The Compare( ) method of String is overloaded. The first version takes two strings and returns a negative number if the first string is alphabetically before the second, a positive number if the first string is alphabetically after the second, and zero if they are equal. The second version works just like the first but is case-insensitive. Example 15-1 illustrates the use of Compare( ).

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;

namespace Example_15_1_  _  _  _Comparing_Strings
{
    class Tester
    {
        public void Run( )
        {
            // create some strings to work with
            string s1 = "abcd";
            string s2 = "ABCD";
            int result; // hold the results of comparisons

            // compare two strings, case-sensitive
            result = string.Compare(s1, s2);
            Console.WriteLine("Compare s1: {0}, s2: {1},
                             result: {2}
", s1, s2, result);

            // overloaded compare, takes Boolean "ignore case"
            //(true = ignore case)
            result = string.Compare(s1, s2, true);
            Console.WriteLine("Compare insensitive.
                               result: {0}
", result);
        }

        static void Main( )
        {
            Tester t = new Tester( );
            t.Run( );
        }
    }
}

The output looks like this:

Compare s1: abcd, s2: ABCD, result: -1
Compare insensitive. result: 0

Example 15-1 begins by declaring two strings, s1 and s2, and initializing them with string literals:

string s1 = "abcd";
string s2 = "ABCD";

Compare( ) is used with many types. A negative return value indicates that the first parameter is less than the second; a positive result indicates that the first parameter is greater than the second; and a zero indicates that they are equal. In Unicode (as in ASCII), a lowercase letter has a smaller value than an uppercase letter; with strings that are identical except for case, lowercase comes first alphabetically. Thus, the output properly indicates that s1 (abcd) is “less than” s2 (ABCD):

compare s1: abcd, s2: ABCD, result: -1

The second comparison uses an overloaded version of Compare( ), which takes a third Boolean parameter, the value of which determines whether case should be ignored in the comparison. If the value of this “ignore case” parameter is true, the comparison is made without regard to case. This time the result is 0, indicating that the two strings are identical:

Compare insensitive. result: 0

There are a couple of ways to concatenate strings in C#. You can use the Concat( ) method, which is a static public method of the String class:

string s3 = string.Concat(s1,s2);

Or you can simply use the overloaded concatenation (+) operator:

string s4 = s1 + s2;

Example 15-2 demonstrates both of these methods.

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;

namespace Example_15_2_  _  _  _Concatenating_Strings
{
    class Tester
    {
        public void Run( )
        {
            string s1 = "abcd";
            string s2 = "ABCD";

            // concatenation method
            string s3 = string.Concat(s1, s2);
            Console.WriteLine("s3 concatenated from
                                 s1 and s2: {0}", s3);

            // use the overloaded operator
            string s4 = s1 + s2;
            Console.WriteLine("s4 concatenated from
                                 s1 + s2: {0}", s4);
        }

        static void Main( )
        {
            Tester t = new Tester( );
            t.Run( );
        }
    }
}

The output looks like this:

s3 concatenated from s1 and s2: abcdABCD
s4 concatenated from s1 + s2: abcdABCD

In Example 15-2, the new string s3 is created by calling the static Concat( ) method and passing in s1 and s2, and the string s4 is created by using the overloaded concatenation operator (+) that concatenates two strings and returns a string as a result.

There are two ways to copy strings; 99.9% of the time you will just write:

oneString = theOtherString;

and not worry about what is going on in memory.

There is a second, somewhat awkward way to copy strings:

myString  = String.Copy(yourString);

and this actually does something subtly different. The difference is somewhat advanced, but here it is in a nutshell.

When you use the assignment operator (=) you create a second reference to the same object in memory, but when you use Copy you create a new string that is initialized with the value of the first string, and then create a reference to that new string.

“Huh?” we hear you cry. Example 15-3 will make it clear.

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;

namespace Example_15_3_  _  _  _Copying_Strings
{
    class Tester
    {
        public void Run( )
        {
            string s1 = "abcd";

            Console.WriteLine("string s1: {0}", s1);
            Console.WriteLine("string s2 = s1; ");

            string s2 = s1;
            Console.WriteLine("s1: {0} s2: {1}", s1, s2);
            Console.WriteLine("s1 == s2? {0}", s1 == s2);
            Console.WriteLine("ReferenceEquals(s1,s2): {0}",
                                   ReferenceEquals(s1, s2));
            Console.WriteLine(" 
string s2 = string.Copy( s1 ); ");

            string s3 = string.Copy(s1);
            Console.WriteLine("s1: {0} s3: {1}", s1, s3);
            Console.WriteLine("s1 == s3? {0}", s1 == s3);
            Console.WriteLine("ReferenceEquals(s1,s3): {0}",
                                    ReferenceEquals(s1, s3));
            Console.WriteLine(" 
s2 = "Hello"; ");

            s1 = "Hello";
            Console.WriteLine("s1: {0} s2: {1}", s1, s2);
            Console.WriteLine("s1 == s2? {0}", s1 == s2);
            Console.WriteLine("ReferenceEquals(s1,s2): {0}",
                                   ReferenceEquals(s1, s2));
        }

        static void Main( )
        {
            Tester t = new Tester( );
            t.Run( );
        }
    }
}

The output looks like this:

string s1: abcd
string s2 = s1;
s1: abcd s2: abcd
s1 == s2? True
ReferenceEquals(s1,s2): True

string s2 = string.Copy( s1 );
s1: abcd s3: abcd
s1 == s3? True
ReferenceEquals(s1,s3): False

s1 = "Hello";
s1: Hello s2: abcd
s1 == s2? False
ReferenceEquals(s1,s2): False

In Example 15-3, you start by initializing one string:

string s1 = "abcd";

You then assign the value of s1 to s2 using the assignment operator:

s2 = s1;

You print their values, as shown in the first section of results, and find that not only do the two string references have the same value, as indicated by using the equality operator (==), but also that ReferenceEquals returns true as well. ReferenceEquals is a static method that returns true when the two objects passed in are the same instance. Therefore, the two references refer to the same object.

On the other hand, if you create s3 and assign its value using String.Copy(s1), although the two values are equal (as shown by using the equality operator) they refer to different objects in memory (as shown by the fact that ReferenceEquals returns false).

Now, returning to s1 and s2, which refer to the same object, if you change either one, for example, when you write:

s1 = "Hello";

s3 goes on referring to the original string, but s1 now refers to a brand-new string.

If you later write:

S3 = "Goodbye";

(not shown in the example), the original string referred to by s1 will no longer have any references to it, and it will be mercifully and painlessly destroyed by the garbage collector.

The .NET String class provides three ways to test for the equality of two strings. First, you can use the overloaded Equals( ) method and ask one string (say, s6) directly whether another string (s5) is of equal value:

Console.WriteLine( "
Does s6.Equals(s5)?: {0}", s6.Equals(s5));

You can also pass both strings to String’s static method, Equals( ):

Console.WriteLine( "Does Equals(s6,s5)?: {0}" string.Equals(s6,s5));

Or you can use the String class’s overloaded equality operator (==):

Console.WriteLine( "Does s6==s5?: {0}", s6 == s5);

In each of these cases, the returned result is a Boolean value (true for equal and false for unequal). Example 15-4 demonstrates these techniques.

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;

namespace Example_15_4_  _  _  _String_Equality
{
    class Tester
    {
        public void Run( )
        {
            string s1 = "abcd";
            string s2 = "ABCD";

            // the string copy method
            string s5 = string.Copy(s2);
            Console.WriteLine("s5 copied from s2: {0}", s5);
            string s6 = s5;
            Console.WriteLine("s6 = s5: {0}", s6);

            // member method
            Console.WriteLine("
Does s6.Equals(s5)?: {0}",
            s6.Equals(s5));

            // static method
            Console.WriteLine("Does Equals(s6,s5)?: {0}",
            string.Equals(s6, s5));

            // overloaded operator
            Console.WriteLine("Does s6==s5?: {0}", s6 == s5);
        }

        static void Main( )
        {
            Tester t = new Tester( );
            t.Run( );
        }
    }
}

The output looks like this:

s5 copied from s2: ABCD
s6 = s5: ABCD
 Does s6.Equals(s5)?: True
Does Equals(s6,s5)?: True
Does s6==s5?: True

The equality operator is the most natural of the three methods to use when you have two string objects.

The String class includes a number of useful methods and properties for finding specific characters or substrings within a string, as well as for manipulating the contents of the string. Example 15-5 demonstrates a few methods, such as locating substrings, finding the index of a substring, and inserting text from one string into another. Following the output is a complete analysis.

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;

namespace Example_15_5_  _  _  _String_Methods
{
    class Tester
    {
        public void Run( )
        {
            string s1 = "abcd";
            string s2 = "ABCD";
            string s3 = @"Liberty Associates, Inc.
 provides custom .NET development,
 on-site Training and Consulting";

            // the string copy method
            string s5 = string.Copy(s2);
            Console.WriteLine("s5 copied from s2: {0}", s5);

            // Two useful properties: the index and the length
            Console.WriteLine("
String s3 is {0} characters long. ", s3.Length);
            Console.WriteLine("The 5th character is {0}
", s3[4]);

            // test whether a string ends with a set of characters
            Console.WriteLine("s3:{0}
Ends with Training?: {1}
", s3,
                              s3.EndsWith("Training"));
            Console.WriteLine("Ends with Consulting?: {0}",
                              s3.EndsWith("Consulting"));

            // return the index of the substring
            Console.WriteLine("
The first occurrence of Training ");
            Console.WriteLine("in s3 is {0}
", s3.IndexOf("Training"));

            // insert the word "excellent" before "training"
            string s10 = s3.Insert(71, "excellent ");
            Console.WriteLine("s10: {0}
", s10);

            // you can combine the two as follows:
            string s11 = s3.Insert(s3.IndexOf("Training"), "excellent ");
            Console.WriteLine("s11: {0}
", s11);
        }
        static void Main( )
        {
            Tester t = new Tester( );
            t.Run( );
        }
    }
}

The output looks like this:

s5 copied from s2: ABCD

String s3 is 94 characters long.
The 5th character is r

s3:Liberty Associates, Inc.
 provides custom .NET development,
 on-site Training and Consulting
Ends with Training?: False

Ends with Consulting?: True

The first occurrence of Training
in s3 is 71

s10: Liberty Associates, Inc.
 provides custom .NET development,
 on-site excellent Training and Consulting

s11: Liberty Associates, Inc.
 provides custom .NET development,
 on-site excellent Training and Consulting

The Length property returns the length of the entire string, and the index operator ([]) is used to access a particular character within a string. Just like arrays, the index numbering in a string starts at zero.

Console.WriteLine(
 "
String s3 is {0} characters long. ",
 s5.Length);
 Console.WriteLine(
 "The 5th character is {0}
", s3[4]);

Here’s the output:

String s3 is 4 characters long.
The 5th character is r

The EndsWith( ) method asks a string whether a substring is found at the end of the string. Thus, you might first ask whether s3 ends with the word Training (which it does not), and then whether it ends with the word Consulting (which it does):

Console.WriteLine("s3:{0}
Ends with Training?: {1}
",
 s3,
 s3.EndsWith("Training") );
Console.WriteLine(
 "Ends with Consulting?: {0}",
 s3.EndsWith("Consulting"));

The output reflects that the first test fails and the second succeeds:

Ends with Training?: False
Ends with Consulting?: True

The IndexOf( ) method locates a substring within a string, and the Insert( ) method inserts a new substring into a copy of the original string. The following code locates the first occurrence of Training in s3:

Console.WriteLine("
The first occurrence of Training ");
Console.WriteLine ("in s3 is {0}
",
 s3.IndexOf("Training"));

The output indicates that the offset is 71:

The first occurrence of Training
in s3 is 71

Then use that value to insert the word excellent, followed by a space, into that string.

Actually, the insertion is into a copy of the string returned by the Insert( ) method and assigned to s10:

string s10 = s3.Insert(71,"excellent ");
Console.WriteLine("s10: {0}
",s10);

Here’s the output:

s10: Liberty Associates, Inc.
 provides custom .NET development,
 on-site excellent Training and Consulting

Finally, you can combine these operations to make a more efficient insertion statement:

string s11 = s3.Insert(s3.IndexOf("Training"),"excellent ");
Console.WriteLine("s11: {0}
",s11);

with the identical result:

s11: Liberty Associates, Inc.
 provides custom .NET development,
 on-site excellent Training and Consulting

The String class has methods for finding and extracting substrings. For example, the IndexOf( ) method returns the index of the first occurrence of a string (or of any character in an array of characters) within a target string. For example, given the definition of the string s1 as:

string s1 = "One Two Three Four";

you can find the first instance of the characters hre by writing:

int index = s1.IndexOf("hre");

This code sets the int variable index to 9, which is the offset of the letters hre in the string s1.

Similarly, the LastIndexOf( ) method returns the index of the last occurrence of a string or substring.

Though the following code:

s1.IndexOf("o");

returns the value 6 (the first occurrence of the lowercase letter o is at the end of the word Two), the method call:

s1.LastIndexOf("o");

returns the value 15 (the last occurrence of o is in the word Four).

The Substring( ) method returns a contiguous series of characters. You can ask it for all the characters starting at a particular offset and ending either with the end of the string or with an offset that you (optionally) provide. Example 15-6 illustrates the Substring( ) method.

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;

namespace Example_15_6_  _  _  _Finding_Substrings
{
    class Tester
    {
        public void Run( )
        {
            // create some strings to work with
            string s1 = "One Two Three Four";

            int index;

            // get the index of the last space
            index = s1.LastIndexOf(" ");
            // get the last word.
            string s2 = s1.Substring(index + 1);

            // set s1 to the substring starting at 0
            // and ending at index (the start of the last word)
            // thus s1 has "one two three"
            s1 = s1.Substring(0, index);

            // find the last space in s1 (after two)
            index = s1.LastIndexOf(" ");

            // set s3 to the substring starting at
            // index, the space after "two" plus one more
            // thus s3 = "three"
            string s3 = s1.Substring(index + 1);
            // reset s1 to the substring starting at 0
            // and ending at index, thus the string "one two"
            s1 = s1.Substring(0, index);

            // reset index to the space between
            // "one" and "two"
            index = s1.LastIndexOf(" ");

            // set s4 to the substring starting one
            // space after index, thus the substring "two"
            string s4 = s1.Substring(index + 1);

            // reset s1 to the substring starting at 0
            // and ending at index, thus "one"
            s1 = s1.Substring(0, index);

            // set index to the last space, but there is
            // none so index now = -1
            index = s1.LastIndexOf(" ");

            // set s5 to the substring at one past
            // the last space. there was no last space
            // so this sets s5 to the substring starting
            // at zero
            string s5 = s1.Substring(index + 1);

            Console.WriteLine("s2: {0}
s3: {1}", s2, s3);
            Console.WriteLine("s4: {0}
s5: {1}
", s4, s5);
            Console.WriteLine("s1: {0}
", s1);
        }

        static void Main( )
        {
            Tester t = new Tester( );
            t.Run( );
        }
    }
}

The output looks like this:

s2: Four
s3: Three
s4: Two
s5: One

s1: One

Example 15-6 is not the most elegant solution possible to the problem of extracting words from a string, but it is a good first approximation, and it illustrates a useful technique. The example begins by creating a string, s1:

string s1 = "One Two Three Four";

The local variable index is assigned the value of the last literal space in the string (which comes before the word Four):

index=s1.LastIndexOf(" ");

The substring that begins one position later is assigned to the new string, s2:

string s2 = s1.Substring(index+1);

This extracts the characters from index+1 to the end of the line (the string “Four”) and assigns the value “Four” to s2.

The next step is to remove the word Four from s1; assign to s1 the substring of s1 that begins at 0 and ends at the index:

s1 = s1.Substring(0,index);

You reassign index to the last (remaining) space, which points you to the beginning of the word Three. You then extract the word Three into string s3. Continue like this until you’ve populated s4 and s5. Finally, display the results:

s2: Four
s3: Three
s4: Two
s5: One
s1: One

A more effective solution to the problem illustrated in Example 15-6 would be to use the String class’s Split( ) method, which parses a string into substrings. To use Split( ), pass in an array of delimiters (characters that indicate where to divide the words). The method returns an array of substrings (which Example 15-7 illustrates). The complete analysis follows the code.

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;

namespace Example_15_7_  _  _  _Splitting_Strings
{
    class Tester
    {
        public void Run( )
        {
            // create some strings to work with
            string s1 = "One,Two,Three Liberty Associates, Inc.";
            // constants for the space and comma characters
            const char Space = ' ';
            const char Comma = ',';
            // array of delimiters to split the sentence with
            char[] delimiters = new char[]
            {
                Space,
                Comma
            };

            int ctr = 1;

            // split the string and then iterate over the
            // resulting array of strings

            String[] resultArray = s1.Split(delimiters);

            foreach (String subString in resultArray)
            {
                Console.WriteLine(ctr++ + ":" + subString);
            }
        }

        static void Main( )
        {
            Tester t = new Tester( );
            t.Run( );
        }
    }
}

The output looks like this:

1: One
2: Two
3: Three
4: Liberty
5: Associates
6:
7: Inc.

Example 15-7 starts by creating a string to parse:

string s1 = "One,Two,Three Liberty Associates, Inc.";

The delimiters are set to the space and comma characters. Then call Split( ) on the string, passing in the delimiters:

String[] resultArray = s1.Split(delimiters);

Split( ) returns an array of the substrings that you can then iterate over using the foreach loop, as explained in Chapter 10:

foreach (String subString in resultArray)

foreach (string subString in s1.Split(delimiters))

Start the foreach loop by initializing output to an empty string, and then create each line of the output in three steps. Start with the incremented value ctr. Then use the += operator to add the colon, then the substring returned by Split( ):

Console.WriteLine(ctr++ + ":" + subString);

With each concatenation, a new copy of the string is made, and all three steps are repeated for each substring found by Split( ).

This repeated copying of the string is terribly inefficient. The problem is that the string type is not designed for this kind of operation. What you want is to create a new string by appending a formatted string each time through the loop. The class you need is StringBuilder.

You can use the System.Text.StringBuilder class for creating and modifying strings. Table 15-2 summarizes the important members of StringBuilder.

Unlike String, StringBuilder is mutable; when you modify an instance of the StringBuilder class, you modify the actual string, not a copy.

Example 15-8 replaces the String object in Example 15-7 with a StringBuilder object.

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;

namespace Example_15_8_  _  _  _StringBuilder
{
    class Tester
    {
        public void Run( )
        {
            // create some strings to work with
            string s1 = "One,Two,Three Liberty Associates, Inc.";

            // constants for the space and comma characters
            const char Space = ' ';
            const char Comma = ',';

            // array of delimiters to split the sentence with
            char[] delimiters = new char[]
            {
                Space,
                Comma
            };

            // use a StringBuilder class to build the
            // output string
            StringBuilder output = new StringBuilder( );
            int ctr = 1;

            // split the string and then iterate over the
            // resulting array of strings
            foreach (string subString in s1.Split(delimiters))
            {
                // AppendFormat appends a formatted string
                output.AppendFormat("{0}: {1}
", ctr++, subString);
            }

            Console.WriteLine(output);
        }
        static void Main( )
        {
            Tester t = new Tester( );
            t.Run( );
        }
    }
}

Only the last part of the program is modified. Rather than using the concatenation operator to modify the string, use the AppendFormat( ) method of StringBuilder to append new formatted strings as you create them. This is much easier and far more efficient. The output is identical:

1: One
2: Two
3: Three
4: Liberty
5: Associates
6:
7: Inc.

Because you passed in delimiters of both comma and space, the space after the comma between Associates and Inc. is returned as a word, numbered 6 in the preceding code. That is not what you want. To eliminate this, you need to tell Split( ) to match a comma (as between One, Two, and Three), a space (as between Liberty and Associates), or a comma followed by a space. It is that last bit that is tricky and requires that you use a regular expression.