Error Invalid option ‘6’ for /langversion; must be ISO-1, ISO-2, 3, 4, 5 or Default

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Pay attention to compiler “type” in the Web.Config file, when changing Framework version:

for 4.5 and C#5 – 

type="Microsoft.CSharp.CSharpCodeProvider...

for 4.6 and C#6 – 

type="Microsoft.CodeDom.Providers.DotNetCompilerPlatform.CSharpCodeProvider, Microsoft.CodeDom.Providers.DotNetCompilerPlatform, Version=1.0.0.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35"

More info in this forum post

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