Return return when adding values ​​to the end of an existing IEnumerable

Question

I just found out about yield return, it seems very nice to me. 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 modify this method to return some ValidationResults from the child method. Without use, the yieldcode 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 with 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 think the first version looks better.

Answer 2

Of course. Just use this:

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

. , retVal , , . :

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" });
    //...
}

Answer 3

Other solutions published so far are good. Here is another way to solve your problem:

• Create two sequences
• combine them
• return 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" });
  ...
}