Why "The value '' is invalid" in nullable field-CodePudding

I have this model of Project with list of Id's, that could be null.

public class Project
{
    [Required] [Key] public Guid id { set; get; }
    ...Other fields
    public List<Guid>? workers { set; get; }
}

I'm using JsonConvert.Serialize to store this list in MySql DB.

protected override void OnModelCreating(ModelBuilder modelBuilder)
    {
        modelBuilder.Entity<Project>(x =>
        {
            x.HasKey(y => y.id);
            x.Property(y => y.workers).HasConversion(to => to == null ? null : JsonConvert.SerializeObject(to),
                from => from == null ? new List<Guid>() : JsonConvert.DeserializeObject<List<Guid>>(from));
        });

Also i have an editing form for Project, where is select tag and option with value="" for nullish value.(@ViewBag.workers - SelectList)

<div >
@if (ViewBag.workers != null)
    {
        <div>
            <label asp-for="workers">Workers</label>
            <select multiple asp-for="workers" [email protected]>
                <option value="">--Select--</option>
            </select>
            <span asp-validation-for="workers"></span>
        </div>
    }
</div>

I can pick and save workers, but when i'm trying to pick default option i have an error

CodePudding user response：

To use <option value="">--Select--</option>, you need to make GUID field as nullable or else value in option tag cannot be null. When you don't have nullable GUID, why do you even need a default option with null value, that does not even make any sense.

If you still want it as it is, you can try with default guid id i.e. 00000000-0000-0000-0000-000000000000