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TIL (meaning, Tonight I Learned) that you can declare non-optional parameters after optional ones in C#.

Just create an indexer with optional parameters:

int this[int a, string b = "b"] { get; set; }

The IL for the set method is this:

instance void set_Item (
int32 a,
[opt] string b,
int32 'value'
) cil managed
{
.param [2] = "b"
}

Note that the "value" is after b.