7.5.1Argument lists

Every function member and delegate invocation includes an argument list which provides actual values or variable references for the parameters of the function member. The syntax for specifying the argument list of a function member invocation depends on the function member category:

• For instance constructors, methods, indexers and delegates, the arguments are specified as an argument-list, as described below. For indexers, when invoking the set accessor, the argument list additionally includes the expression specified as the right operand of the assignment operator.

• For properties, the argument list is empty when invoking the get accessor, and consists of the expression specified as the right operand of the assignment operator when invoking the set accessor.

• For events, the argument list consists of the expression specified as the right operand of the += or -= operator.

• For user-defined operators, the argument list consists of the single operand of the unary operator or the two operands of the binary operator.

The arguments of properties (§10.7), events (§10.8), and user-defined operators (§10.10) are always passed as value parameters (§10.6.1.1). The arguments of indexers (§10.9) are always passed as value parameters (§10.6.1.1) or parameter arrays (§10.6.1.4). Reference and output parameters are not supported for these categories of function members.

The arguments of an instance constructor, method, indexer or delegate invocation are specified as an argument-list:

argument-list: argument argument-list , argument

argument: argument-name_opt argument-value

argument-name: identifier :

argument-value: expression ref variable-reference out variable-reference

An argument-list consists of one or more arguments, separated by commas. Each argument consists of an optional argument-name followed by an argument-value. An argument with an argument-name is referred to as a named argument, whereas an argument without an argument-name is a positional argument. It is an error for a positional argument to appear after a named argument in an argument-list.

The argument-value can take one of the following forms:

• An expression, indicating that the argument is passed as a value parameter (§10.6.1.1).

• The keyword ref followed by a variable-reference (§5.4), indicating that the argument is passed as a reference parameter (§10.6.1.2). A variable must be definitely assigned (§5.3) before it can be passed as a reference parameter. The keyword out followed by a variable-reference (§5.4), indicating that the argument is passed as an output parameter (§10.6.1.3). A variable is considered definitely assigned (§5.3) following a function member invocation in which the variable is passed as an output parameter.

7.5.1.1Corresponding parameters

For each argument in an argument list there has to be a corresponding parameter in the function member or delegate being invoked.

The parameter list used in the following is determined as follows:

• For virtual methods and indexers defined in classes, the parameter list is picked from the most specific declaration or override of the function member, starting with the static type of the receiver, and searching through its base classes.

• For interface methods and indexers, the parameter list is picked form the most specific definition of the member, starting with the interface type and searching through the base interfaces. If no unique parameter list is found, a parameter list with inaccessible names and no optional parameters is constructed, so that invocations cannot use named parameters or omit optional arguments.

• For partial methods, the parameter list of the defining partial method declaration is used.

• For all other function members and delegates there is only a single parameter list, which is the one used.

The position of an argument or parameter is defined as the number of arguments or parameters preceding it in the argument list or parameter list.

The corresponding parameters for function member arguments are established as follows:

• Arguments in the argument-list of instance constructors, methods, indexers and delegates:

• A positional argument where a fixed parameter occurs at the same position in the parameter list corresponds to that parameter.

• A positional argument of a function member with a parameter array invoked in its normal form corresponds to the parameter array, which must occur at the same position in the parameter list.

• A positional argument of a function member with a parameter array invoked in its expanded form, where no fixed parameter occurs at the same position in the parameter list, corresponds to an element in the parameter array.

• A named argument corresponds to the parameter of the same name in the parameter list.

• For indexers, when invoking the set accessor, the expression specified as the right operand of the assignment operator corresponds to the implicit value parameter of the set accessor declaration.

• For properties, when invoking the get accessor there are no arguments. When invoking the set accessor, the expression specified as the right operand of the assignment operator corresponds to the implicit value parameter of the set accessor declaration.

• For user-defined unary operators (including conversions), the single operand corresponds to the single parameter of the operator declaration.

• For user-defined binary operators, the left operand corresponds to the first parameter, and the right operand corresponds to the second parameter of the operator declaration.