[Version 10.0] Feature support for improved interpolated strings#1552
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[Version 10.0] Feature support for improved interpolated strings#1552RexJaeschke wants to merge 8 commits into
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RexJaeschke
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type: feature
RexJaeschke
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January 25, 2026 19:34
RexJaeschke
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Jan 25, 2026
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| For example, $"Hello", $"{cs1}, world!" (given `const string cs1 = $"Hello";`), $"{"Hello," + $"world!"}", and $"xxx{(true ? $"{"X"}" : $"{$"{"Y"}"}")}yyy" are all constant interpolated strings. However, $"{123}" and $"{"abc"}{123.45}" are not. | ||
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| For simplicity, the term *ISE* is used throughout this specification to mean either an *interpolated_string_expression* or an *additive_expression* composed entirely of *interpolated_string_expression*s and binary `+` operators. |
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Is there a better name than ISE? I refer to it in a number of places.
RexJaeschke
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Jan 25, 2026
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| public void AppendFormatted(scoped ReadOnlySpan<char> value); | ||
| public void AppendFormatted(string? value); | ||
| public void AppendFormatted(object? value, int alignment = 0, | ||
| string? format = default); | ||
| public void AppendFormatted(scoped ReadOnlySpan<char> value, |
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These signatures come from the .NET 10 docs; however, scoped isn't supported until C# V11. Is it simply a matter of removing that keyword from here and then restoring it in V11?
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Jan 25, 2026
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| - Have an accessible method with the signature `void AppendLiteral(string s)`, which is called to process a single interpolated string expression literal segment. | ||
| - Have a set of accessible overloaded methods called `AppendFormatted`, one of which is called to process a single interpolation, based on that interpolation’s content. Their signatures are, as follows: |
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Re the handler constructor and the AppendLiteral and AppendFormatted methods, I’ve required that they be accessible rather than public. Is that OK?
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An earlier version of this feature is already present on |
In conversions, restore the IFormattable/FormattableString conversion about the new handler paragraph. Both are valie. In expression, update Better conversion bullets to prefer the handler-preference rules, then the existing rules under a "does not exactly match" bullet. Fix typo in 12.83. Add a bullet in 12.83 to target the IFormattable/Formattable string conversion around the handler.
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This is Rex's adaptation of the following: MS proposal and MS proposal. As the first is trivial and defines a term that is used by the second, Rex combined the two into this one PR.
Rex decided that the minimum set of members a robust handler must have are, as follows: one constructor having two
intparameters, methodAppendLiteral, aToStringmethod to get at the built string, and threeAppendFormattedmethods, so that handler can be called using all interpolated string formats a programmer might throw at it without requiring extra arguments to be passed to the constructor. That is, it can correctly handle formats with or without alignment. I’ve used the default handler member set in this decision. Is this OK?