What are stackalloc array support initializers in C#?
Overview
In C#, stackalloc is used to allocate memory on the stack; however, it can only be used in an unsafe context.
The following restrictions apply when using stackalloc:
- The memory allocated using
stackallocinside a method call is freed when the method returns. - Memory allocated by
stackalloccannot be explicitly freed. - Garbage collection doesn’t work on memory allocated via
stackalloc; therefore, pinning it using afixedstatement is not required.
Enhancements
Following C# 7.0+, stackalloc can be used in an unsafe context, provided that the value returned by a stackalloc expression is stored inside System.Span<T> or System.ReadOnlySpan<T>. The following code presents an example:
class Program {static void Main() {System.Span<int> oddNumbers = stackalloc int[5];for (int i = 0; i < 5; i++) {oddNumbers[i] = i * 2 + 1;}for (int i = 0; i < 5; i++) {System.Console.WriteLine(oddNumbers[i]);}}}
Output
1
3
5
7
9
In the example above, the oddNumber variable is allocated memory on the stack to store five integers using the stackalloc keyword. The first five odd numbers are stored inside oddNumber and are printed on the console.
The stackalloc memory address can also be stored inside a pointer; however, this requires the unsafe context to be used. The following code provides an example:
class Program {static void Main() {unsafe {int * oddNumbers = stackalloc int[5];for (int i = 0; i < 5; i++) {oddNumbers[i] = i * 2 + 1;}for (int i = 0; i < 5; i++) {System.Console.WriteLine(oddNumbers[i]);}}}}
Output
1
3
5
7
9
The example above contains almost the same code as the previous example. However, the oddNumber variable is an integer pointer in this example. Moreover, since we are storing the memory address returned from stackalloc in a pointer variable, we have enclosed the above code in the unsafe context.
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