Non Anemic Super Simple Event Sourced

gistfile1.cs

public class Customer : AggregateRoot {
  
  private string _name;
  
  public Customer(Id customerId, string name)
  {
    Id = customerId;
    Name = name;
    
    ApplyEvent(new CustomerCreated(Id, Name)); //Again, passing already modified values;
  }
  
  public string Name {
    get {
      return _name;
    }
    private set {
      if (string.IsNullOrEmpty(value))
        throw new ArgumentNullException("Name is required"); //Single place where Name gets validated
        
      _name = value;
    }
  }
  
  public void ChangeName(string newName) {
    this.Name = newName;
    
    ApplyEvent(new CustomerNameChanged(this.Id, Name)); //PASSING THE MODIFIED NAME NOT THE PARAMETER
  }
  
  public void When(CustomerNameChanged @event){
    this._name = @event.Name; //The event handler only modifies the internal state by passing the Setter that contains the validation
  }
  
}

This is wrong! You should only change state in the When. Your validating property sounds like feature envy.