Add String support to ReadOnlyMemory<char>

[stephentoub]

Today ReadOnlySpan<char> can reference a System.String (via AsReadOnlySpan()), but ReadOnlyMemory<char> can't. We should fix that so that ReadOnlyMemory<char> can be used as a way to slice strings where the slices can live on the heap.

To me that means:

1. Allowing ReadOnlyMemory<T> to wrap a string in addition to wrapping a T[] and an OwnedMemory<T>. This will mean an extra type check on operations like ReadOnlyMemory<T>.Span, but it can be guarded behind a typeof(T) == typeof(char) check such that the additional branch and few additional instructions will only impact T==char, and string will end up being a very common case for T==char, making it worth it.
2. Adding an AsReadOnlyMemory() extension method forstring just as there's an AsReadOnlySpan() extension.
3. Adding a TryGetString method on MemoryMarshal or some similar place that enables getting the string/offset/count out of a ReadOnlyMemory<char> if it's wrapping one.
4. Whatever string operation methods we want exposed added as extension methods for ReadOnlyMemory<char>, related to https://github.com/dotnet/corefx/issues/20378 and https://github.com/dotnet/corefx/issues/21395

cc: @KrzysztofCwalina, @ahsonkhan, @terrajobst, @jkotas

Replaces https://github.com/dotnet/corefx/issues/20378?

[terrajobst]

Adding a few more folks that asked for StringSegment before

@davidfowl @davkean

[stephentoub]

For API review for items 2 and 3 above:

namespace System
{
    public static class SpanExtensions  // Existing class containing AsReadOnlySpan(this string)... use it, something else, or rename it?
    {
        public static ReadOnlyMemory<char> AsReadOnlyMemory(this string text);
        ...
    }
}

namespace System.Runtime.InteropServices
{
    public static class MemoryMarshal // class added in https://github.com/dotnet/corefx/issues/23879#issuecomment-340861229
    {
        public static bool TryGetString(this ReadOnlyMemory<char> readOnlyMemory, out string value, out int offset, out int count);
        ...
    }
}

Alternative forms of TryGetString to consider instead?

public static string TryGetString(this ReadOnlyMemory<char> readOnlyMemory, out int offset, out int count);
public static (String: string, Offset: int, Count: int) TryGetString(this ReadOnlyMemory<char> readOnlyMemory);

Adding these will be dependent on verifying as part of the work that the extra check for string does not unduly impact other uses of ReadOnlyMemory<char>.

[Joe4evr]

Not a fan of those alternative suggestions for TryGetString(); I think every method whose name starts with Try should return a bool.

[stephentoub]

I think every method whose name starts with Try should return a bool.

Those alternatives were more about the shape; the name could be different.

[terrajobst]

Video

Based on the discussion with @stephentoub we concluded that we should merge the two types:

namespace System
{
    public static class MemoryExtensions
    {
        // Existing class containing AsReadOnlySpan(this string)
        public static ReadOnlyMemory<char> AsReadOnlyMemory(this string text);

        // We already have ReadOnlyMemory<T>.TryGetArray(out ArraySegment<T> segment)
        public static bool TryGetString(this ReadOnlyMemory<char> readOnlyMemory, out string value, out int offset, out int count);
    }
}

[karelz]

FYI: The API review discussion was recorded - see https://youtu.be/HnKmLJcEM74?t=1163 (28 min duration)

[stephentoub]

Fixed by dotnet/corefx#25120