C# Language Specification

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:

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  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 (§5.3) before its value can be obtained, or otherwise a compile-time error occurs.
  
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  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 (§7.5.4) is performed, and the result of the invocation becomes the value of the property access expression.
  
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  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 (§7.5.4) 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.
  

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