When to use a Cast or Convert

See Diff Between Cast and Convert on another forum

Answer

The Convert.ToInt32(String, IFormatProvider) underneath calls the Int32.Parse (read remarks).
So the only difference is that if a null string is passed it returns 0, whereas Int32.Parse throws an ArgumentNullException.
It is really a matter of choice whichever you use.

Personally, I use neither, and tend to use the TryParse functions (e.g. System.Int32.TryParse()).

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UPDATE

Link on top is broken, see this answer on StackOverflow.

Cast when it's really a type of int, Convert when it's not an int but you want it to become one.

For example int i = (int)o; when you know o is an int

int i = Convert.ToInt32("123") because "123" is not an int, it's a string representation of an int.