public void SetValueOfPrivateProperty() { var transaction = new Transaction(); var expectedClientId = this.fixture.Create<string>(); #region SOLUTION (commented) //var clientIdProperty = transaction.GetType().GetProperty("ClientId", BindingFlags.NonPublic | BindingFlags.Instance); //clientIdProperty.SetValue(transaction, expectedClientId); #endregion transaction.GetClientId().Should().Be(expectedClientId); }