The promotion rules apply to expressions containing values of two or more simple types and to simple-type values passed as arguments to methods. Each value is promoted to the appropriate type in the expression. (Actually, the expression uses a temporary copy of each promoted value—the types of the original values remain unchanged.) Figure 7.4 lists the simple types alphabetically and the types to which each can be promoted. Values of all simple types also can be implicitly converted to type object. We demonstrate such implicit conversions in Chapter 19.

By default, C# does not allow you to implicitly convert values between simple types if the target type cannot represent every value of the original type (e.g., the int value 2000000 cannot be represented as a short, and any floating-point number with nonzero digits after its decimal point cannot be represented in an integer type such as long, int or short).

To prevent a compilation error in cases where information may be lost due to an implicit conversion between simple types, the compiler requires you to use a cast operator to force the conversion. This enables you to “take control” from the compiler. You essentially say, “I know this conversion might cause loss of information, but for my purposes here, that’s fine.” Suppose you create a method Square that calculates the square of an int argument. To call Square with the whole part of a double argument named doubleValue, you’d write Square((int) doubleValue). This method call explicitly casts (converts) the value of doubleValue to an integer for use in method Square. Thus, if doubleValue’s value is 4.5, the method receives the value 4 and returns 16, not 20.25.