How to cast a datarow field to a generic type?

Question

I would like to make the following function work:

    T GetRowVal<T>(System.Data.DataRow dr, string fieldName)
    {
        var type=typeof(T);
        var val = dr[fieldName];
        if (type.IsGenericType && type.GetGenericTypeDefinition() == typeof(Nullable<>))
            if (val == DBNull.Value || val == null) return null;
        return (T)val;
    }

Of course, not having constraints on the T type it doesn't want to return null even if it should be allowed in this case. How can I correct this function? (Or if there is a better way to do it, please let me know).

For know, the approach I would use:

    T GetRowVal<T>(System.Data.DataRow dr, string fieldName)
    {
        var type=typeof(T);
        var val = dr[fieldName];
        if (type.IsGenericType && type.GetGenericTypeDefinition() == typeof(Nullable<>))
            if (val == DBNull.Value || val == null) return default(T);
        return (T)val;
    }

New solution :)

return dr.Field<T>(fieldName);

Not sure it is nicer in terms of performances, but probably looks cleaner.

The implementation can be found on MS Github:

https://github.com/Microsoft/referencesource/blob/master/System.Data.DataSetExtensions/System/Data/DataRowExtensions.cs

score 1 · Accepted Answer · answered Jun 02 '17 at 19:20

1

your only option is return default(T);

answered Jun 02 '17 at 19:20

TakeMeAsAGuest

957
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11

score 0 · Answer 2 · answered Jun 02 '17 at 19:23

0

I found a similar question with what seems to be a good response here:

Nullable type as a generic parameter possible?

Nullable<T> GetRowVal<T>(System.Data.DataRow dr, string fieldName)

as the signature could work.

answered Jun 02 '17 at 19:23

The issue is that it would require a cast after or a call to .Value if the type is not Nullable I guess. – Jean Jun 02 '17 at 20:00

How to cast a datarow field to a generic type?

2 Answers2