/// <inheritdoc />
/// <remarks>Does nothing if <see cref="For"/> is <c>null</c>.</remarks>
public override void Process(TagHelperContext context, TagHelperOutput output)
{
if (For == null)
{
// Regular HTML <select/> element. Just make sure Items wasn't specified.
if (Items != null)
{
var message = Resources.FormatSelectTagHelper_CannotDetermineContentWhenOnlyItemsSpecified(
"<select>",
nameof(For).ToLowerInvariant(),
nameof(Items).ToLowerInvariant());
throw new InvalidOperationException(message);
}
// Pass through attribute that is also a well-known HTML attribute.
if (!string.IsNullOrEmpty(Multiple))
{
output.CopyHtmlAttribute(nameof(Multiple), context);
}
}
else
{
// Note null or empty For.Name is allowed because TemplateInfo.HtmlFieldPrefix may be sufficient.
// IHtmlGenerator will enforce name requirements.
var metadata = For.Metadata;
if (metadata == null)
{
throw new InvalidOperationException(Resources.FormatTagHelpers_NoProvidedMetadata(
"<select>",
nameof(For).ToLowerInvariant(),
nameof(IModelMetadataProvider),
For.Name));
}
bool allowMultiple;
if (string.IsNullOrEmpty(Multiple))
{
// Base allowMultiple on the instance or declared type of the expression.
var realModelType = For.Metadata.RealModelType;
allowMultiple =
typeof(string) != realModelType && typeof(IEnumerable).IsAssignableFrom(realModelType);
}
else if (string.Equals(Multiple, "multiple", StringComparison.OrdinalIgnoreCase))
{
allowMultiple = true;
// Copy exact attribute name and value the user entered. Must be done prior to any copying from a
// TagBuilder. Not done in next case because "true" and "false" aren't valid for the HTML 5
// attribute.
output.CopyHtmlAttribute(nameof(Multiple), context);
}
else if (!bool.TryParse(Multiple.ToLowerInvariant(), out allowMultiple))
{
throw new InvalidOperationException(Resources.FormatTagHelpers_InvalidValue_ThreeAcceptableValues(
"<select>",
nameof(Multiple).ToLowerInvariant(),
Multiple,
bool.FalseString.ToLowerInvariant(),
bool.TrueString.ToLowerInvariant(),
nameof(Multiple).ToLowerInvariant())); // acceptable value (as well as attribute name)
}
// Ensure GenerateSelect() _never_ looks anything up in ViewData.
var items = Items ?? Enumerable.Empty <SelectListItem>();
var tagBuilder = Generator.GenerateSelect(
ViewContext,
For.Metadata,
optionLabel: null,
name: For.Name,
selectList: items,
allowMultiple: allowMultiple,
htmlAttributes: null);
if (tagBuilder != null)
{
output.SelfClosing = false;
output.Merge(tagBuilder);
}
}
}