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Created attachment 23413 [details]
Workbook demonstrating behavior
Evaluating `((string) null) + ( (string) null)` in Workbooks shows an evaluation of `null`, but it should evaluate to an empty string (""). Also:
var s = (string)null;
"" + s;
evaluates to `null` but should evaluate to `""`.
FWIW, "A" + s evaluates properly to "A"
According to "https://github.com/dotnet/csharplang/blob/master/spec/expressions.md#arithmetic-operators"
"These overloads of the binary + operator perform string concatenation. If an operand of string concatenation is null, an empty string is substituted. Otherwise, any non-string argument is converted to its string representation by invoking the virtual ToString method inherited from type object. If ToString returns null, an empty string is substituted."