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Author: sharpchen

docs/document/Modern CSharp/docs/Attribute/Understanding String Formatting.md

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+# Understanding String Formatting
+
+## Padding & Alignment
+
+Composite formatting syntax supports three semantic for
+
+- the padding length of each interpolation 
+- the direction of each padding for interpolation
+- the format of each interpolation
+
+What's padding? It fills out a string to a specified length using spaces. 
+If the specified length is less than the length of the string, string remains the same.
+You can specify the direction to be left or right.
+
+```cs
+// pad spaces on left
+"123".PadLeft(20);
+//                 123
+```
+
+The second syntax in composite formatting is a optional integer for the interpolation:
+
+- specify direction of padding by `-`(pad spaces on left) and pad on right by default
+- length of padding
+
+```cs
+string.Format("{0,20}", 123);
+//                 123
+string.Format("{0,-20}", 123);
+// 123                ^ ends here
+```
+
+## Format Convention
+
+### Numeric Format
+
+- `G` for **G**eneral
+- `C` for **C**urrency with decimal precision supported
+- `B` for **B**inary numeric
+- `D` for padding **integers** to specified trailing digit count
+- `E` for **E**xponential format
+    - `e` for lowercase, `1.23e+02` for example.
+- `F` for **F**ixed-point numeric
+- `N` for **N**umeric
+    - formats to `ddd,ddd.ddd...`, trailing precision specifier is allowed
+- `P` for **P**ercentage
+- `X` for he**X**adecimal
+    - `x` for lowercase hexadecimal
+    - trailing number after `X` or `x` left pads to length with `0`
+- `R` for **R**ound-trip
+    - supported for  `Double`, `Single`, `Half` and `BigInteger` only.
+    - ensures the converted string represents the exact precision of the number.
+
+
+## `ToString` & `IFormattable`