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We all love format strings for generating human readable text. Certainly, they're a better choice than string concatenation. Even if I have a blind spot that keeps me from remembering the printf format tokens, I still prefer to use format strings.
Jasmine has a co-worker that does use format strings, but they also miss using string concatenation. So they've found a way to combine them, and avoid the perils of making readable code that describes the format of their text.
myField.DataFormatString = "{0:#,##0." + new string('0', 2) + ";(#,##0." + new string('0', 2) + ")}";
Now, in their defense, I can see how this code started its life. At some point, they wanted to make the number of decimal places a parameter. And sure, this is an easy way to do that and retain the intent, though this whole thing could be cleaned up with just a simple decimalPlaces = new string('0', 2), which would make the concatenation less confusing to read.
Related, while I was checking the documentation on the .NET formatting strings, I learned that they have a per-mille format specifier which requires writing a unicode sigil on most keyboards.
I'm sorry, but seeing string perMilleFmt = "#0.## " + '\u2030'; in my code would be TRWTF.