4.1.1.7 - Strings, character codes and dates (VB.NET)

String length, positions and substrings

A string is an ordered sequence of characters; its position convention determines how you access and extract them. VB.NET's .NET string methods use zero-based positions. In the examples here, each displayed character occupies one position; more complex Unicode text can contain characters represented by multiple UTF-16 code units.

Dim label As String = "AA-042"
Dim length As Integer = label.Length
Dim separator As Integer = label.IndexOf("-"c)
Dim prefix As String = label.Substring(0, separator)
Dim digits As String = label.Substring(separator + 1)
Dim first As Char = label(0)
Dim message As String = prefix & ":" & digits

length is 6, separator is 2, prefix is "AA", digits is "042", first is "A"c and message is "AA:042". Substring(start, length) extracts a stated number of positions; Substring(start) extracts everything from that start to the end. Do not confuse the requested length with an ending index. Substring(3, 2) returns "04".

IndexOf returns the first matching position, or -1 if no match exists. Check the result before using it as an index when a separator is not guaranteed. Use StringComparison.Ordinal when you want an exact culture-independent string search, for example label.IndexOf("AA", StringComparison.Ordinal). With the Char search above, we are locating the literal hyphen.

Concatenation joins strings. Prefer & for this in VB.NET, making the text operation explicit. "12" & "3" produces "123", not numeric 15. Strings are immutable: assigning a concatenation or substring result does not edit an existing string in place. Valid indices for a nonempty string of length n are 0 through n−1; an empty string has no valid character index. Extracting beyond the end is an error.

Characters and their codes

A character code is a numeric representation of a character; it is not the same as interpreting that character as a numeric digit. In VB.NET, AscW converts a character to its UTF-16 code-unit value and ChrW converts a suitable code-unit value back to a character. For the basic Latin examples used here, the familiar Unicode and ASCII values agree.