Question

Why var a = 7; would set a type to a certain type (i.e. int instead of byte)? Are there any rules/defaults/checks made on the fly by C# compiler?

Answer 1

It’s not clear what you mean by “on the fly” – but the C# compiler simply follows the rules laid down in the spec. For a declaration of the kind:

var a = expression;

the type of a is the type of expression. The expression 7 is of type int, although it’s also known to be a constant within the range of byte, allowing:

byte a = 7;

to compile. The availability of that conversion to byte doesn’t change the type of the expression 7 though, so int is what the C# compiler uses for the type of a.

Note that I’d recommend against using var for constants like this. It ends up with code which can get pretty confusing around the boundaries of int, uint, long etc. var is meant to help with anonymous types, and also to help make code more readable. When it makes code less readable, just don’t use it.