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I am accepting a form from the user with a bunch of inputs and I am constructing a string from those inputs. I am trimming each input and 2 of the inputs (GID & Clone) are not required fields so I'm doing null checks before building the string to avoid null object errors.

I wrote this method and it works, but I was wondering if there is a less repetitive way of doing this.

private string ConstructSeedReference(SeedViewModel model)
{
    string SeedRefId;

    if (!String.IsNullOrEmpty(model.Seed.GID) && !String.IsNullOrEmpty(model.Seed.Clone))
    {
        SeedRefId = String.Format("{0}{1}{2}{3}{4}{5}{6}{7}{8}",
        model.Customer.CustomerCode.TrimEnd(), model.Seed.CollectionCodeId.TrimEnd(), model.Seed.SpeciesId.TrimEnd(),
        model.Seed.Zone.TrimEnd(), model.Seed.Elevation.TrimEnd(), model.Seed.ColYear.TrimEnd(), model.Seed.OrchardId, model.Seed.GID.TrimEnd(), model.Seed.Clone.TrimEnd());
        return SeedRefId;
    } 
    else if (!String.IsNullOrEmpty(model.Seed.GID) && String.IsNullOrEmpty(model.Seed.Clone))
    {
        SeedRefId = String.Format("{0}{1}{2}{3}{4}{5}{6}{7}",
        model.Customer.CustomerCode.TrimEnd(), model.Seed.CollectionCodeId.TrimEnd(), model.Seed.SpeciesId.TrimEnd(),
        model.Seed.Zone.TrimEnd(), model.Seed.Elevation.TrimEnd(), model.Seed.ColYear.TrimEnd(), model.Seed.OrchardId, model.Seed.GID.TrimEnd());
        return SeedRefId;
    }
    else if (String.IsNullOrEmpty(model.Seed.GID) && !String.IsNullOrEmpty(model.Seed.Clone))
    {
        SeedRefId = String.Format("{0}{1}{2}{3}{4}{5}{6}{7}",
        model.Customer.CustomerCode.TrimEnd(), model.Seed.CollectionCodeId.TrimEnd(), model.Seed.SpeciesId.TrimEnd(),
        model.Seed.Zone.TrimEnd(), model.Seed.Elevation.TrimEnd(), model.Seed.ColYear.TrimEnd(), model.Seed.OrchardId, model.Seed.Clone.TrimEnd());
        return SeedRefId;
    }
    else
    {
        SeedRefId = String.Format("{0}{1}{2}{3}{4}{5}{6}",
        model.Customer.CustomerCode.TrimEnd(), model.Seed.CollectionCodeId.TrimEnd(), model.Seed.SpeciesId.TrimEnd(),
        model.Seed.Zone.TrimEnd(), model.Seed.Elevation.TrimEnd(), model.Seed.ColYear.TrimEnd(), model.Seed.OrchardId);
        return SeedRefId;
    }
}
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String.Format("{0}{1}{2}{3}{4}{5}{6}", ...);

Since you are just doing concatenation, string.Concat is more appropriate here.

if (!String.IsNullOrEmpty(model.Seed.GID) && !String.IsNullOrEmpty(model.Seed.Clone))
{
    SeedRefId = String.Format("{0}{1}{2}{3}{4}{5}{6}{7}{8}",
    model.Customer.CustomerCode.TrimEnd(), model.Seed.CollectionCodeId.TrimEnd(), model.Seed.SpeciesId.TrimEnd(),
    model.Seed.Zone.TrimEnd(), model.Seed.Elevation.TrimEnd(), model.Seed.ColYear.TrimEnd(), model.Seed.OrchardId, model.Seed.GID.TrimEnd(), model.Seed.Clone.TrimEnd());
    return SeedRefId;
} 

Between your ifs and else, there is a lot of repeated code. And, we can observe a pattern there:

  • The first 7 elements are always present.
  • The last 2 elements are present if it is not null/empty
  • The elements are always in the same order.

The method can be reduced to this:

private string ConstructSeedReference(SeedViewModel model)
{
    var reference = string.Concat(
        model.Customer.CustomerCode.TrimEnd(), 
        model.Seed.CollectionCodeId.TrimEnd(), 
        model.Seed.SpeciesId.TrimEnd(),
        model.Seed.Zone.TrimEnd(), 
        model.Seed.Elevation.TrimEnd(), 
        model.Seed.ColYear.TrimEnd(), 
        model.Seed.OrchardId);

    if (!string.IsNullOrEmpty(model.Seed.GID))
    {
        reference += model.Seed.GID.TrimEnd();
    }

    if (!string.IsNullOrEmpty(model.Seed.Clone))
    {
        reference += model.Seed.Clone.TrimEnd();
    }

    return reference;
}

EDIT: Just reread your question, if your concern is only about the NullReferenceException. You can simply use null-coalescing operator ?.:

private string ConstructSeedReference(SeedViewModel model)
{
    return string.Concat(
        model.Customer.CustomerCode.TrimEnd(),
        model.Seed.CollectionCodeId.TrimEnd(),
        model.Seed.SpeciesId.TrimEnd(),
        model.Seed.Zone.TrimEnd(),
        model.Seed.Elevation.TrimEnd(),
        model.Seed.ColYear.TrimEnd(),
        model.Seed.OrchardId,
        model.Seed.GID?.TrimEnd(),
        model.Seed.Clone?.TrimEnd());
}
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