Drag and Drop on Designer gives error

Dragging a simple file from Explorer to designer surface throws error.

File contents:

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

namespace ConsoleApp1
{
	public class Example
	{
		public int Bar { get; set; }
	}
}

Error: Method not found: 'Microsoft.CodeAnalysis.SyntaxTree… See attached .jpg.

[dcastenholz]

This behavior is exhibited on 2 different machines. I uninstalled and reinstalled on one of them. No change in behavior.

[dcastenholz]

Dragged the previously attached .cs file from the solution explorer onto the model surface and dropped it.