How can I bind a comma separated list in a URL to an array of objects? - asp.net-core

Question

I have a class named VerseRangeReference that has the properties Chapter, FirstVerse and LastVerse.
I have decorated it with a TypeConverterAttribute [TypeConverter(typeof(VerseRangeReferenceConverter))]
I have an action on a controller like this
public Task<ViewResult> Verses(VerseRangeReference[] verses)
But the value of verses is always a single element with the value null. Here is my type converter
public class VerseRangeReferenceConverter : TypeConverter
{
public override bool CanConvertFrom(ITypeDescriptorContext context, Type sourceType)
{
return sourceType == typeof(string);
}
public override object ConvertFrom(ITypeDescriptorContext context, CultureInfo culture, object value)
{
if (value == null)
throw new ArgumentNullException(nameof(value));
if (value.GetType() == typeof(string))
{
string source = (string)value;
return VerseRangeReference.ParseMultiple(source);
}
return null;
}
}
The result of VerseRangeReference.ParseMultiple(source) is a valid array of instances of VerseRange.

Answer 1

I had to implement a custom model binder. If someone can think of a way to do this with a TypeConverter then I will accept that answer instead because model binders are more complicated.
public class VerseRangeReferenceArrayModelBinder : IModelBinder
{
public Task BindModelAsync(ModelBindingContext bindingContext)
{
string modelName = bindingContext.ModelName;
ValueProviderResult valueProviderResult = bindingContext.ValueProvider.GetValue(modelName);
if (valueProviderResult != ValueProviderResult.None)
{
VerseRangeReference[] verseRangeReferences = VerseRangeReference.ParseMultiple(valueProviderResult.FirstValue);
bindingContext.Result = ModelBindingResult.Success(verseRangeReferences);
}
return Task.CompletedTask;
}
}
public class VerseRangerReferenceArrayModelBinderProvider : IModelBinderProvider
{
public IModelBinder GetBinder(ModelBinderProviderContext context)
{
if (context.Metadata.ModelType == typeof(VerseRangeReference[]))
return new BinderTypeModelBinder(typeof(VerseRangeReferenceArrayModelBinder));
return null;
}
}
This must be registered.
services.AddMvc(options =>
{
options.ModelBinderProviders.Insert(0, new VerseRangerReferenceArrayModelBinderProvider());
})
.SetCompatibilityVersion(CompatibilityVersion.Version_2_1);

Answer 2

You can use a type converter to bind a comma separated string to a sequence of values. However, the type converter should convert from the string to the sequence directly. This means that the type converter should be configured for something like IEnumerable<T> or T[]. To simplify matters I will continue my explanation for IEnumerable<int> but if you want to use arrays instead you should just make sure that the type converter converts to an array instead of something that implements IEnumerable<T>.
You can configure a type converter for IEnumerable<int> using TypeDescriptor.AddAttributes:
TypeDescriptor.AddAttributes(
typeof(IEnumerable<int>),
new TypeConverterAttribute(typeof(EnumerableIntTypeConverter)));
This configures EnumerableIntTypeConverter as a type converter that can convert IEnumerable<int>.
This call has to be made when the process starts and in the case of ASP.NET Core this can conveniently be done in the Startup.Configure method.
Here is the EnumerableIntTypeConverter that converts the comma separated string of numbers to a list of ints:
internal class EnumerableIntTypeConverter : TypeConverter
{
private const char Separator = ',';
public override bool CanConvertFrom(ITypeDescriptorContext context, Type sourceType)
=> sourceType == typeof(string);
public override object ConvertFrom(ITypeDescriptorContext context, CultureInfo culture, object value)
{
if (!(value is string #string))
throw new NotSupportedException($"{GetType().Name} cannot convert from {(value != null ? value.GetType().FullName : "(null)")}.");
if (#string.Length == 0)
return Enumerable.Empty<int>();
var numbers = new List<int>();
var start = 0;
var end = GetEnd(#string, start);
while (true)
{
if (!int.TryParse(
#string.AsSpan(start, end - start),
NumberStyles.AllowLeadingSign,
culture,
out var number))
throw new FormatException($"{GetType().Name} cannot parse string with invalid format.");
numbers.Add(number);
if (end == #string.Length)
break;
start = end + 1;
end = GetEnd(#string, start);
}
return numbers;
}
private static int GetEnd(string #string, int start)
{
var end = #string.IndexOf(Separator, start);
return end >= 0 ? end : #string.Length;
}
}
The parsing uses System.Memory to avoid allocating a new string for each number in the list. If your framework doesn't have the int.TryParse overload that accepts a Span<char> you can use string.Substring instead.