The object Type

      Many programming languages and class hierarchies provide a root type, from which all other objects in the hierarchy are derived. C# and .NET are no exception. In C#, the object type is the ultimate parent type from which all other intrinsic and user-defined types are derived. This means that you can use the object type for two purposes:

      • You can use an object reference to bind to an object of any particular subtype. For example, in Chapter 8, “Operators and Casts,” you see how you can use the object type to box a value object on the stack to move it to the heap; object references are also useful in reflection, when code must manipulate objects whose specific types are unknown.

      • The object type implements a number of basic, general-purpose methods, which include Equals, GetHashCode, GetType, and ToString. Responsible user-defined classes might need to provide replacement implementations of some of these methods using an object-oriented technique known as overriding, which is discussed in Chapter 4, “Inheritance.” When you override ToString, for example, you equip your class with a method for intelligently providing a string representation of itself. If you don’t provide your own implementations for these methods in your classes, the compiler picks up the implementations in object, which might or might not be correct or sensible in the context of your classes.

      You examine the object type in more detail in subsequent chapters.

      The string Type

      C# recognizes the string keyword, which under the hood is translated to the .NET class, System.String. With it, operations like string concatenation and string copying are a snap:

      Despite this style of assignment, string is a reference type. Behind the scenes, a string object is allocated on the heap, not the stack; and when you assign one string variable to another string, you get two references to the same string in memory. However, string differs from the usual behavior for reference types. For example, strings are immutable. Making changes to one of these strings creates an entirely new string object, leaving the other string unchanged. Consider the following code (code file StringSample/Program.cs):

      The output from this is as follows:

      Changing the value of s1 has no effect on s2, contrary to what you’d expect with a reference type! What’s happening here is that when s1 is initialized with the value a string, a new string object is allocated on the heap. When s2 is initialized, the reference points to this same object, so s2 also has the value a string. However, when you now change the value of s1, instead of replacing the original value, a new object is allocated on the heap for the new value. The s2 variable still points to the original object, so its value is unchanged. Under the hood, this happens as a result of operator overloading, a topic that is explored in Chapter 8. In general, the string class has been implemented so that its semantics follow what you would normally intuitively expect for a string.

      String literals are enclosed in double quotation marks ("."); if you attempt to enclose a string in single quotation marks, the compiler takes the value as a char and throws an error. C# strings can contain the same Unicode and hexadecimal escape sequences as chars. Because these escape sequences start with a backslash, you can’t use this character unescaped in a string. Instead, you need to escape it with two backslashes (\\):

      Even if you are confident that you can remember to do this all the time, typing all those double backslashes can prove annoying. Fortunately, C# gives you an alternative. You can prefix a string literal with the at character (@) and all the characters after it are treated at face value; they aren’t interpreted as escape sequences:

      This even enables you to include line breaks in your string literals:

      In this case, the value of jabberwocky would be this:

      C# 6 defines a new string interpolation format that is marked by using the $ prefix. You’ve previously seen this prefix in the section “Working with Variables.” You can change the earlier code snippet that demonstrated string concatenation to use the string interpolation format. Prefixing a string with $ enables you to put curly braces into the string that contains a variable – or even a code expression. The result of the variable or code expression is put into the string at the position of the curly braces:

      NOTE Strings and the features of string interpolation are covered in detail in Chapter 10, “Strings and Regular Expressions.”

Controlling Program Flow

      This section looks at the real nuts and bolts of the language: the statements that allow you to control the flow of your program rather than execute every line of code in the order it appears in the program.

      Conditional Statements

      Conditional statements enable you to branch your code depending on whether certain conditions are met or what the value of an expression is. C# has two constructs for branching code: the if statement, which tests whether a specific condition is met, and the switch statement, which compares an expression with several different values.

      The if Statement

      For conditional branching, C# inherits the C and C++ if.else construct. The syntax should be fairly intuitive for anyone who has done any programming with a procedural language:

      If more than one statement is to be executed as part of either condition, these statements need to be joined into a block using curly braces ({.}). (This also applies to other C# constructs where statements can be joined into a block, such as the for and while loops):

      If you want to, you can use an if statement without a final else statement. You can also combine else if clauses to test for multiple conditions (code file IfStatement/Program.cs):

      There is no limit to how many else ifs you can add to an if clause.

      Note that the previous example declares a string variable called input, gets the user to enter text at the command

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