[TestFixture]
public class ApiControllersShould
{
  [Test]
  public void UseDtoParameterTypes()
  {
    // Define some data
    var methods = Methods.InAssemblyOf("Api Controller Methods",
                    t => t.Where(p => p.DeclaringType.BaseType == typeof(ApiControllerBase)
                    && p.CustomAttributes.Any(a => a.AttributeType == typeof(RouteAttribute))));

    // Apply convention to data
    Convention.Is(new MethodParametersAreDto("DanceScore.DataContracts"), methods);
  }
}

public class MethodParametersAreDto : IConvention
{
  private readonly string _namespacePrefix;

  public MethodParametersAreDto(string namespacePrefix)
  {
    _namespacePrefix = namespacePrefix;
  }

  public void Execute(Methods data, IConventionResultContext result)
  {
    var methods = data.MethodsToVerify
                    .Where(m => !m.GetParameters()
                    .All(p => p.ParameterType.Namespace.StartsWith("System")
                    || p.ParameterType.Name.StartsWith(_namespacePrefix)));

    result.Is(ConventionReason, methods);
  }

  public string ConventionReason
  {
    get { return "All Method Parameters MUST be in System or DataContracts Namespace"; }
  }
}