Use Generic class with SqlDataReader GetValue

Question

I have this code that works ok with a database with no NULL values

public T GetField<T>(SqlDataReader dr, string fieldName)
{
    return (T)dr.GetValue(dr.GetOrdinal(fieldName));
}

Then I want to control the DBNull values, because they already exist in some tables and I upgraded the function like this:

public T GetField<T>(SqlDataReader dr, string fieldName) where T : new()
{
    var value = dr.GetValue(dr.GetOrdinal(fieldName));   
    return value is DBNull ? new T() : (T)value;
}

But now, I cannot ask for GetField<string> because string has no constructor with 0 parameters.

I have tried to create another function like the Abstract one, with no restrictions and only with string type like this

public string GetField<string>(SqlDataReader dr, string fieldName)
{
    var value = dr.GetValue(dr.GetOrdinal(fieldName));   
    return value is DBNull ? "" : value.ToString();
}

but the answer is that they are ambiguous definitions, because T also includes string, even with the restriction applied.

I could create independent functions for each datatype, but I rather prefer the abstract solution because it is way much cleaner.

Did I miss something?

Thanks!

It's not clear what you mean by "another function like the abstract one" - which abstract what? You haven't shown any abstract methods or classes... did you mean generic? (Generic and abstract mean very different things in C#.) — Jon Skeet
– Jon Skeet, Commented Nov 19, 2014 at 17:15

Jon Skeet · Accepted Answer · 2014-11-19 17:28:23Z

5

Your final method would fail because it's trying to declare a type parameter called string.

I suggest you add another overload without the constraint on T, but with a default value:

public T GetField<T>(SqlDataReader dr, string fieldName, T defaultValue)
{
    var value = dr.GetValue(dr.GetOrdinal(fieldName));   
    return value is DBNull ? defaultValue : (T) value;
}

Then you'd call it as:

string x = foo.GetField(reader, "field", "");

... with the added benefit that the method isn't string-specific.

edited Nov 19, 2014 at 17:28

answered Nov 19, 2014 at 17:14

Jon Skeet

1.5m893 gold badges9.3k silver badges9.3k bronze badges

Sign up to request clarification or add additional context in comments.

2 Comments

Jon Skeet Over a year ago

@JeroenMostert: Yup, fixed.

Beronien Over a year ago

The default value is a nice feature to have and I have included it, thanks!

Jeroen Mostert · Accepted Answer · 2014-11-19 17:25:40Z

2

Here's one approach:

public T GetField<T>(SqlDataReader dr, string fieldName)
{
    var value = dr.GetValue(dr.GetOrdinal(fieldName));
    return value is DBNull ? default(T) : (T) value;
}

This has the added benefit of working with nullable types. Then you'd call that as:

string x = foo.GetField<string>(reader, "field") ?? "";

Which is deliberately contrived in this case -- Jon's version with a default suits you better if usually you want something other than null returned.

answered Nov 19, 2014 at 17:25

Jeroen Mostert

29k2 gold badges60 silver badges92 bronze badges

1 Comment

Beronien Over a year ago

That was exactly was I was looking for. Mixing both solutions give me the flexibility to answer all my scenarios. Many thanks!

Collectives™ on Stack Overflow

Use Generic class with SqlDataReader GetValue

2 Answers 2

2 Comments

1 Comment

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

2 Comments

1 Comment

Your Answer

Sign up or log in

Post as a guest

Related