4.2. Some Instance Methods and the Length Property

Thus far we have not emphasized the use of objects, or even noted what is an object. Strings are a special type in C#. We have used string literals as parameters to functions and we have used the special concatenation operator +. In fact strings are objects. Like other objects, strings have a general notation for functions that are specially tied to the particular type of object. These functions are called instance methods. They always act on an object of the particular class, but a reference to the object is not placed inside the parameter list, but before the method name and a dot as in:

csharp> string s = "hello";
csharp> s.ToUpper();
"HELLO"

ToUpper (converting to upper case) is particular action that makes sense with strings. It take s (the string object reference before the dot in this example) and returns a new string (in upper case). Since this action depends only on the string itself, no further parameters are necessary, and the parentheses after the method name are empty. The general method syntax is

object-reference.methodName (further-parameters )

More string methods are listed below, some with further parameters.

Data can also be associated with object properties. A property of a string is its length (an int). References to property values use dot notation but do not have a parameter list at the end:

csharp> string s = "Hello";
csharp> s.Length;
5
csharp> "".Length;
0

Be careful: Though 5 is the length of s in the example above, the last character in s is s[4]. Using s[5] would generate an IndexOutOfRangeException.

String objects have associated string methods which can be used to manipulate string values. There are an enormous number of string methods, but here are just a few of the most common ones to get you started. The string object to which the method is being applied is referred to as this string in the descriptions. After the methods, the length property is also listed. In the heading this object is not shown explicitly, so be careful when applying these methods and the length property: In actual use they must be preceded by a reference to a string, followed by a dot, as shown in all the examples. The reference to this string can be a variable name, a literal, or any expression evaluating to a string.

4.2.1. Summary of String Length and Some Instance Methods

int IndexOf(string target)

Returns the index of the beginning of the first occurrence of the string target in this string object. Returns -1 if target not found. Example:

csharp> string greeting = "Bonjour", part = "jo";
csharp> greeting.IndexOf(part);
3
csharp> greeting.IndexOf("jot");
-1

string Substring(int start)

Returns the substring of this string object starting from index start through to the end of the string object. Example:

csharp> string name = "Sheryl Crow";
csharp> name.Substring(7);
"Crow"

string Substring(int start, int len)

Returns the substring of this string object starting from index start, including a total of len characters. Example:

csharp> string name = "Sheryl Crow";``
csharp> name.Substring(3,5);
"ryl C"

string ToUpper()

Return a string like this string, except all in upper case. Example:

csharp> "Hi Jane!".ToUpper();
"HI JANE!"

string ToLower()

Return a string like this string, except all in lower case. Example:

csharp> "Hi Jane!".ToLower();
"hi jane!"

int Length

Property referring to the length of this string object. Example:

csharp> string greeting = "Bonjour";
csharp> greeting.Length;  //no parentheses
7

Further string methods are introduced in More String Methods.

4.2.2. Testing Strings For Equality

Strings can be tested for equality like numbers, with ==: two equal signs, not the one equal sign used for assignment. The case of letters matters:

csharp> string s = "Hello"; // initial value assigned
csharp> string t = "HELLO";
csharp> s == t;  // equality test
false
csharp> s.ToUpper() == t;
true
csharp> string u = "High".Substring(0,2); // assign
csharp> u == "Hi"; // equality test
true
csharp> u == "High";
false

Hence string expressions can be used in if statements.