Avoid boxing nullable values in BindingValue.

[MarchingCube]

What does the pull request do?

Deals with Nullable<T> problems when used as a binding value.

What is the current behavior?

Due to dotnet/runtime#50915 we are boxing TransformedBounds? when it is passed to binding code.
This happens quite frequently since each visual has a property using this struct, plus we update it a lot when resizing.

What is the updated/expected behavior with this PR?

How was the solution implemented (if it's not obvious)?

Checklist

• Added unit tests (if possible)?
• Added XML documentation to any related classes?
• Consider submitting a PR to https://github.com/AvaloniaUI/Documentation with user documentation

Breaking changes

Obsoletions / Deprecations

Fixed issues

[ThomasGoulet73]

@MarchingCube Doesn't this disable BindingValue validation for projects that reference Avalonia.Base built in Release ? I think some people still want the validation.

[MarchingCube]

Yeah, this is basically a compromise. Although this code is mostly used internally so I was not expecting too many people using this directly.

If you have an idea on how to fix this case of nullable value type boxing - let me know. Especially since JIT can produce different asm per supported runtime (I think only net6 is capable of eliding boxing for this pattern)

[maxkatz6]

I also believe it's not related to value validation in the user code.

[MarchingCube]

Yeah, this is mostly an internal sanity check to ensure that we are not creating BindingValue<T> with errors using this ctor (since there are other variants that are meant to be used instead).

[ThomasGoulet73]

I'm not aware of the inner workings of BindingValue so I don't really know if or when it is called but if you want to avoid boxing and keep the validation, I guess you could use something similar to this:

if (typeof(T).IsValueType)
{
	if (value.GetType() == typeof(BindingValue<object>))
	{
		throw new Exception();
	}
}
else
{
	if (value is UnsetValueType)
	{
		throw new Exception();
	}

	if (value is DoNothingType)
	{
		throw new Exception();
	}

	if (value is BindingValue<object>)
	{
		throw new Exception();
	}
}

This avoids boxing by comparing the type of T when T is a value type and using is when T is a reference type. When T is a reference type, value will be a reference type or an already boxed value type.

Here's a quick benchmark:
Object with int value

Method	Mean	Error	StdDev	Ratio	RatioSD	Gen 0	Gen 1	Gen 2	Allocated
Old	5.249 ns	0.1022 ns	0.1093 ns	1.00	0.00	0.0038	-	-	24 B
New	5.271 ns	0.0956 ns	0.0848 ns	1.00	0.02	0.0038	-	-	24 B

Nullable int with int value

Method	Mean	Error	StdDev	Ratio	Gen 0	Gen 1	Gen 2	Allocated
Old	117.87 ns	0.850 ns	0.754 ns	1.00	0.0114	-	-	72 B
New	40.70 ns	0.303 ns	0.269 ns	0.35	0.0038	-	-	24 B