If you want to specify the name of the format argument, you have to specify the name of the following argument also:
string.Format(format:"abbccc", arg0:1, arg1:22);
That's not very useful, as the names "arg0" and "arg1" doesn't say anything at all about the arguments.
Also, there are only overloads up to "arg2", so if you have more arguments, you have to put them in an array to name the argument:
string.Format(format:"abbccc", args:new object[] { 1, 2, 3, 4 });
You can simply skip naming the arguments:
string.Format("abbccc", 1, 22);
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