Question

        string str = "({{0}})";
        int i = 0;
        string str2 = string.Format(str, i++);
        string str3 = string.Format(str, i++);

why is str3 ({0}) instead of ({1})?

Answer 1

You escaped the curly braces so they have no special meaning. From the documentation:

To specify a single literal brace character in format, specify two leading or trailing brace characters; that is, "{{" or "}}".

You can simplify your program and still demonstrate the problem:

Console.WriteLine("{{0}}", 1);

Output:

{0}

See it working online: ideone

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To get the desired output you need to use {{ followed by {0} and then finally }}:

string str = "({{{0}}})";