Dictionary<,> is the equivalent. While it doesn't have a Get(...) method, it does have an indexed property called Item which you can access in C# directly using index notation:
class Test {
Dictionary<int,String> entities;
public String getEntity(int code) {
return this.entities[code];
}
}
If you want to use a custom key type then you should consider implementing IEquatable<> and overriding Equals(object) and GetHashCode() unless the default (reference or struct) equality is sufficient for determining equality of keys. You should also make your key type immutable to prevent weird things happening if a key is mutated after it has been inserted into a dictionary (e.g. because the mutation caused its hash code to change).