How to convert numeric string object to nullable numeric

Question

With the following example, two calls are made to ConvertNumericStringObj, it sends back a Type int object both times.

string strValue = "123";
object obj = ConvertNumericStringObj(typeof(int), strValue);
object obj = ConvertNumericStringObj(typeof(int?), strValue);   

public static object ConvertNumericStringObj(Type conversion, object value)
{
    var t = conversion;
    if (t.IsGenericType && t.GetGenericTypeDefinition().Equals(typeof(Nullable<>)))
    {
        if (value == null)
        {
            return null;
        }
        t = Nullable.GetUnderlyingType(t);
    }
    return Convert.ChangeType(value, t);
}

My question is: Is there someway to pass in the string and int? Type and convert it so it returns a int? object?

You cannot have a boxed Nullable<T>. The runtime has special boxing behavior for nullables. If an object reference is a boxed T then it can always be converted to a T?. — Mike Zboray
– Mike Zboray, Commented Oct 16, 2014 at 5:15
See mike z's comment. That said, if you can change the method, you could make it generic and have it return T instead of object. That would get around the boxing restriction on Nullable<T>. Then you also would have to continue the code in the Nullable<T> special case that already exists (which is there because Convert.ChangeType won't convert to Nullable<T>), to convert to the underlying type and rewrap it in a Nullable<T> to return it. It's a lot of trouble to go to, but if you have to have do things this way, it can be done. — Peter Duniho
– Peter Duniho, Commented Oct 16, 2014 at 5:23
Thanks Peter, I think your right. Too much trouble in the end I need to compare these values as well, which nullables don't do. So this just means handling the nulls carefully and converting to their underlining Type. — Hank
– Hank, Commented Oct 16, 2014 at 5:34

dexterlo · Accepted Answer · 2014-10-16 05:39:34Z

1

If you want the types to potentially be either int or int?, then what you're looking for is "generics". This should get you what you want.

public static T ConvertNumericStringObj<T>(string value)
{
    var t = typeof (T);
    if (t.IsGenericType && t.GetGenericTypeDefinition() == typeof(Nullable<>))
    {
        if (string.isNullOrEmpty(value))
            return default(T);

        t = Nullable.GetUnderlyingType(t);
    }
    return (T)Convert.ChangeType(value, t);
}

However, I'm curious why you wouldn't just return a nullable integer resulting from int.TryParse() instead.

public static int? ConvertNumericStringObj(string value)
{
    int? x;

    if (int.TryParse(value , out x)
        return x;

    return null;
}

answered Oct 16, 2014 at 5:39

dexterlo

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2 Comments

Hank Over a year ago

Thank you, I am not very fluent on generics, but not sure that this work however, this would mean that when calling I'd have to set the `T' Type explicitly wouldn't it? Which unfortunately is not what I'm after.

dexterlo Over a year ago

You might want to rethink what you're trying to do with this method. How would you consume the return value, if you don't know what underlying type to expect? You should either always return a nullable type (i.e. pass in int? to the generic method above) or always know what specific type (e.g. int or int?) when you call the generic method.

Zeeshan · Accepted Answer · 2014-10-16 05:41:12Z

0

Yes you can. Try Int32.TryParse.

public static int? ConvertNumericStringObj(string strValue)
{
    int x;
    if (Int32.TryParse(strValue , out x)
        return x;
    return null;
}

But I wonder, if you necessarily need to pass int? ?

Edit: as OP asked it to be a little generic, try extension method (roughly) like,

public static T? ConvertNumericStringObj<T>(string strValue) 
{
    if (string.IsNullOrEmpty(strValue))
        return null;
    return (T) Convert.ChangeType(strValue, typeof(T));
}

This way, you can then use as:

int? x = strX.ConvertNumericStringObj();

edited Oct 16, 2014 at 5:41

answered Oct 16, 2014 at 5:24

Zeeshan

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3 Comments

Ben Voigt Over a year ago

All you've done is take away the type parameter. Now it can't handle other types.

Zeeshan Over a year ago

@BenVoigt, I don't think, OP cared for other types. he said, My question is: Is there someway to pass in the string and int? ..

Hank Over a year ago

My mistake in the title, my apologies on that, I did mean for it to be generic.

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