yield return when appending values on to the end of an existing IEnumerable

Question

I just found out about yield return, I seems really nice. I use it in a method like this:

public IEnumerable<ValidationResult> Validate(ValidationContext vc)
{
    if (Name == "Arbitary")
        yield return new ValidationResult("Bad Name.", new[] { "Name" });
    else if (Email == "BadEmail")
        yield return new ValidationResult("Bad Email.", new [] {"Email" });
    // further rules follow.
}

However, I need to change this method to return some ValidationResults from a child method. Without using yield, the code would look like this:

public override IEnumerable<ValidationResult> Validate(ValidationContext vc)
{
    // TryValidateObject fills the ICollection you pass it.
    List<ValidationResult> retVal = new List<ValidationResult>();
    Validator.TryValidateObject(this, vc, retVal, true);

    if (Name == "Arbitary")
        retVal.Add(new ValidationResult("Bad Name.", new[] { "Name" }));
    else if (Email == "BadEmail")
        retVal.Add(new ValidationResult("Bad Email.", new[] { "Email" }));

    return retVal;
}

Is it possible to rewrite this using yield?

Answer 1

Are you looking for this?

public override IEnumerable<ValidationResult> Validate(ValidationContext vc)
{
    // TryValidateObject fills the ICollection you pass it.
    List<ValidationResult> retVal = new List<ValidationResult>();
    Validator.TryValidateObject(this, vc, retVal, true);
    foreach (var item in retVal)
        yield return item;
    if (Name == "Arbitary")
        yield return new ValidationResult("Bad Name.", new[] { "Name" });
    else if (Email == "BadEmail")
        yield return new ValidationResult("Bad Email.", new[] { "Email" });       
}

If so, I believe the first version looks nicer.

Answer 2

The other solutions posted so far are good. Here's another way to solve your problem:

• Create two sequences
• concatenate them
• return the concatenation.

So:

public override IEnumerable<ValidationResult> Validate(ValidationContext vc)
{
  return ResultsFromValidator(vc).Concat(AdditionalResults());
}
private IEnumerable<ValidationResult> ResultsFromValidator(ValidationContext vc)
{
  List<ValidationResult> retVal = new List<ValidationResult>();
  Validator.TryValidateObject(this, vc, retVal, true);
  return retVal;
}
private IEnumerable<ValidationResult> AdditionalResults()
{
  if (Name == "Arbitrary")
    yield return new ValidationResult("Bad Name.", new[] { "Name" });
  ...
}

Answer 3

Sure. Just use this:

foreach (var item in retVal) {
    yield return item;
}

After that, you could also continue with yield returns. So you could return your retVal after you called the method and then continue as in your first sample. Like this:

public override IEnumerable<ValidationResult> Validate(ValidationContext vc)
{
    List<ValidationResult> retVal = new List<ValidationResult>();
    Validator.TryValidateObject(this, vc, retVal, true);
    foreach (var item in retVal) {
        yield return item;
    }

    if (Name == "Arbitary")
        yield return new ValidationResult("Bad Name.", new[] { "Name" });
    else if (Email == "BadEmail")
        yield return new ValidationResult("Bad Email.", new[] { "Email" });
    //...
}