String interpolation & format specifiers

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 InvariantCulture, 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. Formatting always uses InvariantCulture (deterministic across hosts).
• 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").

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"

See also#

• Numeric types & literal suffixes — the numeric values you format
• Convert — Convert.ToString and the coercion builtins