"Dim a=b". What type of declaration is this? How can it be used in c#?

It can't be used in c#, it is a VB code line.
What it does is declare a new variable a and assign it the value of b. Since VB is not type safe, it doesn't car what type a or b are.

C# is type safe. The closest you can get to this are:

dynamic a = b;

or

var a=b;

But it is much, much better to declare a using the correct datatype in the first place, depending on what the datatype of b could be.:

int a = b;

Button a = b;

Control a = b;