An expression is classified as one of the following:
?A value. Every value has an associated type.
?A variable. Every variable has an associated type, namely the declared
type of the variable.
?A namespace. An expression with this classification can only appear as the
left-hand side of a memberaccess
(?4.5.4). In any other context, an expression classified as a namespace
causes a compile-time
error.
?A type. An expression with this classification can only appear as the
left-hand side of a member-access
(?4.5.4), or as an operand for the as operator (?4.9.10), the is operator
(?4.9.9), or the typeof
operator (?4.5.11). In any other context, an expression classified as a
type causes a compile-time error.
?A method group, which is a set of overloaded methods resulting from a
member lookup (?4.3). A
method group may have an associated instance expression. When an instance
method is invoked, the
result of evaluating the instance expression becomes the instance
represented by this (?4.5.7). A
method group is only permitted in an invocation-expression (?4.5.5) or a
delegate-creation-expression
(?4.5.10.3). In any other context, an expression classified as a method
group causes a compile-time
error.
?A property access. Every property access has an associated type, namely
the type of the property.
Furthermore, a property access may have an associated instance expression.
When an accessor (the get
or set block) of an instance property access is invoked, the result of
evaluating the instance expression
becomes the instance represented by this (?4.5.7).
?An event access. Every event access has an associated type, namely the
type of the event. Furthermore,
an event access may have an associated instance expression. An event access
may appear as the lefthand
operand of the += and -= operators (?4.13.3). In any other context, an
expression classified as an
event access causes a compile-time error.
?An indexer access. Every indexer access has an associated type, namely
the element type of the
indexer. Furthermore, an indexer access has an associated instance
expression and an associated
argument list. When an accessor (the get or set block) of an indexer access
is invoked, the result of
evaluating the instance expression becomes the instance represented by this
(?4.5.7), and the result of
evaluating the argument list becomes the parameter list of the invocation.
?Nothing. This occurs when the expression is an invocation of a method
with a return type of void. An
expression classified as nothing is only valid in the context of a
statement-expression (?5.6).
The final result of an expression is never a namespace, type, method group,
or event access. Rather, as noted
above, these categories of expressions are intermediate constructs that are
only permitted in certain contexts.
A property access or indexer access is always reclassified as a value by
performing an invocation of the getaccessor
or the set-accessor. The particular accessor is determined by the context
of the property or indexer
access: If the access is the target of an assignment, the set-accessor is
invoked to assign a new value
(?4.13.1). Otherwise, the get-accessor is invoked to obtain the current
value (?4.1.1).
14.1.1 Values of expressions
Most of the constructs that involve an expression ultimately require the
expression to denote a value. In such
cases, if the actual expression denotes a namespace, a type, a method
group, or nothing, a compile-time error
occurs. However, if the expression denotes a property access, an indexer
access, or a variable, the value of
the property, indexer, or variable is implicitly substituted:
?The value of a variable is simply the value currently stored in the
storage location identified by the
variable. A variable must be considered definitely assigned (?2.3) before
its value can be obtained, or
otherwise a compile-time error occurs.
?The value of a property access expression is obtained by invoking the
get-accessor of the property. If the
property has no get-accessor, a compile-time error occurs. Otherwise, a
function member invocation
(?4.4.3) is performed, and the result of the invocation becomes the value
of the property access
expression.
?The value of an indexer access expression is obtained by invoking the
get-accessor of the indexer. If the
indexer has no get-accessor, a compile-time error occurs. Otherwise, a
function member invocation
(?4.4.3) is performed with the argument list associated with the indexer
access expression, and the
result of the invocation becomes the value of the indexer access expression.