title: NUnit 3.0 TestCase const custom object arguments
body: <p>I've written the class <code>SomeObject</code> and I want to define a <code>const</code> instance of this object to keep/reuse in my <code>TestCase</code>s. How should I rewrite the code below to achieve this behavior?</p>
  
  <pre><code>[TestFixture]
  public class SomeObjectTests
  {
      private const SomeObject item0 = new SomeObject(0.0); // doesn't work
  
      [TestCase(item0, ExpectedResult = 0.0)]
      public double TestSomeObjectValue(SomeObject so)
      {
          return so.Value;
      }
  
      [TestCase(item0, ExpectedResult = "0.0")]
      public string TestSomeObjectValueString(SomeObject so)
      {
          return so.ValueString;
      }
  }
  </code></pre>
  
  <p>I get the following error message:</p>
  
  <blockquote>
    <p>A const field of a reference type other than string can only be initialized with null.</p>
  </blockquote>