C# - using library error

i have .dll file in folder project is. add reference library project , when try use(import) using qrlib; following: the type or namespace name 'qrlib' not found(are missing using directive or an assembly reference?)

given library file, it's not me implemented it, have use (and pretty sure there no problems it). might problem ?

you can try change framework version 4, dlls cannot referenced in .netframework(client profile).

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