Default value behaviour – nullable value types point in list is inconsistent with other places and incorrect

[bravequickcleverfibreyarn]

• For value types, the array elements are initialized with the default value, the 0-bit pattern; the elements will have the value 0.
• All the reference types (including the non-nullable), have the values null.
• For nullable value types, HasValue is set to false and the elements would be set to null.

From Types|Default constructors

For a nullable_type the default value is an instance for which the HasValue property is false and the Value property is undefined. The default value is also known as the null value of the nullable type.

From Default values of C# types (C# reference)

Any nullable value type | An instance for which the HasValue property is false and the Value property is undefined. That default value is also known as the null value of a nullable value type.

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FPOV is it correct to state that value hidden in Nullable<T> is default (not null) rather than presenting behavior characteristics of Value property. That since initialization/default is discussed which by nature relates to fields.

On the other hand private readonly bool hasValue stays in darkness of black box for consumer so there is little antinomy in such approach.

Maybe just right would be to state 0-bit pattern obedience along with behavior of Value property. Many other structs are black boxes that return via properties so I guess there is no need to tell explicitly that after default initialization any bool property returns false.

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Document Details

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• ID: a02d7544-6f92-5582-9d28-7160a5bc916a
• Version Independent ID: 7e3f884a-94b6-ea27-70ea-060fe51f51c2
• Content: Arrays - C# Programming Guide
• Content Source: docs/csharp/programming-guide/arrays/index.md
• Product: dotnet-csharp
• Technology: csharp-fundamentals
• GitHub Login: @BillWagner
• Microsoft Alias: wiwagn

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Associated WorkItem - 148959

[BillWagner]

Thanks for noting this @Uzivatel919

I want to think about how to present this in a way that is both consistent and usable. There are a few related behaviors with respect to arrays and types (especially nullable types). I think the updates are somewhere along these lines:

• When an array is constructed, the storage for all the elements is initialized to the 0 bit pattern. That means:
  • All struct types are initialized to their 0 values. (Note that in C# 10, once structs can have a default constructor, that may be different than the instance generated from the default constructor.)
  • All reference types are initialized to null. This includes non-nullable value types. An array of a non-nullable reference type has all null elements.
  • Nullable value types are initialized to the 0 bit pattern. That means HasValue is false, and Value is undefined.

The reason for this behavior is to balance the performance needs of not calling a parameterless constructor for each element with having well defined behavior for freshly allocated arrays.

[BillWagner]

Fixing this as part of the work on collection expressions. #36356