Question

MSDN says:

“Without the suffix m, the number is treated as a double, thus generating a compiler error.”

What does the “M” in:

decimal current = 10.99M;

stand for?

Is it any different than:

decimal current = (decimal)10.99

Answer 1

M makes the number a decimal representation in code.

To answer the second part of your question, yes they are different.

decimal current = (decimal)10.99

is the same as

double tmp = 10.99;
decimal current = (decimal)tmp;

Now for numbers larger than sigma it should not be a problem but if you meant decimal you should specify decimal.

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Update:

Wow, i was wrong. I went to go check the IL to prove my point and the compiler optimized it away.

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Update 2:

I was right after all!, you still need to be careful. Compare the output of these two functions.

class Program
{
    static void Main(string[] args)
    {
        Console.WriteLine(Test1());
        Console.WriteLine(Test2());
        Console.ReadLine();
    }

    static decimal Test1()
    {
        return 10.999999999999999999999M;
    }
    static decimal Test2()
    {
        return (decimal)10.999999999999999999999;
    }
}

The first returns 10.999999999999999999999 but the seccond returns 11

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Just as a side note, double will get you 15 decimal digits of precision but decimal will get you 96 bits of precision with a scaling factor from 0 to 28. So you can represent any number in the range ((-2⁹⁶ to 2⁹⁶) / 10^{(0 to 28)})