Cannot find `ZipArchive` in the “System.IO.Compression” namespace

Question

My question is related to I didn't find "ZipFile" class in the "System.IO.Compression" namespace

But I have referenced the DLL's for my 4.5.1 webforms project:

Properties of my project give me: Target framework: .Net Framework 4.5.1. and the web.config:

<compilation debug="true" targetFramework="4.5" />

What am I missing?

The solution was to manually reference the assemblies in the web.config But why? Why wasn't the checkbox in the add reference dialog insufficient?

<assemblies>
    <add assembly="System.IO.Compression, Version=4.0.0.0, Culture=neutral, PublicKeyToken=B77A5C561934E089" />
    <add assembly="System.IO.Compression.FileSystem, Version=4.0.0.0, Culture=neutral, PublicKeyToken=B77A5C561934E089" />
</assemblies>

Taken from this answer to "Installed .Net 4.5 but can't use ZipFile class in Visual C#"

Solution

you can update References in Project file directly (Unload Project and Edit this or open .csproj file with notepad and edit)

Before edit must same as:

<Reference Include="System.IO.Compression" />
<Reference Include="System.IO.Compression.FileSystem" />

After your edit:

<Reference Include="System.IO.Compression, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089, processorArchitecture=MSIL" />
<Reference Include="System.IO.Compression.FileSystem, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089, processorArchitecture=MSIL" />

load again your project in Visual Studio and rebuild it. I hope it can help you.