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

Question

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.

Are you saying you have `Type someType = typeof(T)` and you want to use them to get a `T myT`? — Daniel A. White, Sep 05 '11 at 23:45
You can't safely do that. Those assignments are checked at compile tim. — Daniel A. White, Sep 05 '11 at 23:47
Under what circumstances do you expect to receive a null nullable decimal? — Eric Lippert, Sep 05 '11 at 23:56
possible duplicate of [Convert.ChangeType() fails on Nullable Types](http://stackoverflow.com/questions/3531318/convert-changetype-fails-on-nullable-types) — lesderid, Aug 05 '13 at 19:16

score 24 · Answer 1 · edited May 23 '17 at 11:53

24

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);

edited May 23 '17 at 11:53

Community

1
1

answered Aug 05 '13 at 18:03

Dror

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score 5 · Answer 2 · answered Aug 17 '14 at 19:31

5

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);
}

answered Aug 17 '14 at 19:31

Ziad

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I also added a check for null or whitespace but this is a great answer thank you string.IsNullOrWhiteSpace(value.ToString()) – workabyte Nov 04 '14 at 15:43

score 3 · Accepted Answer · answered Sep 05 '11 at 23:36

3

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?

answered Sep 05 '11 at 23:36

Daniel A. White

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OK but for example, imagine that my typeConverter is a nullable type (decimal?), in Type object is there a property can get only decimal type (not decimal?) ? – Darf Sep 05 '11 at 23:38
can you expand your question with more details? – Daniel A. White Sep 05 '11 at 23:39

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

3 Answers3

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