Question

public static object GetObject(int x)
{
    return new object { };
}
public static object GetObject(string x)
{
    return new object { };
}
public static void ProcessObject<T>(T x) where T : int, string <= got error here: 
{
    object o = GetObject(x);
}

Got error “A type used as a constraint must be an interface, a non-sealed class or a type parameter.”

How can I rewrite the code to get it work without write ProcessObject(int x) and ProcessObject(string x) twice?

Answer 1

So what you have now (according to accepted answer and your comments) is:

public static void ProcessObject<T>(T x)
{
    object o;
    if (typeof(T) == typeof(int))
        o = GetObject((int)(object)x);
    else if (typeof(T) == typeof(string))
        o = GetObject((string)(object)x);
    else
        throw new Exception();
    // do stuff with o
}

I’d recommend making public int and string overloads, but to prevent code duplication, internally call another method:

public static void ProcessObject(int x)
{
    ProcessObject(GetObject(x));
}
public static void ProcessObject(string x)
{
    ProcessObject(GetObject(x));
}
private static void ProcessObject(object o)
{
    // do stuff with o
}

This makes your public methods’ input values clear: int and string are the only acceptable types, while still not duplicating your actual logic (// do stuff with o).

You might dislike that the two public ProcessObject methods are duplicates of each other, (on the surface anyway; under the hood, they’re calling two different GetObject overloads) but I think it’s the best option.