the namespace name "Azure" not be found in vs 2012

[Jason_Li_dev]

I created a simple to read the Secret key in Azure，

I referenced some dll in project

Azure.Core.dll

Azure.Identity.dll

Azure.Security.KeyVault.Secrets.dll

but this cannot build。

Error message：The type or namespace name 'Azure' could not be found (are you missing a using directive or an assembly reference?)

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