What is the difference between Span<T> and Memory<T>?

Both are zero-allocation views into a contiguous block of memory, but they differ in where they can live.

Span is a ref struct — stack-only:

public static int SumDigits(ReadOnlySpan<char> s) {
    int sum = 0;
    foreach (var c in s) if (char.IsDigit(c)) sum += c - '0';
    return sum;
}
SumDigits("order-2026-04".AsSpan(6, 4));  // no alloc

Can't be:

• Stored in a class field
• Captured in a closure
• Held across await or yield return

Memory is a normal struct — heap-friendly. Use when you need to pass a buffer across async boundaries:

public async Task ProcessAsync(ReadOnlyMemory<byte> buffer) {
    await Task.Delay(10);                // ✓ Memory survives await
    var span = buffer.Span;              // pin/unpin only when needed
    DoSync(span);
}

Rule: prefer Span<T> for tight synchronous loops, Memory<T> when crossing async or storing a buffer. The conversion is cheap (memory.Span).