ARROW-7289: [C#] ListType constructor argument is redundant

[HashidaTKS]

ListType constructor is redundant and should be seperated.

[github-actions]

https://issues.apache.org/jira/browse/ARROW-7289

[HashidaTKS]

sorry, I'll fix the commit history.

[HashidaTKS]

I fixed it.

[eerhardt]

Can this constructor just call into the constructor above? Similar to the way it works in C++:

arrow/cpp/src/arrow/type.h

Lines 591 to 597 in 81909dc

	// List can contain any other logical value type
	explicit ListType(const std::shared_ptr<DataType>& value_type)
	: ListType(std::make_shared<Field>("item", value_type)) {}
	explicit ListType(const std::shared_ptr<Field>& value_field) : BaseListType(type_id) {
	children_ = {value_field};
	}

[HashidaTKS]

Thanks for your suggestion.
I modified it.

[kou]

It almost looks good to me.
Could you also check my comments?

[kou]

Should we throw an exception for null valueDataType?
NullType isn't usable.

@eerhardt What do you think about this?

[HashidaTKS]

The line was removed by the new commit but same check exists in Field.
Should we throw an exception there?

https://github.com/apache/arrow/blob/master/csharp/src/Apache.Arrow/Field.cs#L44

[eerhardt]

I don't see this behavior in either the C++ or Java implementation.

I'd say it is up to you whether to leave the Field constructor as-is, or change it to throw on a null type. I could go either way on it.

[kou]

OK. We can do it as a follow-up task when we want to do.
We can merge this.

[kou]

It seems that int64 case is redundant.
Should we check both string and int64 cases?

[HashidaTKS]

Indeed, I remove it.

[eerhardt]

Looks good to me. Thanks @HashidaTKS!

[kou]

+1