How do I create a Dictionary that holds different types in C#

Well, you could use Dictionary<string, dynamic> in C# 4 / .NET 4 – but other than that, you can’t do it with exactly the code shown because there’s no type which is implicitly convertible to int, string and double. (You could write your own one, but you’d have to list each type separately.)

You could use Dictionary<string, object> but then you’d need to cast the results:

int a = (int) Storage.Get("age");
string b = (string) Storage.Get("name");
double c = (double) Storage.Get("bmi");

Alternatively, you could make the Get method generic:

int a = Storage.Get<int>("age");
// etc