How to Convert a String to a Nullable Type Which is Determined at Runtime?

Question

I have the below code and I'd need to convert a string to a type which is also specified from String:

 Type t = Type.GetType("System.Nullable`1[[System.DateTime, mscorlib, Version=2.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089]]");

            object d = Convert.ChangeType("2012-02-23 10:00:00", t);

I get the below error messagE:

Invalid cast from 'System.String' to 'System.Nullable`1[[System.DateTime, mscorlib, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089]]'.

How would that be nicely possible?

I know one ugly way would be to check whether the type is nullable using if:

    Type possiblyNullableType = Type.GetType("System.Nullable`1[[System.DateTime, mscorlib, Version=2.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089]]");

    var underlyingType = Nullable.GetUnderlyingType(possiblyNullableType);

    Object result;

    // if it's null, it means it wasn't nullable
    if (underlyingType != null)
    {
        result = Convert.ChangeType("2012-02-23 10:00:00", underlyingType);
    }

Would there be any better way?

Thanks,

score 34 · Accepted Answer · answered Feb 24 '12 at 09:37

34

There are two problems.

Firstly, Convert.ChangeType just plain does not support nullable types.

Secondly, even if it did, by boxing the result (assigning it to an object), you'd already be converting it to a DateTime.

You could special case nullable types:

string s = "2012-02-23 10:00:00";
Type t = Type.GetType("System.Nullable`1[[System.DateTime, mscorlib, Version=2.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089]]");
object d;

if (t.IsGenericType && t.GetGenericTypeDefinition() == typeof(Nullable<>))
{
    if (String.IsNullOrEmpty(s))
        d = null;
    else
        d = Convert.ChangeType(s, t.GetGenericArguments()[0]);
}
else
{
    d = Convert.ChangeType(s, t);
}

answered Feb 24 '12 at 09:37

Why would you need to check t.IsGenericType? the t.GetGenericTypeDefinition() == typeof(Nullable<>) part would cover that; wouldn't it? – The Light Feb 24 '12 at 12:09
2

@William `GetGenericTypeDefinition()` throws an exception if the type isn't generic. – Feb 24 '12 at 13:41
Thank you so much for this elegant solution, you saved my day! – Ron Michael Mar 12 '20 at 17:36

score 14 · Answer 2 · answered Feb 24 '12 at 10:07

I wrote the below generic helper method which works in most scenarios (not tested with generic types):

static void Main(string[] args)
{
    Object result =
        ConvertValue(
            "System.Nullable`1[[System.DateTime, mscorlib, Version=2.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089]]",
            "2012-02-23 10:00:00");
}

public static Object ConvertValue(string typeInString, string value)
{
    Type originalType = Type.GetType(typeInString);

    var underlyingType = Nullable.GetUnderlyingType(originalType);

    // if underlyingType has null value, it means the original type wasn't nullable
    object instance = Convert.ChangeType(value, underlyingType ?? originalType);

    return instance;
}

Thank you. This is the most elegant and working solution. – Segmentation Fault Feb 27 '17 at 13:42 — Segmentation Fault, Feb 27 '17 at 13:42

score 2 · Answer 3 · answered Jul 31 '16 at 13:36

public static T GetValue<T>(string Literal, T DefaultValue)
    {
        if (Literal == null || Literal == "" || Literal == string.Empty) return DefaultValue;
        IConvertible obj = Literal;
        Type t = typeof(T);
        Type u = Nullable.GetUnderlyingType(t);

        if (u != null)
        {
            return (obj == null) ? DefaultValue : (T)Convert.ChangeType(obj, u);
        }
        else
        {
            return (T)Convert.ChangeType(obj, t);
        }
    }

score 0 · Answer 4 · edited May 23 '17 at 12:10

0

Something like this? Unless you really need to do it dynamically.

if (string.IsNullOrEmpty(input))
{
   return new DateTime?();
}
else
{
   return new DateTime?(DateTime.Parse(input));
}

Possibly you could check to see if your type is one of the 'nullable' types and then maybe you could find something useful here:

Convert string to nullable type (int, double, etc...)

edited May 23 '17 at 12:10

Community

1
1

answered Feb 24 '12 at 09:32

Paddy

33,309
15
79
114

I can't specify DateTime, it's determined at runtime. – The Light Feb 24 '12 at 09:33
`return new DateTime?();` should be replaced by `return null;`. – ken2k Feb 24 '12 at 09:35
Perhaps if you could supply some more information about what you're trying to do as opposed to what you're currently doing we could advise? – Paddy Feb 24 '12 at 09:36

How to Convert a String to a Nullable Type Which is Determined at Runtime?

4 Answers4