Porting code to System.Memory

With PR https://github.com/mono/monodevelop/pull/5570, we now have support for using the new Span/Memory APIs.

What does this entail? Lots of performance niceties.

This guide will go through common patterns that can be ported over to the new APIs with a mapping for each method and how it should be ported.

Porting guidelines

To get a ReadOnlySpan<char> from a string, use AsSpan().

To get a string from a ReadOnlySpan<char>, use ToString().

We currently do not have stackalloc span support until we use C# 7.3, so use "mystring".AsSpan() where you need a local span.

Do not use span1 == span2, unless you know what you want is referential equality! Use Span.Equals or Span.SequenceEquals

String API	ReadOnlySpan API
==	SequenceEquals
Substring	Slice
Trim*	Trim*
Starts/EndsWith	Starts/EndsWith
Equals	Equals
IndexOfAny(char[], i)	Slice(i).IndexOfAny(char[]) + i

Note: any StringComparison other than Ordinal and OrdinalIgnoreCase will convert to a string internally, so you won't get any benefit for equality there.

Common patterns

Slice manipulation - i.e. Substring+Trim

Normally, a string substring then a trim would allocate two strings.

By using Span APIs, you can prevent one intermediate string allocation:

string a = "testcase ";
a = a.Substring(0, "test".Length).Trim(); // one allocation for Substring, one for Trim
// a = "case"

Would be with Span APIs:

string a = "testcase ";
var span = a.AsSpan();
span = span.Slice(0, "test".Length).Trim();
// The above also be contracted to span.AsSpan(0, "test".Length).Trim()
a = span.ToString(); // just one string allocation
// a = "case"

IndexOfAny(char[], i)

char[] chars = new[] { 'a', 'b' };
int start = 1;
int i = "abc".IndexOfAny (chars, start);
if (i == -1)
    Console.WriteLine("Not found");
// i = 1;

This is tricker with Span APIs, the correct form is:

char[] chars = new[] { 'a', 'b' };
int start = 1;
int i = "abc".AsSpan(start).IndexOfAny(chars);
if (i == -1)
    Console.WriteLine("Not found");
i += start;
// i = 1