This book is intended for C# programmers. It does not explain basic C# concepts, so you should have some familiarity with C# programming before you read it. The book does not assume that you are an expert C# developer, however. If you know basic techniques such as creating classes, writing methods, and working with arrays and lists, then you should be able to work through the problems and understand their solutions.

This book includes 100 problems to challenge you and help you develop your C# programming skills. The solutions rely on general programming methods rather than C# language minutiae, so you should have a chance to solve them even if you haven't been programming for decades. They demonstrate useful techniques, not the answers to trivia questions.

Many of the examples also demonstrate important development concepts, such as comparing floating point values, handling integer overflow, avoiding unnecessary recursion, working across time zones, and building simulations. Some examples also show how to use specific advanced C# and .NET Framework tools such as the yield statement, the Transact Parallel Library (TPL), and .NET cryptography classes.

The following paragraphs describe the book's chapters in some detail:

Chapter 1, Mathematics, describes problems that ask you to perform mathematical tasks. These include numerical tasks such as calculating statistical functions, factorials, and binomial coefficients. They also include some combinatorial tasks such as finding all of the permutations or combinations from a set of items.

Chapter 2, Geometry, presents problems that perform geometric calculations. These ask you to find values such as the roots of equations, the points where lines and circles intersect, and the areas of polygons. Many of these problems ask you to draw shapes such as lines, circles, arrowheads, polygons, and stars.

Chapter 3, Dates and Times, includes problems that deal with dates and times. They pay special attention to programs that include multiple time zones or that include times when daylight saving time (or the equivalent in other countries) starts or ends.

Chapter 4, Randomization, poses problems that use randomization to produce various kinds of randomized data. For example, they ask you to generate random floating point numbers within a range, pick random items from a list or array, and generate random passwords.

Chapter 5, Strings, describes problems that manipulate strings. For example, they require you to convert between numbers and roman numerals and between bytes and hexadecimal strings, detect palindromic substrings, validate passwords for correctness, and use the Soundex algorithm to represent word's sound.

Chapter 6, Files and Directories, covers problems that deal with the filesystem. Its problems ask you to remove blank lines from a file, calculate a directory's total size, detect duplicate files, and generate thumbnails for image files in a directory.

Chapter 7, Advanced C# and .NET Features, includes problems that deal with more advanced features such as Language Integrated Query (LINQ), Parallel LINQ (PLINQ), Transact Parallel Library (TPL), and the yield statement.

Chapter 8, Simulations, asks you to write programs that perform simulations. Those range from simple numeric or text-based simulations (such as Dawkins' weasel and finding hailstone sequences), to positional simulations (such as Langton's ant and Life), to simulations that model gravitational force and acceleration.

Chapter 9, Cryptography, describes cryptographic problems. Some, such as the problems that deal with Caesar and Vigenère ciphers, are interesting mostly for fun and historical perspective. Others, such as those that deal with prime numbers, hashing, and string encryption, demonstrate strong, modern cryptographic methods.

You may not learn something new fromeveryproblem in the book, but hopefully you'll learn something from many of them.

Programming is best learned by doing rather than by reading, so I strongly encourage you to try to solve a problem before you look at my example solution later in the book. I've tried to include all of the information that you need to solve each problem in its description, so you should be able to take a crack at it before you look at my solution.

Even if you get stuck on a problem, the beginning of the solution's description may help you figure out how to solve it. If you experience an Ah ha! moment while you're reading about the solution, stop and give it another try.

Keep in mind that there are always multiple ways to solve any particular problem. Some solutions may be better than others, but most problems can be solved by several different approaches, so don't think your solution is wrong just because it's different from mine. In fact, yours may be better than mine.

The easiest way to read this book is from front to back, tackling each problem in order. The chapters are arranged so the less confusing topics come first. Similarly, the problems within each chapter are arranged with the less demanding first. Some of the material described in one problem's solution may also be used in a later solution, so reading the problems and their solutions in order makes sense.

However, the problems and their example solutions are described separately, so you can jump around if you like. For example, if you are particularly interested in steganography, you can jump straight to Chapter 9, Cryptography. The worst that will happen is that an example solution may refer you to an earlier solution for some of its details. You can then go back and read the earlier solution for further details.

private int FindError(string string1, string string2)
{
    int error = 0;
    for (int i = 0; i < string1.Length; i++)
        error += Math.Abs((int)string1[i] - (int)string2[i]);
    return error;
}

{
    string testSubstring = string1.Substring(startPos, length);
    int testPos = string2.IndexOf(testSubstring);
    if (testPos < 0) break;

    bestLength = length;
    bestSubstring = testSubstring;
}

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