Well in C# rules are more defined than in C++...
Increment in C#, C and C++[
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In C#, you first example will show 13 and 34 and your second example will show 13 and 8.
In C++, it would be undefined behavior. Both the value of a and c would be indetermined. In practice, the value will depend on the order the compiler will do operations.
C/C++ compiler can essentially evaluate the 3 sub expression in any order and can save back the result of postfix operator at any time and it might load variables value from memory once or multiple times. Thus as soon as the same variable is modified more than once in an expression without sequence point between the 2 operations, the result would be undefined.
In practice a would probably be 11, 12 or 13 in first example and c between 31 and 34 as undefined behavior is usually one of the possible interpretation. For the second example c would probably be between 7 and 12.
By the way, in C/C++ the value will depend on the compiler and often it might also depends on the optimization.
Update
Some more information for C++:
Sequence point - Wikipedia[
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