How to use Convert.ChangeType() when conversionType is a nullable int

7
votes

I mean, I want to convert this:

string a = 24;
Convert.ChangeType(a, typeof(decimal?))

But it throws me an error.

UPDATE 1:

I've got a Type object where can be decimal?, int?, .. many nullable types. Then with the Type object, I need to convert the string value in type object.

3
c#
Are you saying you have Type someType = typeof(T) and you want to use them to get a T myT? – Daniel A. White
Exactly, that's what I am looking for. – Darf
You can't safely do that. Those assignments are checked at compile tim. – Daniel A. White
Under what circumstances do you expect to receive a null nullable decimal? – Eric Lippert
possible duplicate of Convert.ChangeType() fails on Nullable Types – lesderid

3 Answers

23
votes

See an excellent answer here:

public static T GetValue<T>(string value)
{
   Type t = typeof(T);
   t = Nullable.GetUnderlyingType(t) ?? t;

   return (value == null || DBNull.Value.Equals(value)) ? 
      default(T) : (T)Convert.ChangeType(value, t);
}

E.g.:

string a = 24;
decimal? d = GetValue<decimal?>(a);
5
votes

This is based on Dror's answer but has slightly less overhead when dealing with null values:

public static T GetValue<T>(string value)
{
   if(value == null || DBNull.Value.Equals(value))
       return default(T);

   var t = typeof(T);
   return (T)Convert.ChangeType(value, Nullable.GetUnderlyingType(t) ?? t);
}
3
votes

You can't do this since Nullable<T> don't implement IConvertable.

You can do this although.

string a = 24;
decimal? aAsDecimal = (decimal)Convert.ChangeType(a, typeof(decimal));

Might I also interest you in TryParse?