What the purpose of this = default(...) in struct constructor?

Question

During an research the purpose of this reassignment possibility with structs I ran into following puzzle: Why it is needed to do this = default(...) at the beginning of some struct constructor. It's actually zeroes already zeroed memory, isn't it?

See an example from .NET core:

public CancellationToken(bool canceled)
{
    this = default(CancellationToken);
    if (canceled)
    {
        this.m_source = CancellationTokenSource.InternalGetStaticSource(canceled);
    }
}

Answer 1

It's actually zeroes already zeroed memory, isn't it?

No. When you create custom constructor for struct, it's your responsibility to set each and every field of struct to some value.

Just default .ctor() will fill it with zeroes - and that's one of the reasons why you are not allowed to implement own default .ctor for struct in C# (while CLR technically allows it, but it's a different topic to discuss).

So technique here is to call default(), which will create separate instance filled with zeroes, when using 'this = ' assignment will copy all fields from right to left (as it is struct), which will satisfy you need to initialize every field of struct. Then, you can do whatever you want.

In most cases it might be better (from code readability) to use something like this instead:

public MyStruct(..) : this() {
...
}