Need an efficient helper to check if an instance of T is a default(T)

[VSadov]

The need arises once in a while to check if a value at hand is not initialized.

See for example comments on dotnet/corefx#16122 (comment)

Also, If/when parameterless constructors for structs are introduced, the need for IsDefault check could become more apparent.

[VSadov]

CC @jkotas

[jamesqo]

Yes, definitely, this would be awesome. A cost-free API for checking whether some generic type is a class (and not a nullable) would be nice too, if it can be implemented straightforwardly.

[jkotas]

The need arises once in a while to check if a value at hand is not initialized.

Do you have more examples where this would be needed? It is not a rocket science to write such helper - take address of the value and its size, and check whether it is all zeros (using unsafe code, potentially with the usual loop unrolling, etc.).

A cost-free API for checking whether some generic type is a class (and not a nullable) would be nice too

Agree. It should be separate issue.

[JonHanna]

If it could work on a boxed value, I'd use it in System.Linq.Expressions.Compiler.ILGen.TryEmitConstant so that any value type not already handled could have a constant that happened to be the default emitted the same way default expressions are. As is one would have to create a default value of the given type through reflection before using RuntimeHelpers.Equals() and that allocation just to test is probably not worth it.

[sakno]

I've implemented prototype of such method as a part of my library so the code can be reused. Source code is here and the public method is here.
I did that according with the following assumptions:

1. The value to be checked is passed by value in the method so I have guarantees that stack-allocated value will not be moved by GC event if value type has fields of reference types
2. sizeof IL instruction always can be replaced by constant value at JIT compilation. All checks in my implementation based on assumption that JIT removes dead code because all subsequent checks based on the value returned by sizeof.
3. The code contains a few optimized paths for data types of size 1, 2, 4 and 8. In this case JIT replaces my code with simple ceq instruction.
4. For large structs (more than 8 bytes) I use memory aligned zero check accelerated by Vector if underlying hardware supports it

Additionally, I have BitwiseEquals method that works in similar way and it demonstrates good performance.

However, I used inline IL for implementation to keep the full control over generated code. But I'm sure that the same behavior can be achieved using S.R.CS.Unsafe class

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