Using ?: how can I prevent item being store in collection if condition fails? I don't want to store null

Is there a way to prevent an item being stored in collection if condition fails. In this example I store null but it still counts as a collection item and can be enumerated. In collection I want to have only values from failed checks and no null values.

if (part.IsFailure || customer.IsFailure || provider.IsFailure)
{
  return BadRequest(new InvalidArgumentDetails(new List<InvalidArgumentFieldDetail>()
  {
    part.IsFailure ? new InvalidArgumentFieldDetail(nameof(part.Id), part1.Error.ErrorCode.ToString(), part.Error.Message, null) : null,
    customer.IsFailure ? new InvalidArgumentFieldDetail(nameof(customer.Id), customer.Error.ErrorCode.ToString(), customer.Error.Message, null) : null,
    provider.IsFailure ? new InvalidArgumentFieldDetail(nameof(provider.Id), provider.Error.ErrorCode.ToString(), provider.Error.Message, null) : null,
  }));
}

>Solution :

Don’t try to put everything on one line of code. You can conditionally add elements to the list by using if statements. For example:

var result = new List<InvalidArgumentFieldDetail>();
if (part.IsFailure)
    result.Add(new InvalidArgumentFieldDetail(nameof(part.Id), part1.Error.ErrorCode.ToString(), part.Error.Message, null));
if (customer.IsFailure)
    result.Add(new InvalidArgumentFieldDetail(nameof(customer.Id), customer.Error.ErrorCode.ToString(), customer.Error.Message, null));
if (provider.IsFailure)
    result.Add(new InvalidArgumentFieldDetail(nameof(provider.Id), provider.Error.ErrorCode.ToString(), provider.Error.Message, null));
if (result.Any())
    return BadRequest(new InvalidArgumentDetails(result));

Alternatively, if you want to build the list with null elements (which your quick edit and your recent comment imply) then you can simply filter those elements out afterward:

if (part.IsFailure || customer.IsFailure || provider.IsFailure)
{
  return BadRequest(new InvalidArgumentDetails(new List<InvalidArgumentFieldDetail>()
  {
    // your existing conditions to add three elements
  }.Where(x => x != null)));
}   // ^--- filter out null elements here

Any way you look at it, you only have two options:

1. Don’t add null elements to the list.
2. Add null elements to the list and then remove them.