`Add-Type` C# 6+ features throwing errors

Powershell uses CodeDomProvider to compile their assemblies. The version provided with the framework just supports C# 5, so no new features are available by default.

However, if you provide another CodeDomProvider, you can compile any language, also C#6. There is a CodeDomProvider available for Roslyn (the new .NET compiler). You can download it from NuGet and include the assembly using Add-Type. Then create an instance of the compiler and pass that in the -CodeDomProvider attribute.

To expand upon Patrick Hoffmans's solution, I was a bit uncomfortable about using the reflection method in unbob's solution as that could potentially break in the future.

I worked out the following powershell code instead which uses .NET named classes and interfaces:

#requires -Version 5

# download https://www.nuget.org/packages/Microsoft.CodeDom.Providers.DotNetCompilerPlatform/ and extract with 7-zip to a location, enter that location on the next line
$DotNetCodeDomLocation = 'C:\Utils\microsoft.codedom.providers.dotnetcompilerplatform.2.0.1'
Add-Type -Path "$DotNetCodeDomLocation\lib\net45\Microsoft.CodeDom.Providers.DotNetCompilerPlatform.dll"

# using Invoke-Expression moves this class definition to runtime, so it will work after the add-type and the ps5 class interface implementation will succeed
# This uses the public interface ICompilerSettings instead of the private class CompilerSettings
Invoke-Expression -Command @"
class RoslynCompilerSettings : Microsoft.CodeDom.Providers.DotNetCompilerPlatform.ICompilerSettings
{
    [string] get_CompilerFullPath()
    {
        return "$DotNetCodeDomLocation\tools\RoslynLatest\csc.exe"
    }
    [int] get_CompilerServerTimeToLive()
    {
        return 10
    }
}
"@
$DotNetCodeDomProvider = [Microsoft.CodeDom.Providers.DotNetCompilerPlatform.CSharpCodeProvider]::new([RoslynCompilerSettings]::new())

This can then be used as in the following examples:

• To directly add a type to the powershell instance, with assembly reference example (requires the roslyn compiler and the above code to be bundled with your script):

  Add-Type -CodeDomProvider $DotNetCodeDomProvider -TypeDefinition $your_source_code_block -ReferencedAssemblies @([System.Reflection.Assembly]::GetAssembly([hashtable]).Location)
  

• To compile the code to a dll for loading in future/other scripts (only requires the resulting dll file to be bundled with your script):

  $DotNetAssemblyParameters = [System.CodeDom.Compiler.CompilerParameters]::new(
      @([System.Reflection.Assembly]::GetAssembly([hashtable]).Location),
      'path_and_name_for_saved.dll',
      $false
  )
  # you can adjust more compilation settings here if you want, see
  # https://docs.microsoft.com/en-us/dotnet/api/system.codedom.compiler.compilerparameters?view=netframework-4.7.2
  $compilationResults = $DotNetCodeDomProvider.CompileAssemblyFromSource(
      $DotNetAssemblyParameters,
      $your_source_code_block
  )
  

The compiled dll can then be used with a simple Add-Type:

Add-Type -Path 'path_and_name_for_saved.Dll'

This allows for you to use the latest .NET compiler in powershell either inline with your main script if you bundle the CodeDomProvider dll and the roslyn compiler, or you can compile the C# code out to a dll so it doesn't have to be recompiled each time the script is run, allowing for easier portability and a faster script run time.