Question

What is this called?

double  d1 = 0d;
decimal d2 = 0L;
float   d3 = 0f;

And where can I find a reference of characters I can use? If I want to cast 0 to short, which letter I need?

Answer 1

The best source is the C# specification, specifically section Literals.

The relevant bits:

The type of an integer literal is determined as follows:

• If the literal has no suffix, it has the first of these types in which its value can be represented: int, uint, long, ulong.
• If the literal is suffixed by U or u, it has the first of these types in which its value can be represented: uint, ulong.
• If the literal is suffixed by L or l, it has the first of these types in which its value can be represented: long, ulong.
• If the literal is suffixed by UL, Ul, uL, ul, LU, Lu, lU, or lu, it is of type ulong.

If no real_type_suffix is specified, the type of the real literal is double. Otherwise, the real type suffix determines the type of the real literal, as follows:

• A real literal suffixed by F or f is of type float. […]

• A real literal suffixed by D or d is of type double. […]

• A real literal suffixed by M or m is of type decimal. […]

That means the letter (or letters) is called “suffix”. There is no way to represent short this way, so you have to use (short)0, or just short x = 0;.