a block of memory of sufficient size to contain 100 elements of type int is allocated on the stack, not the heap; the address of the block is stored in the pointer fib. This memory is not subject to garbage collection and therefore does not have to be pinned (via fixed). The lifetime of the memory block is limited to the lifetime of the method in which it is defined (there is no way to free the memory before the method returns).

is only valid in local variable initializers.

Because types are involved,  requires unsafe context. See Unsafe Code and Pointers.

is similar to  in the C run-time library.

Note* - From MSDN