Mishmash instance and static in C#? What happens?

First of all, this will not compile!

namespace Whatever.Models
{
   public static class Flapdoodle
   {
      public string Bar()
      {
         return "Bar";
      }
   }
}

 
 

Static classes must have to have static methods, but what if we have something like this instead for our Flapdoodle class?

namespace Whatever.Models
{
   public class Flapdoodle
   {
      public static string Foo()
      {
         return "Foo";
      }
      
      public string Bar()
      {
         return "Bar";
      }
   }
}

 
 

Some observations about this are...

1. Alright, this cannot compile.
   using Microsoft.VisualStudio.TestTools.UnitTesting;
   using Whatever.Models;
   namespace Whatever.Tests
   {
      [TestClass]
      public class UnitTest
      {
         [TestMethod]
         public void Test()
         {
            Assert.AreEqual(Flapdoodle.Bar(), "Bar");
         }
      }
   }
   
    
2. And, neither can this:
   using Microsoft.VisualStudio.TestTools.UnitTesting;
   using Whatever.Models;
   namespace Whatever.Tests
   {
      [TestClass]
      public class UnitTest
      {
         [TestMethod]
         public void Test()
         {
            Flapdoodle bosh = new Flapdoodle();
            Assert.AreEqual(bosh.Foo(), "Foo");
         }
      }
   }
    
3. This compiles and the test passes too.
   using Microsoft.VisualStudio.TestTools.UnitTesting;
   using Whatever.Models;
   namespace Whatever.Tests
   {
      [TestClass]
      public class UnitTest
      {
         [TestMethod]
         public void Test()
         {
            Assert.AreEqual(Flapdoodle.Foo(), "Foo");
            Flapdoodle bosh = new Flapdoodle();
            Assert.AreEqual(bosh.Bar(), "Bar");
         }
      }
   }
    

By the way, you cannot have a static struct but you can put static methods in a struct that is not static. The behavior is the same as what is documented above for a similar situation with an instance class (Flapdoodle) which has a mix of static and instance methods.