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Run-time evaluation of argument listsСодержание книги
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During the run-time processing of a function member invocation (§7.5.4), the expressions or variable references of an argument list are evaluated in order, from left to right, as follows: · For a value parameter, the argument expression is evaluated and an implicit conversion (§6.1) to the corresponding parameter type is performed. The resulting value becomes the initial value of the value parameter in the function member invocation. · For a reference or output parameter, the variable reference is evaluated and the resulting storage location becomes the storage location represented by the parameter in the function member invocation. If the variable reference given as a reference or output parameter is an array element of a reference-type, a run-time check is performed to ensure that the element type of the array is identical to the type of the parameter. If this check fails, a System.ArrayTypeMismatchException is thrown. Methods, indexers, and instance constructors may declare their right-most parameter to be a parameter array (§10.6.1.4). Such function members are invoked either in their normal form or in their expanded form depending on which is applicable (§7.5.3.1): · When a function member with a parameter array is invoked in its normal form, the argument given for the parameter array must be a single expression that is implicitly convertible (§6.1) to the parameter array type. In this case, the parameter array acts precisely like a value parameter. · When a function member with a parameter array is invoked in its expanded form, the invocation must specify zero or more positional arguments for the parameter array, where each argument is an expression that is implicitly convertible (§6.1) to the element type of the parameter array. In this case, the invocation creates an instance of the parameter array type with a length corresponding to the number of arguments, initializes the elements of the array instance with the given argument values, and uses the newly created array instance as the actual argument. The expressions of an argument list are always evaluated in the order they are written. Thus, the example class Test static void Main() { produces the output x = 0, y = 1, z = 2 The array co-variance rules (§12.5) permit a value of an array type A[] to be a reference to an instance of an array type B[], provided an implicit reference conversion exists from B to A. Because of these rules, when an array element of a reference-type is passed as a reference or output parameter, a run-time check is required to ensure that the actual element type of the array is identical to that of the parameter. In the example class Test static void Main() { the second invocation of F causes a System.ArrayTypeMismatchException to be thrown because the actual element type of b is string and not object. When a function member with a parameter array is invoked in its expanded form, the invocation is processed exactly as if an array creation expression with an array initializer (§7.6.10.4) was inserted around the expanded parameters. For example, given the declaration void F(int x, int y, params object[] args); the following invocations of the expanded form of the method F(10, 20); correspond exactly to F(10, 20, new object[] {}); In particular, note that an empty array is created when there are zero arguments given for the parameter array. When arguments are omitted from a function member with corresponding optional parameters, the default arguments of the function member declaration are implicitly passed. Because these are always constant, their evaluation will not impact the evaluation order of the remaining arguments. Type inference When a generic method is called without specifying type arguments, a type inference process attempts to infer type arguments for the call. The presence of type inference allows a more convenient syntax to be used for calling a generic method, and allows the programmer to avoid specifying redundant type information. For example, given the method declaration: class Chooser public static T Choose<T>(T first, T second) { it is possible to invoke the Choose method without explicitly specifying a type argument: int i = Chooser.Choose(5, 213); // Calls Choose<int> string s = Chooser.Choose("foo", "bar"); // Calls Choose<string> Through type inference, the type arguments int and string are determined from the arguments to the method. Type inference occurs as part of the binding-time processing of a method invocation (§7.6.5.1) and takes place before the overload resolution step of the invocation. When a particular method group is specified in a method invocation, and no type arguments are specified as part of the method invocation, type inference is applied to each generic method in the method group. If type inference succeeds, then the inferred type arguments are used to determine the types of arguments for subsequent overload resolution. If overload resolution chooses a generic method as the one to invoke, then the inferred type arguments are used as the actual type arguments for the invocation. If type inference for a particular method fails, that method does not participate in overload resolution. The failure of type inference, in and of itself, does not cause a binding-time error. However, it often leads to a binding-time error when overload resolution then fails to find any applicable methods. If the supplied number of arguments is different than the number of parameters in the method, then inference immediately fails. Otherwise, assume that the generic method has the following signature: Tr M<X1…Xn>(T1 x1 … Tm xm) With a method call of the form M(E1 …Em) the task of type inference is to find unique type arguments S1…Sn for each of the type parameters X1…Xn so that the call M<S1…Sn>(E1…Em)becomes valid. During the process of inference each type parameter Xi is either fixed to a particular type Si or unfixed with an associated set of bounds. Each of the bounds is some type T. Initially each type variable Xi is unfixed with an empty set of bounds. Type inference takes place in phases. Each phase will try to infer type arguments for more type variables based on the findings of the previous phase. The first phase makes some initial inferences of bounds, whereas the second phase fixes type variables to specific types and infers further bounds. The second phase may have to be repeated a number of times. Note: Type inference takes place not only when a generic method is called. Type inference for conversion of method groups is described in §7.5.2.13 and finding the best common type of a set of expressions is described in §7.5.2.14. The first phase For each of the method arguments Ei:
The second phase The second phase proceeds as follows:
Input types If E is a method group or implicitly typed anonymous function and T is a delegate type or expression tree type then all the parameter types of T are input types of E with type T. Output types If E is a method group or an anonymous function and T is a delegate type or expression tree type then the return type of T is an output type of E with type T. Dependence An unfixed type variable Xi depends directly on an unfixed type variable Xj if for some argument Ek with type Tk Xj occurs in an input type of Ek with type Tk and Xi occurs in an output type of Ek with type Tk. Xj depends on Xi if Xj depends directly on Xi or if Xi depends directly on Xk and Xk depends on Xj. Thus “ depends on ” is the transitive but not reflexive closure of “ depends directly on ”. Output type inferences An output type inference is made from an expression E to a type T in the following way: · If E is an anonymous function with inferred return type U (§7.5.2.12) and T is a delegate type or expression tree type with return type Tb, then a lower-bound inference (§7.5.2.9) is made from U to Tb. · Otherwise, if E is a method group and T is a delegate type or expression tree type with parameter types T1…Tk and return type Tb, and overload resolution of E with the types T1…Tk yields a single method with return type U, then a lower-bound inference is made from U to Tb. · Otherwise, if E is an expression with type U, then a lower-bound inference is made from U to T. · Otherwise, no inferences are made.
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