Rules and Guidelines

There are some additional rules and guidelines that you should know about when defining a method that specifies default values for some of its parameters:

■ You can specify default values for the parameters of methods, constructor methods, and parameterful properties (C# indexers). You can also specify default values for parameters that are part of a delegate definition. Then, when invoking a variable of this delegate type, you can omit the arguments and accept the default values.

■ Parameters with default values must come after any parameters that do not have default val- ues. That is, after you define a parameter as having a default value, then all parameters to the right of it must also have default values. For example, in the definition of my M method, I would get a compiler error if I removed the default value ("A") for s. There is one exception to this rule: a params array parameter (discussed later in this chapter) must come after all parameters (including those that have default values), and the array cannot have a default value itself.

■ Default values must be constant values known at compile time. This means that you can set default values for parameters of types that C# considers to be primitive types, as shown in Table 5-1 in Chapter 5, “Primitive, Reference, and Value Types.” This also includes enumerated types, and any reference type can be set to null. For a parameter of an arbitrary value type, you can set the default value to be an instance of the value type, with all its fields containing zeroes. You can use the default keyword or the new keyword to express this; both syntaxes produce identical Intermediate Language (IL) code. Examples of both syntaxes are used by my M method for setting the default value for the dt parameter and guid parameter, respectively.

■ Be careful not to rename parameter variables because any callers who are passing arguments by parameter name will have to modify their code. For example, in the declaration of my M method, if I rename the dt variable to dateTime, then my third call to M in the earlier code will cause the compiler to produce the following message: error CS1739: The best over­ load for 'M' does not have a parameter named 'dt'.

■ Be aware that changing a parameter’s default value is potentially dangerous if the method is called from outside the module. A call site embeds the default value into its call. If you later change the parameter’s default value and do not recompile the code containing the call site, then it will call your method passing the old default value. You might want to consider using a default value of 0/null as a sentinel to indicate default behavior; this allows you to change your default without having to recompile all the code with call sites. Here is an example.

// Don’t do this:

private static String MakePath(String filename = "Untitled") { return String.Format(@"C:\{0}.txt", filename);

}

// Do this instead:

private static String MakePath(String filename = null) {

// I am using the null­coalescing operator (??) here; see Chapter 19 return String.Format(@"C:\{0}.txt", filename ?? "Untitled");

}

■ You cannot set default values for parameters marked with either the ref or out keywords because there is no way to pass a meaningful default value for these parameters.

There are some additional rules and guidelines that you should know about when calling a method by using optional or named parameters:

■ Arguments can be passed in any order; however, named arguments must always appear at the end of the argument list.

■ You can pass arguments by name to parameters that do not have default values, but all required arguments must be passed (by position or by name) for the compiler to compile the code.

■ C# doesn’t allow you to omit arguments between commas, as in M(1, ,DateTime.Now), because this could lead to unreadable comma-counting code. Pass arguments by way of their parameter name if you want to omit some arguments for parameters with default values.

■ To pass an argument by parameter name that requires ref/out, use syntax like the following.

// Method declaration:

private static void M(ref Int32 x) { ... }

// Method invocation: Int32 a = 5;

M(x: ref a);

Date: 2016-03-03; view: 780