Reflection support for Tuple and ValueTuple

[steveharter]

Background and Motivation

When using Tuple and ValueTuple, metadata is hard to access:

• The name of the members ("Item1", "Item2", etc) must be obtained through reflection. For Tuple, they are the property names. For ValueTuple, they are field names. Also, Tuple is an immutable reference type while ValueTuple is a mutable value type which further complicates the usage patterns.
• When more than 7 elements are used, the Rest property\field must be used, which is another Tuple, and thus the total count is not obvious and must be calculated.
• The "TransformName" or friendly parameter name used with ValueTuple must be obtained from reflection in a non-trivial manner by inspecting [TupleElementNames].

At runtime, the ITuple interface is useful to abstract away the Rest property\field and provide a count, although it doesn't support the "TransformName". However, similar behavior is not available through reflection. See dotnet/csharplang#1906.

Proposed API

• Add extension methods to the existing System.Reflection.Extensions assembly.
  • This avoids runtime knowledge and pay-to-play for consumers.
• These look-up the various metdata and return an new TupleInfo value type.
  • This is somewhat slow and no extra caching is performed (like similar cases elsewhere). Caching should be done at a higher level, if necessary.
• The extensions should work for both runtime-based and designtime-based reflection types (via MetadataLoadContext).

API (in System.Reflection namespace):

public static class EventInfoExtensions
{
+    public static TupleInfo[] GetTupleInfo(this EventInfo eventInfo);
}

+public static class FieldInfoExtensions
{
+    public static TupleInfo[] GetTupleInfo(this FieldInfo fieldInfo);
}

public static class MemberInfoExtensions
{
+    public static TupleInfo[] GetTupleInfo(this MemberInfo member);
}

public static class MethodInfoExtensions
{
+    public static TupleInfo[] GetTupleInfo(this MethodInfo method);
}

+public static class ParameterInfoExtensions
{
+    public static TupleInfo[] GetTupleInfo(this ParameterInfo parameter);
}

public static class PropertyInfoExtensions
{
+    public static TupleInfo[] GetTupleInfo(this PropertyInfo property);
}

public partial struct TupleInfo
{
    public readonly string Name { get; }
    public readonly string? TransformName { get; }
    public readonly System.Type Type { get; }
}

InvalidOperationException is thrown if the various metadata type (PropertyInfo, etc) is not a Tuple or ValueTuple.

Prototype at master...steveharter:ReflectionExt

[jeffhandley]

We won't get to this during .NET 6; moving to .NET 7.

[steveharter]

This is a lower-priority issue with no reported asks from the community. Moving to future.