Can't find ObservableCollection in System.Collections.ObjectModel

Two WPF projects, same machine, browse Collections.ObjectModel

one has ObservableCollections and says

// Type: System.Collections.ObjectModel.ObservableCollection`1
// Assembly: System, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089
// Assembly location: C:\Program Files\Reference Assemblies\Microsoft\Framework\.NETFramework\v4.0\Profile\Client\System.dll

The other (was upgraded from a 3.0 project,) does not and says

// Type: System.Collections.ObjectModel.Collection`1
// Assembly: 开发者_开发问答mscorlib, Version=2.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089
// Assembly location: C:\Windows\Microsoft.NET\Framework\v2.0.50727\mscorlib.dll

Why is the second reference to using System.Collections.ObjectModel so different?

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You are looking at 2 different types. Types in different assemblies can be in the same namespace.

Since your upgraded project doesn't include a reference to the System.dll assembly (in 3.0 it was in Windowsbase.dll), it won't have the ObservableCollection. If you add a reference to that assembly, you will have access to that collection.

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Look at the end of the System assembly "v4.0\Profile\Client\System.dll" vs "Framework\v2.0.50727\mscorlib.dll"

That project is set to use the client profile. Go into project properties and select the full .Net 4 profile instead.

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I have multiple projects. The main project was set to .net 4 but the referenced project was still .net 3.5 and it was referencing the 2.0 mscorlib.