Just randomly got this today. Not sure what I did to cause it as I haven't really messed with anything since last time when it worked fine. All I did today was add 3 categories.
Assets\Opsive\UltimateInventorySystem\Editor\Utility\DatabaseNamesScriptGenerator.cs(21,33): error CS1069: The type name 'CSharpCodeProvider' could not be found in the namespace 'Microsoft.CSharp'. This type has been forwarded to assembly 'System.CodeDom, Version=4.0.0.0, Culture=neutral, PublicKeyToken=cc7b13ffcd2ddd51' Consider adding a reference to that assembly.
Assets\Opsive\UltimateInventorySystem\Editor\Utility\DatabaseNamesScriptGenerator.cs(21,33): error CS1069: The type name 'CSharpCodeProvider' could not be found in the namespace 'Microsoft.CSharp'. This type has been forwarded to assembly 'System.CodeDom, Version=4.0.0.0, Culture=neutral, PublicKeyToken=cc7b13ffcd2ddd51' Consider adding a reference to that assembly.