Some improvements for records with ToString() and formatting

[Sergio0694]

Opening this as a follow up from a conversation with @333fred on the C# Discord server.

This discussion is about a couple ideas for improvements to records in C# 10, both for performance and for better interoperability with the new InterpolatedStringBuilder type in .NET 6. Specifically, it pertains to some possible optimizations that could be done to the generated ToString() method for records, as well as more efficient support for formatting.

Faster ToString() method

If we use as an example a record such as record Person(string FirstName, string LastName, int Age), we get this (sharplab):

public override string ToString()
{
    StringBuilder stringBuilder = new StringBuilder();
    stringBuilder.Append("Person");
    stringBuilder.Append(" { ");
    if (PrintMembers(stringBuilder))
    {
        stringBuilder.Append(" ");
    }
    stringBuilder.Append("}");
    return stringBuilder.ToString();
}

protected virtual bool PrintMembers(StringBuilder builder)
{
    builder.Append("FirstName");
    builder.Append(" = ");
    builder.Append((object)FirstName);
    builder.Append(", ");
    builder.Append("LastName");
    builder.Append(" = ");
    builder.Append((object)LastName);
    builder.Append(", ");
    builder.Append("Age");
    builder.Append(" = ");
    builder.Append(Age.ToString());
    return true;
}

This is not efficient, both because StringBuilder doesn't pool the underlying buffer, and for all the various other disadvantages also listed here. It would be much more efficient if the generated code for ToString() on C# 10 leveraged InterpolatedStringBuilder instead, to get all the benefits such as reduced allocations, no boxing, pooled underlying buffers, etc. I understand that this would also mean that PrintMembers could no longer be used (as it just takes a StringBuilder), so there might be a small code size increase in ToString() that might or might not be a concern, but worth pointing out.

Better support for formatting

In the context of the improved string interpolation support in C# 10, I think it'd make sense if records in C# 10 also got an autogenerated ISpanFormattable implementation, or some equivalent mechanism to allow them to be used into interpolated string holes with much better efficiency. Having them implement ISpanFormattable seems like the most straightforward option, as InterpolatedStringBuilder and other types already have explicit support for this interface anyway.

Quoting Fred here:

"Just make records better when used in formatting"

Cheers! 😄

[jcouv]

This was discussed in LDM 5/3/2021. See upcoming notes.

In short, the ToString implementation on records is meant to be minimal (no localization) and not performance-oriented. We recommend users implement ISpanFormattable and other ToString overload themselves if they want. Most likely the default format would not be good for such scenarios anyways...

[alrz]

I wish we could remove that if by moving the space to PrintMembers (if appropriate). Not exactly sure if that could work but the current codegen is kinda clunky.

Also no need to load builder in each line. just use Append return value?

[jcouv]

In order to avoid printing double spaces or trailing comma, some boolean needs to be passed around, but as far as I know, it cannot be removed.

As for chaining the Append calls, that sounds like a good idea.

[alrz]

What I'm suggesting is to replace return true with Append(" "). Need to think more why that might not work. But that way we could chain Append calls all the way.