2.0Beta1: Determine T from T?

[n8wrl]

Good afternoon!

I have a method that adds a SQL parameter with the right SQLtype based
on a CLR type:

public void AddParam<T>(string name, T value)
{
...
}

I translate 'T' to a valid DbType. The problem is when T is Nullable,
as in

AddParam<int?>(name, x);

How can I get the 'int' from 'int?' inside AddParam() so I can
translate the type & handle NULLs properly?

Things I've tried:

* Nullable.ToObject<T>(value) returns a boxed object.
* value ?? value returns a T?
* Nullable.ValueOrDefault<T>(value) returns a boxed object.
* Try a Nullable-specific AddParam<T>(...) where T : Nullable but that
complains about static structures as a constraint.
* Assign value directly to the SQL procedure but I get a "No mapping
exists from object type System.Nullable to a known managed provider
native type."

Thanks!

-Brian

 

[Sylvain Lafontaine]

Why aren't you using a SqlTypes like SqlInt32 instead of « int? » ?

int? has not be introduced with the intent of replacing SqlTypes like
SqlInt32.

S. L.

 

[BrianS]

At some point I have to interface SQL types to 'real' types on business
objects - types clients are familiar with, like int, string, etc. This
is the place I chose to do that.

Thanks for the reply!

 

[Alexander Shirshov]

Brian,

Probably I'm missing something, but I don't understand what are you trying
to achieve.

Since you're passing a value, you already have type information in it and
you can simply call GetType():

void AddParam(string name, object value)
{
if (value.GetType() == typeof(string))
{
...
}
....
}

If you want to give callers some degree of control over parameter's data
type (like in AddParam("int_column", "123")) then you could add an overload
with datatype parameter:

void AddParam(string name, object value, Type dataType)


HTH,
Alexander

 

[Jon Skeet [C# MVP]]

Probably I'm missing something, but I don't understand what are you trying
to achieve.

Since you're passing a value, you already have type information in it and
you can simply call GetType():

void AddParam(string name, object value)
{
if (value.GetType() == typeof(string))
{
...
}
....
}

Note that that's a lot slower (and less readable, IMO) than:

if (value is string)

or

string valAsString = value as string;
if (valAsString != null)
{
....
}

 

[Alexander Shirshov]

Note that that's a lot slower (and less readable, IMO) than:

if (value is string)

Is uniformity important for readability? "is" will not work for value types.

And I don't think the performance is an issue for the original poster. DB
operations are many orders of magnitude slower and I don't think anyone
would be adding query parameters by thousands.

 

[Daniel O'Connell [C# MVP]]

Is uniformity important for readability? "is" will not work for value
types.

I believe you are mistaken. "as" will not work for value types, but is will
work perfectly well. "is" will(atleast in the 2.0 beta) issue a warning
informing you that the expression will always be true if you apply is to a
value type variable(ie meaning a non-boxed value type), but it still works.

 

[BrianS]

You're probably right. I was trying to use generics to avoid boxing and
a dozen type-specific methods that are all pretty much the same. But
since It's assigned to SqlParameter.Value we box anyway.

The other problem is even if value is an int?, for example, 'if (value
is Nullable)' returns false, so it's not easy to figure that out in a
generic <T> way.

Thanks for the input!

-Brian

 

[Alexander Shirshov]

Yes, I quoted and commented not the sentence I intended. Thanks for
correcting.