String interpolation & format specifiers

Build a string from literal text and embedded expressions with $"…{expr}…". A hole may carry a .NET format specifier after a colon ({amount:F2}), applied via IFormattable in the current culture — exactly C#.

Summary#

$"…{expr}…" builds a string from literal text and embedded expressions. A hole may carry a .NET format specifier after a colon — {amount:F2} — applied via IFormattable in the current culture, exactly like C#'s string.Format.

Signature#

$"literal {expr} more {expr:format} text"

Description#

Each {expr} hole is converted to its string form and concatenated with the surrounding literal text.
A :format after the expression applies a .NET format string to a formattable value (numbers, dates): {total:F2} → two decimals, {n:N0} → thousands separators, {ratio:P1} → a percent, {when:yyyy-MM-dd} → a date. A hole with no specifier writes the value as its ToString() does.
Every hole formats in the CURRENT culture, as C#'s do: the viewer's [PreferredCulture], else app.DefaultCulture, else the invariant culture — never the host machine's, so the text is the same on every server and in every browser. Under app.DefaultCulture = "sv-SE", $"{1234.5m}" is 1234,5. Write string.Format(CultureInfo.InvariantCulture, "{0}", x) for text a machine reads (Culture formatting — ToString(format, culture)).
A non-formattable value (a string) ignores the specifier and coerces as usual — matching C#'s string.Format.
The formatted conversion is also available directly as the 2-arg Convert.ToString(value, "F2").

Which other interpolated forms are there?#

All of C#'s forms: $@"…" (or @$"…") keeps backslashes as written and may span lines; $"""…""" is a raw interpolated string (no escapes, quotes written plainly); {x,5} right-aligns the formatted value in five characters and {x,-5} left-aligns it, combining with a format as {x,6:D4}. A hole may break across lines.

string Row(int qty, string sku) => $@"C:\stock\{sku}" + $"""  "{sku}" """ + $"[{qty,5}][{qty,-5}][{qty,6:D4}]";

Examples#

string Money(decimal amount) { return $"Total: {amount:F2}"; }   // "Total: 1234.50"
string Thousands(int n) { return $"{n:N0}"; }                    // "1,234,567"
string Percent(decimal ratio) { return $"{ratio:P1}"; }          // "12.3 %"
string Plain(int n) { return $"n = {n}"; }                       // "n = 5" (no specifier)
string Direct(decimal d) { return Convert.ToString(d, "F3"); }   // "3.142"