Performance cliff with StringBuilder.Append(AppendInterpolatedStringHandler)

[johnthcall]

Description

In my teams code base we are occasionally appending long segments to a stringbuilder and line splitting them via +.
I noticed that when concatting an interpolated string and non-interpolated string as the input to Append that it falls back to String.Concat. This can mean that an small change to the string builder append input can result losing the performance of AppendInterpolatedStringHandler.

This uses AppendInterpolatedStringHandler:

var sb = new StringBuilder();
sb.Append($"Key {key} = " + 
    $"{value}");

This falls back to string.Concat

var sb = new StringBuilder();
sb.Append($"Key {key} = " + 
    value});

Configuration

.Net 6.0.2, windows 11, x64

Regression?

No

Data

https://sharplab.io/#v2:EYLgtghglgdgPgAQEwEYCwAoBAGABAlAOgBUBTADwBcBuTTBAZnyVwGFcBvTXH/JhACy4AsgAoCeANakAngBpcAZUoAnWAHNcANwgAbAK6kAlJ269zOlbgDOwXAF5cMUgHclqjQCF9UXQBNSFVEjWgxzc1tCAEEABxjSGD9RABIAIgBpWU5pGQBfB1xU3ABqXDNwivC0jh0DUlzUkPLcXObGfCFhJHEUKVkFZTUYTVrDEy4wyssbO0dnN0GvH39A4NDKmei4hKS0zJls2XzHItLmjYrR43XeVoxcoA==

Analysis

The following three snippets all compile down to the same code so there must be something in roslyn detecting that the full string is interpolated and using AppendInterpolatedStringHandler in that case.
https://sharplab.io/#v2:EYLgtghglgdgPgAQEwEYCwAoBAGABAlAOgBUBTADwBcBuTTBAZnyVwGFcBvTXH/JhACy4AsgwAUBPAGtSATwA0uAMqUATrADmuAG4QANgFdSASk7deF3atwBnNbgC8uACQAiANJzOM2QF9HuK64ANS45hYRkS6uHLqGpL6utBgWvuF8+ELCAhIo0nKKKuowWnFGplwpUVa29k5unrLecv5OQaHpUdX6Rsmp6YyZIgCsAJK5+QrKapo6PSZmVZE1dtb1Hl4cPq2cZQlJ6WkYvkA==

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[stephentoub]

This isn't about the implementation of StringBuilder.Append(AppendInterpolatedStringHandler). The code sb.Append($"Key {key} = " + $"{value}"); uses the AppendInterpolatedStringHandler overload. The code sb.Append($"Key {key} = " + value}); doesn't. I think you're actually asking for a change to the C# compiler / C# language with how it does concatenation of interpolated strings (with non-interpolated strings)?
@333fred

[333fred]

This would require a language change. We considered and rejected allowing string values to be converted to handler types: https://github.com/dotnet/csharplang/blob/main/proposals/csharp-10.0/improved-interpolated-strings.md#allow-string-types-to-be-convertible-to-handlers-as-well, and this is very similar to the request of allowing string values and interpolated strings to be +'d together. I think this would probably be a better place for an analyzer to suggest converting added interplated strings or a single interpolated string.

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Issue Details

---

Description

In my teams code base we are occasionally appending long segments to a stringbuilder and line splitting them via +.
I noticed that when concatting an interpolated string and non-interpolated string as the input to Append that it falls back to String.Concat. This can mean that an small change to the string builder append input can result losing the performance of AppendInterpolatedStringHandler.

This uses AppendInterpolatedStringHandler:

var sb = new StringBuilder();
sb.Append($"Key {key} = " + 
    $"{value}");

This falls back to string.Concat

var sb = new StringBuilder();
sb.Append($"Key {key} = " + 
    value});

Configuration

.Net 6.0.2, windows 11, x64

Regression?

No

Data

https://sharplab.io/#v2:EYLgtghglgdgPgAQEwEYCwAoBAGABAlAOgBUBTADwBcBuTTBAZnyVwGFcBvTXH/JhACy4AsgAoCeANakAngBpcAZUoAnWAHNcANwgAbAK6kAlJ269zOlbgDOwXAF5cMUgHclqjQCF9UXQBNSFVEjWgxzc1tCAEEABxjSGD9RABIAIgBpWU5pGQBfB1xU3ABqXDNwivC0jh0DUlzUkPLcXObGfCFhJHEUKVkFZTUYTVrDEy4wyssbO0dnN0GvH39A4NDKmei4hKS0zJls2XzHItLmjYrR43XeVoxcoA==

Analysis

The following three snippets all compile down to the same code so there must be something in roslyn detecting that the full string is interpolated and using AppendInterpolatedStringHandler in that case.
https://sharplab.io/#v2:EYLgtghglgdgPgAQEwEYCwAoBAGABAlAOgBUBTADwBcBuTTBAZnyVwGFcBvTXH/JhACy4AsgwAUBPAGtSATwA0uAMqUATrADmuAG4QANgFdSASk7deF3atwBnNbgC8uACQAiANJzOM2QF9HuK64ANS45hYRkS6uHLqGpL6utBgWvuF8+ELCAhIo0nKKKuowWnFGplwpUVa29k5unrLecv5OQaHpUdX6Rsmp6YyZIgCsAJK5+QrKapo6PSZmVZE1dtb1Hl4cPq2cZQlJ6WkYvkA==

Author:	johnthcall
Assignees:	-
Labels:	area-System.Runtime, tenet-performance, untriaged
Milestone:	-

[tannergooding]

Thanks for opening the issue. Based on the above, I'd recommend opening a new issue for an analyzer covering this scenario.