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A qualified identifier, in C#, is a string that includes a single identifier or a sequence of identifiers that are separated by dot (.). It is declared within a name space and can include one or more name spaces or types.
A qualified identifier is used mainly to uniquely specify a type or type member by allowing the inclusion of the name space in which the identifier is declared. It enables two identifiers that have a common name and are declared within two different nested name spaces. It guarantees type-safety of the data by eliminating the ambiguity caused by multiple identifiers defined within the same scope. It allows a nested name space to be defined without lexically nesting several name space declarations.
While developing a large application that is integrated with multiple modules with many identifiers, the possibility of an identifier having the same name can occur in multiple regions of code. By specifying the identifier in a hierarchical manner, a qualified identifier helps in expressing the semantic category of the code underlying the identifier and addresses the issue of ambiguity in resolving the identifier name.
A qualified identifier is a program element (interface, type, variable, name space, etc.) that has a fully qualified name. A fully qualified name is the complete hierarchical path of an identifier, starting from its global name space.
For example, an integer variable, "Total," can be declared within a name space, "Calculator," which has multiple members that can help calculate an arithmetic logic. The qualified identifier that is used to refer to the integer variable would be "Calculator.Total".
If two members of a nested name space have the same fully-qualified name, this signals an error because they contribute to the same declaration space.