title: Assert that all properties but some are null
body: <p>I have mapping methods, that maps model <code>A</code> to model <code>B</code>. I write unit tests for all of there. I want to somehow nicely handle situation when destination model has a lot of properties, but most of them are <code>null</code>. The test purpose is to test that some small number of properties were correctly mapped and others were not mapped at all (no other than designed properties were set) without having to specify every property.</p>
  
  <p>I cannot change the <code>B</code> class because it is usually service contract that I am calling with only few properties out of many.</p>
  
  <p>Can you please help how to simplify the unit test?</p>
  
  <p>I am using .NET Core 2.1 and NUnit.</p>
  
  <p>Example of current state:</p>
  
  <pre><code>public class B
  {
      public string Id { get; set; }
      public string Prop1 { get; set; }
      public string Prop2 { get; set; }
      public string Prop3 { get; set; }
      public string Prop4 { get; set; }
      public string Prop5 { get; set; }
      // a lot of other properties
  }
  </code></pre>
  
  <p>Mapping method:</p>
  
  <pre><code>public B Map(A a)
  {
      return new B
      {
          Id = a.Id // only Id property is set
      };
  }
  </code></pre>
  
  <p>Test:</p>
  
  <pre><code>[Test]
  public void MapTest()
  {
      var a = new A { Id = "123" };
      var b = mapping.Map(a);
  
      Assert.That(b, Is.Not.Null);
      Assert.That(b.Id, Is.EqualTo(a.Id));
  
      Assert.That(b.Prop1, Is.Null); // this is what i want to simplify
      Assert.That(b.Prop2, Is.Null);
      Assert.That(b.Prop3, Is.Null);
      Assert.That(b.Prop4, Is.Null);
      Assert.That(b.Prop5, Is.Null);
      // a lot of other properties
  }
  </code></pre>