Test.cs

public enum TradeStatusEnum { Created, Approved }

public class Party 
{
  //ctor etc.
  
  public string Name { get; protected set; }
}

public class Trade
{
    //ctor etc.
    
    public TradeStatusEnum TradeStatus { get; protected set; }
    public DateTimeOffset TradeTime { get; protected set; }
    public Party CounterParty {get; protected set; }
}

public class Tests
{
  [Test]
  public void TheTest()
  {
    var data = new Trade()
                    .SetWith(x => x.TradeStatus, TradeStatusEnum.Approved)
                    .UpdateWith(x => x.TradeTime, time => time.AddDays(3.5))
                    .SetWith(x => x.Party.Name, "Jon Doe");
                    
    
    //Do your test
  }
}

TestUtils.cs

public static class TestUtils
{
  public static T SetWith<T, R>(this T instance, Expression<Func<T, R>> chainSelector, R value)
  {
    var selectorString = chainSelector.ToString().Substring(chainSelector.ToString().IndexOf('.') + 1);
    Impromptu.InvokeSetChain(instance, selectorString, value);
    
    return instance;
  }

  public static T UpdateWith<T, R>(this T instance, Expression<Func<T, R>> chainSelector, Func<R, R> valueTransformer)
  {
    var selectorString = chainSelector.ToString().Substring(chainSelector.ToString().IndexOf('.') + 1);
    var value = Impromptu.InvokeGetChain(instance, selectorString);
    Impromptu.InvokeSetChain(instance, selectorString, valueTransformer(value));

    return instance;
  }
}