Generic Type Parameter constraints in C# .NET

Consider the following Generic class:

public class Custom<T> where T : string
{
}

This produces the following error:

'string' is not a valid constraint. A type used as a constraint must be an interface, a non-sealed class or a type parameter.

Is there another way to constrain which types my generic class can use?

Also, can I constrain to multiple types?

E.G.

T can only be string, int or byte