Module default_csharp 

The value a C# member takes when the stream does not carry it.

Table 9 of DDS-XTypes 1.3, as C#. crate::default_c is the same table as C, crate::default_java as Java and crates/wire-probe/src/default.rs as the reflective column; that module carries the clause quotations and the reasoning for the three kinds Table 9 does not list. Deliberately a fifth derivation and not a call into any of them: the columns are compared against each other, and a shared helper would make them agree about a mistake.

Why it is a generated method and not a literal

A literal cannot carry an aggregate. “A structure without any of the optional members and with other members set to their default values” is a new S() and one assignment per member, and writing that at every position a member can go missing would put the same answer in the generated file as many times as the type is reached — where two copies can differ.

Why it answers with the value instead of filling one

C’s defaulter is void zd_def_T(T *v): it writes through a pointer. C# has ref and out and neither is usable at the call sites the walk produces — a property cannot be passed as either (CS0206), and a Nullable<T> member is not an out T. So the method returns Table 9’s value and the caller assigns it, which is the same decision one step later, and it is what makes the reader’s shape work: every generated decoder opens with T v = zd_def_T();, so a position the stream does not carry is already what Table 9 says before a single octet is read.

What C# settles that Java could not

Every zero row of Table 9 is spelled by the member’s own width, because the mapping is one to one — an octet is a byte and an unsigned long is a uint, where the Java column has to write both as the next signed type up. What is left is the two rows C# spells differently from every other column: a float needs its f suffix because a real literal is a double and there is no implicit narrowing even for a constant, and an enumeration’s default is a real enum member rather than an int.

A sequence<T> is a T[] and its “zero-length sequence” is new T[0] and not a null: null is what an @optional member’s absence means here, and one value cannot mean both. §7.2.2.4.4.4.7 asks for a zero-length sequence, and §7.2.2.4.4.4.8’s “Implementations shall not provide a default value” for an @optional member is the null.

Functions

csharp_defaulters_called_by

The defaulters members call, and exactly those.

csharp_defaulters_over

Build the defaulter for each of types, once per name.

default_into

place = zd_def_<T>();

defaulter_from_decoder

The defaulter that goes with a decoder, by name.

defaulter_name

The defaulter method name for a type.

member_default

One member’s default: its @default, or its type’s.