How to convert a non-nullable type to a nullable type?

Question

Is it possible to convert a non-nullable value type known only at runtime to nullable? In other words:

public Type GetNullableType(Type t)
{
    if (t.IsValueType)
    {
        return typeof(Nullable<t>);
    }
    else
    {
        throw new ArgumentException();
    }
}

Obviously the return line gives an error. Is there a way to do this? The Type.MakeGenericType method seems promising, but I have no idea how to get a unspecified generic Type object representing Nullable<T> . Any ideas?

Answer 1

you want typeof(Nullable<>).MakeGenericType(t)

Note: Nullable<> without any supplied arguments is the unbound generic type; for more complex examples, you would add commas to suit - ie KeyValuePair<,> , Tuple<,,,> etc.

Answer 2

You're on the right track. Try this:

if (t.IsValueType)
{
    return typeof(Nullable<>).MakeGenericType(t);
}
else
{
    throw new ArgumentException();
}

Answer 3

Type GetNullableType(Type type) {
    // Use Nullable.GetUnderlyingType() to remove the Nullable<T> wrapper
    // if type is already nullable.
    type = Nullable.GetUnderlyingType(type);
    if (type.IsValueType)
        return typeof(Nullable<>).MakeGenericType(type);
    else
        return type;
} 

Answer 4

The easiest solution is to return the UnderlyingSystemType of the first of the GenericTypeArguments. So in this example, a Nullable Datetime? is returning as the property type and I need to convert it to a Datetime type so it can be added to a DataTable. This should work with int? and double? etc.

if (Nullable.GetUnderlyingType(prop.PropertyType) != null) {
  tb.Columns.Add(prop.Name, prop.PropertyType.GenericTypeArguments.First().UnderlyingSystemType);
} else {
  tb.Columns.Add(prop.Name, prop.PropertyType);
}