Question

I’m trying to write a method for coverting a given object to an instance of a given type. I started with this:

private static T TryCast<T>(object o)
{
    return (T) o;
}

Going in, I know that isn’t going to work, but it illustrates the concept. Now, I’m going to start having problems when I have types that won’t cast automatically, like string –> DateTime. I was trying to use the Convert Class to deal with these cases, but I just get a compile time error instead of a runtime error. The following code gets the compile error “Cannot cast expression of type ‘string’ to type ‘T’

private static T TryCast<T>(object o)
{
    var typeName = typeof (T).FullName;

    switch (typeName)
    {
        case "System.String":
            return (T) Convert.ToString(o);
        default:
            return (T) o;
    }
}

I’m also aware of Convert.ChangeType(), but I’m wondering if it will handle edge cases that I would otherwise handle in the above switch, like the stated string –> DateTime that I’d normally just use Convert.ToDateTime for.

private static T TryCast<T>(object o)
{
    return (T)Convert.ChangeType(o, typeof(T));
}

So, what is my best option? If somebody can give me a workable approach, I can take it from there.

Answer 1

Convert.ChangeType should handle edge cases; it delegates to IConvertible.

To answer the question, the compiler doesn’t know that T is string.
Therefore, it doesn’t let you cast between to unrelated types (just like you can’t cast Button to TextBox).

You can work around that by casting to object first:

return (T)(object)o.ToString();

Now, each individual conversion is allowed by the compiler (it’s either a direct upcast or a direct downcast), and you know that the whole thing will work because T is String.