C# pass through generic as parameter

Question

I have a method like that:

private Contact GetContactFromDbReaderWithSingleResult(DbDataReader dbReader)
{
    if (dbReader.HasRows)
    {
        return new DataReaderReflection().DataReaderToEntity<Contact>(dbReader)[0];
    }

    return null;
}

and then I could have a quite similar method like that:

private Email GetEmailFromDbReaderWithSingleResult(DbDataReader dbReader)
{
    if (dbReader.HasRows)
    {
        return new DataReaderReflection().DataReaderToEntity<Email>(dbReader)[0];
    }

    return null;
}

The signature of the DataReaderToEntity-Method is:

public List<T> DataReaderToEntity<T>(IDataReader dr) where T : new()

Now I want to create a generic variant GetGenericEntityFromDbReaderWithSingleResullt.

My approach looks like that, but it's not correct:

private object? GetEntityFromDbReaderWithSingleResult<T>(DbDataReader dbReader)
{
    if (dbReader.HasRows)
    {
        return new DataReaderReflection().DataReaderToEntity<typeof(T)>(dbReader)[0];
    }

    return null;
}

Still learning to deal with generics though... What am I doing wrong?

Thank in advance!

And the methods return-type should probably be just `T`, not `object?`. — MakePeaceGreatAgain, Aug 10 '17 at 10:11

AGB · Accepted Answer · 2017-08-10T10:35:57.030

4

Change your return type to T instead of object and remove the typeof:

private T GetEntityFromDbReaderWithSingleResult<T>(DbDataReader dbReader) where T : new()
{
    if (dbReader.HasRows)
    {
        return new DataReaderReflection().DataReaderToEntity<T>(dbReader)[0];
    }

    return default(T);
}

edited Aug 10 '17 at 10:35

answered Aug 10 '17 at 10:11

AGB

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@HimBromBeere T might be a value type though which would not be nullable. – AGB Aug 10 '17 at 10:16
@HimBromBeere No you can't. Your code even would compile. You would get a message like **"Cannot convert null to type parameter 'T' because it could be a value type. Consider using 'default(T)' instead."** Have a look at [How can I return NULL from a generic method in C#?](https://stackoverflow.com/questions/302096/how-can-i-return-null-from-a-generic-method-in-c) – Mighty Badaboom Aug 10 '17 at 10:17
Tried that before, does not work, compiler says: 'T' must be a non-abstract type with a public parameterless constructor in order to use it as parameter 'T' in the generic type or method 'DataReaderReflection.DataReaderToEntity(IDataReader)' – iquellis Aug 10 '17 at 10:30
Your method needs to have where T : new() as well. I've updated my answer. – AGB Aug 10 '17 at 10:36
Yes, thanks. I deleted my correction post after you changed your example. Thank you so much! – iquellis Aug 10 '17 at 10:40

C# pass through generic as parameter

1 Answers1