API Proposal: C# 10 interpolated strings support, part 3

[333fred]

Background and Motivation

Part 1 and part 2 added APIs to support the improved interpolated string literal feature in C# 10. After a bit more design in the space, we want to make a few minor changes to the names and add a new attribute to help improve the authoring and usage experience for users.

The general driving goal here was to standardize the names builder types use. It is expected that our guidance to API authors will be "Your interpolated string builder types should start with the words InterpolatedString and end with Builder". This convention will help make it clearer for users when something is a valid builder type.

Updated API: InterpolatedStringDefaultBuilder

We rename InterpolatedStringBuilder to InterpolatedStringDefaultBuilder. This is being done to avoid confusion with the new attribute.

-    public ref struct InterpolatedStringBuilder
+    public ref struct InterpolatedStringDefaultBuilder

Proposed API: InterpolatedStringBuilderAttribute

This attribute marks a type as being a valid builder type. This is a change from the existing proposal, where the validity of the conversion was being determined from doing a bunch of overload resolution. This should make errors more sane and helpful to end users.

Open question: Should we allow this on interfaces as well? With abstract statics in interfaces, it would theoretically be possible to use type parameter constrained to an interface as a builder type, but that would only be valid if the interface can be attributed with this.

namespace System.Runtime.CompilerServices
{
    [AttributeUsage(AttributeTargets.Class | AttributeTargets.Struct, AllowMultiple = false, Inherited = false)]
    public sealed class InterpolatedStringBuilderAttribute : Attribute
    {
        public InterpolatedStringBuilderAttribute()
        {
        }
    }
}

We then update the structs defined in Parts 1 and 2 as follows, to add the attribute and also standardize the naming of the other struct types with the rules mentioned above:

namespace System.Runtime.CompilerServices
{
+   [InterpolatedStringBuilder]
    public ref struct InterpolatedStringDefaultBuilder ...

+   [InterpolatedStringBuilder]
-   public ref struct InterpolatedSpanBuilder ...
+   public ref struct InterpolatedStringSpanBuilder ...
}

namespace System.Text
{
    public sealed class StringBuilder
    {
+       [InterpolatedStringBuilder]
-       public struct InterpolatedAppendFormatBuilder ...
+       public struct InterpolatedStringAppendFormatBuilder ...
    }
}

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Issue Details

---

Background and Motivation

Part 1 and part 2 added APIs to support the improved interpolated string literal feature in C# 10. After a bit more design in the space, we want to make a few minor changes to the names and add a new attribute to help improve the authoring and usage experience for users.

The general driving goal here was to standardize the names builder types use. It is expected that our guidance to API authors will be "Your interpolated string builder types should start with the words InterpolatedString and end with Builder". This convention will help make it clearer for users when something is a valid builder type.

Updated API: InterpolatedStringDefaultBuilder

We rename InterpolatedStringBuilder to InterpolatedStringDefaultBuilder. This is being done to avoid confusion with the new attribute.

-    public ref struct InterpolatedStringBuilder
+    public ref struct InterpolatedStringDefaultBuilder

Proposed API: InterpolatedStringBuilderAttribute

This attribute marks a type as being a valid builder type. This is a change from the existing proposal, where the validity of the conversion was being determined from doing a bunch of overload resolution. This should make errors more sane and helpful to end users.

Open question: Should we allow this on interfaces as well? With abstract statics in interfaces, it would theoretically be possible to use type parameter constrained to an interface as a builder type, but that would only be valid if the interface can be attributed with this.

namespace System.Runtime.CompilerServices
{
    [AttributeUsage(AttributeTargets.Class | AttributeTargets.Struct, AllowMultiple = false, Inherited = false)]
    public sealed class InterpolatedStringBuilderAttribute : Attribute
    {
        public InterpolatedStringBuilderAttribute()
        {
        }

        public bool UseLazyComputation { get; }
    }
}

UseLazyComputation controls the execution time of the interpolated string components. If false, the code $"{MyFunction1()}{MyFunction2()}" is executed as:

var tmp1 = MyFunction1();
var tmp2 = MyFunction2();
var builder = BuilderType.Create(0, 2);
builder.AppendFormatted(tmp1);
builder.AppendFormatted(tmp2);
builder

If UseLazyComputation is true, it would instead be executed as:

var builder = BuilderType.Create(0, 2);
builder.AppendFormatted(MyFunction1());
builder.AppendFormatted(MyFunction2());
builder

Note the move from temporary variables, executed before the builder is created, to execution inline with the AppendFormatted calls. This gives the builder author better control over exactly when things are executed and gives the user better insight into what the semantics of the code the compiler will generate is.

We then update the structs defined in Parts 1 and 2 as follows:

namespace System.Runtime.CompilerServices
{
+   [InterpolatedStringBuilder]
    public ref struct InterpolatedStringDefaultBuilder ...

+   [InterpolatedStringBuilder(UseLazyComputation = true)]
    public ref struct InterpolatedSpanBuilder ...
}

namespace System.Text
{
    public sealed class StringBuilder
    {
+       [InterpolatedStringBuilder]
        public struct InterpolatedAppendFormatBuilder ...
    }
}

Author:	333fred
Assignees:	-
Labels:	api-suggestion, area-System.Runtime, blocking, untriaged
Milestone:	-

[KalleOlaviNiemitalo]

public bool UseLazyComputation { get; }

+   [InterpolatedStringBuilder(UseLazyComputation = true)]
    public ref struct InterpolatedSpanBuilder ...

Does UseLazyComputation need a set accessor as well?

[333fred]

Does UseLazyComputation need a set accessor as well?

That would be helpful, wouldn't it 😅

[FilipToth]

I don't really see much use in this API. The user can do all that themselves and execute the methods whenever they want. Do you have any use-case scenarios where this would be simpler or more efficient that you could share with us?

[333fred]

I don't really see much use in this API. The user can do all that themselves and execute the methods whenever they want. Do you have any use-case scenarios where this would be simpler or more efficient that you could share with us?

I would suggest reading through the linked part one and two proposals, and the language feature they are here to support. Most of these apis will not be used directly by consumers.

[333fred]

LDM met today, and rejected the idea of UseLazyComputation. The property and explanation has been removed from the proposal.

[michalczerwinski]

One thing which would be really useful to include in this proposal is some way of splitting string format into constant literals.
I stumbled upon this problem trying to implement feature for EF Core to allow string.Format/interpolation to be translated into SQL.

Currently the string.Format parsing is encapsulated in ValueStringBuilder.AppendFormatHelper. If we can make it public or at least some part of it it would be beneficial.

[bartonjs]

Video

After a long discussion we think the best convention we'll come up with is the suffix "InterpolatedStringHandler"

Therefore, the renames should be

• InterpolatedStringHandlerArgumentAttribute
• InterpolatedStringHandlerAttribute
• DefaultInterpolatedStringHandler
• MemoryExtensions+TryWriteInterpolatedStringHandler
• StringBuilder+AppendInterpolatedStringHandler

The attribute is otherwise good as proposed

namespace System.Runtime.CompilerServices
{
    [AttributeUsage(AttributeTargets.Class | AttributeTargets.Struct, AllowMultiple = false, Inherited = false)]
    public sealed class InterpolatedStringHandlerAttribute : Attribute
    {
        public InterpolatedStringHandlerAttribute()
        {
        }
    }
}