Generic conversion with Int64

I have a generic method in a MySQL connector witch make generic conversion. I simplify it for the question, cause it makes lot of things. At the end, it make a generic conversion and work fine, but I have a problem to convert Int64 to Int32.

object var1 = (long)64;
int? var2 = Select<int?>(var1); // E开发者_如何转开发RROR Int32Converter from Int64

public T Select<T>(object value)
{
  return (T)System.ComponentModel.TypeDescriptor.GetConverter(typeof(T)).ConvertFrom(value);
}

How can I fix it ?

Can't you use Convert.ChangeType instead?

int value = Convert.ChangeType(var1, TypeCode.Int32);

or

int value = Convert.ToInt32(var1);

note that these will throw exceptions if var is outside the allowable range of Int32

You can't convert from a larger type to a narrower type without an explicit cast.

The problem is that ChangeType isn't compatible with nullable value types (eg. int?).

If you test for Nullable<T> and then convert to the non-nullable type instead it should work. eg.

object var1 = (long)64;
int? var2 = Select<int?>(var1); 

public T Select<T>(object value)
{
    var type = typeof(T);
    if (type.InheritsOrImplements(typeof(Nullable<>)))
        type = type.GetGenericArguments().First();
    // Non-nullable type can be cast as Nullable equivalent
    return (T)TypeDescriptor.GetConverter(type).ConvertFrom(value);
}

BTW... InheritsOrImplements is a handy extension method for Type

public static bool InheritsOrImplements(this Type child, Type parent)
{
    if (child == null || parent == null) return false;

    parent = resolveGenericTypeDefinition(parent);
    if (parent.IsAssignableFrom(child)) return true;

    var currentChild = child.IsGenericType ? child.GetGenericTypeDefinition() : child;
    while (currentChild != typeof(object))
{
        if (parent == currentChild || hasAnyInterfaces(parent, currentChild)) return true;
        currentChild = currentChild.BaseType != null && currentChild.BaseType.IsGenericType
            ? currentChild.BaseType.GetGenericTypeDefinition() 
            : currentChild.BaseType;

        if (currentChild == null) return false;
    }
return false;
}