I don't know C#, but what you quote refers to a database NULL. The term "coalesc" is something I associate with databases as well.

A (relational) database NULL is certainly not equivalent to a Perl undef. Ignoring lots of details, the basic difference is:

    undef binary-OP defined-value  ->  defined-value
    NULL binary-OP not-NULL        ->  NULL
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In other words, once you have a NULL, if you apply operations to it (except a few special ones), you remain having a NULL. undef is quite different - it happily behaves like a defined value (0, or the empty string), and once you start applying (most) operations on it, it becomes defined.

    NULL + 1 == NULL
    undef + 1 == 1
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