[C#] Implement MapType

[Platob]

Describe the enhancement requested

Hello,

i started dev on new implementation of MapType, im wondering how it should be built ?

I think is a list of Struct which are like KeyValuePair c# class

using System.Collections.Generic;

namespace Apache.Arrow.Types
{
    public class MapType : NestedType
    {
        public override ArrowTypeId TypeId => ArrowTypeId.Map;
        public override string Name => "map";

        public Field KeyField => Fields[0];
        public Field ValueField => Fields[0];

        public IArrowType ValueDataType => Fields[0].DataType;

        public MapType(IArrowType keyType, IArrowType valueType, bool nullable)
            : this(new Field("key", keyType, false), new Field("value", keyType, false), nullable)
        {
        }

        public MapType(Field keyField, Field valueField, bool nullable)
            : this(new StructType(new List<Field>() { keyField, valueField }), nullable)
        {
        }

        public MapType(StructType structType, bool nullable) : this(new ListType(new Field("key_value", structType, nullable)), nullable)
        {
        }

        public MapType(ListType listType, bool nullable) : this(new Field("key_values", listType, nullable))
        {
        }

        public MapType(Field keyValueField) : base(keyValueField)
        {
        }

        public override void Accept(IArrowTypeVisitor visitor) => Accept(this, visitor);
    }
}

Component(s)

C#

[westonpace]

I think is a list of Struct which are like KeyValuePair c# class

Yes, sounds like you have it correct. Here is the comment from schema.fbs:

/// A Map is a logical nested type that is represented as
///
/// List<entries: Struct<key: K, value: V>>
///
/// In this layout, the keys and values are each respectively contiguous. We do
/// not constrain the key and value types, so the application is responsible
/// for ensuring that the keys are hashable and unique. Whether the keys are sorted
/// may be set in the metadata for this field.
///
/// In a field with Map type, the field has a child Struct field, which then
/// has two children: key type and the second the value type. The names of the
/// child fields may be respectively "entries", "key", and "value", but this is
/// not enforced.
///
/// Map
///
/// - child[0] entries: Struct
/// -- child[0] key: K
/// -- child[1] value: V
///
/// Neither the "entries" field nor the "key" field may be nullable.
///
/// The metadata is structured so that Arrow systems without special handling
/// for Map can make Map an alias for List. The "layout" attribute for the Map
/// field must have the same contents as a List.