Directies for application domains security zone c#

When building the following code I fet the error
"The type or namespace name 'Zone' could not be found (are you missing a using directive or an assembly reference?)      

object[] hostEvidence = { new Zone(SecurityZone.Internet) }; 
            Evidence internetEvidence = new Evidence(hostEvidence, null); 
            AppDomain myDomain = AppDomain.CreateDomain("MyDomain"); 
            myDomain.ExecuteAssembly("SecondAssembly.exe", internetEvidence);

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