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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.
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.
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"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 = 1MSBuildProjectService.ToMSBuildPath