String.Format("{0,-10}",str);//这个表示第一个参数str字符串的宽度为10，左对齐
String.Format("{0,10}",str);//这个表示第一个参数str字符串的宽度为10，右对齐
参考：From MSDN
Format Item Syntax
Each format item takes the following form and consists of the following components:
{index[,alignment][:formatString]}
The matching braces ("{" and "}") are required.
Index Component
The mandatory index component, also called a parameter specifier, is a number starting from 0 that identifies a corresponding item in the list of objects. That is, the format item whose parameter specifier is 0 formats the first object in the list, the format item whose parameter specifier is 1 formats the second object in the list, and so on.
Multiple format items can refer to the same element in the list of objects by specifying the same parameter specifier. For example, you can format the same numeric value in hexadecimal, scientific, and number format by specifying a composite format string like this: "{0:X} {0:E} {0:N}".
Each format item can refer to any object in the list. For example, if there are three objects, you can format the second, first, and third object by specifying a composite format string like this: "{1} {0} {2}". An object that is not referenced by a format item is ignored. A runtime exception results if a parameter specifier designates an item outside the bounds of the list of objects.
Alignment Component
The optional alignment component is a signed integer indicating the preferred formatted field width. If the value of alignment is less than the length of the formatted string, alignment is ignored and the length of the formatted string is used as the field width. The formatted data in the field is right-aligned if alignment is positive and left-aligned if alignment is negative. If padding is necessary, white space is used. The comma is required if alignment is specified.
Format String Component
The optional formatString component is a format string that is
appropriate for the type of object being formatted. Specify a