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I'm creating a generic object from DB data:

object[] data = new object[dataReader.FieldCount];
dataReader.GetValues(data);
T t = (T)Activator.CreateInstance(typeof(T), data);

But, types with no constructor error on the third line. I want to add an if:

if (typeof(T).GetConstructor(data.TypesOf()) != null)

data.TypesOf() is actually an array - Type[] - that contains all types of the objects in data.

What is the equivalent to data.TypesOf() that really works?

Or do I need to iterate data and build it myself?

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2
  • data is always an object[] - it has no relation to typeof(T) whatsoever, since it is just the values from the reader. What are you trying to represent with data.TypesOf() ? Commented May 7, 2013 at 8:30
  • @MarcGravell. data.TypesOf() is a Type[] array which contains the types of data. In order, at that. Added to question. Commented May 7, 2013 at 9:01

4 Answers 4

Reset to default
6

I'm assuming your object[] is containing values that are, for example, an int, a string and a float, and you are trying to resolve a constructor of the form public T(int,string,float). To get the types, you could use:

var types = Array.ConvertAll(data, x => x.GetType());

But that won't actually help you much here, since that is pretty-much what Activator.CreateInstance(typeof(T), data) has already tried to do - so if Activator failed, I don't see that it is obvious that you're going to do any better - unless the key difference is that the constructor is non-public and you are going to supply some BindingFlags.

Personally, though, I would suggest that it is easier to bind by name than by position; there are tools like dapper that will do all of that for you, allowing you to use simply:

var data = conn.Query<SomeType>(sql, args).ToList();

for example:

int userId = 12345;
var users = conn.Query<User>("select * from Users where Id = @userId",
      new {userId}).SingleOrDefault();
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3 Comments

Thanks for the tool. I was actually trying to avoid ORM since I understood it costs in performance and load on DB. I'd like to hear your stand on this, though not here since it's off topic.
@JNF I don't think that is off topic - and your concern is reasonable, but unfounded. We actually wrote dapper to avoid all the performance problems with ORMs - it is a micro-ORM, not a full ORM. It doesn't do anything more than materialization (turning row data into objects), and it does that brutally efficiently (it uses raw IL generation internally, which it caches (the IL), etc). We profile it extensively: it is basically exactly the same speed as hand-rolled IDataReader code, but without the human errors.
@JNF but yes, in the general case, ORMs can be slow - Entity Framework and NHibernate in particular have a lot of complexity and overhead (although I've seen both teams try to improve performance lately). The micro-ORMs are simply in a whole different performance area, though.
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As far as I understood you are trying to get types of elememnts of an object array. So you can do something like:

 var ctorArgsTypes = data.Select(d => d.GetType()).ToArray()

 var ctor = typeof(T).GetConstructor(ctorArgsType);

 // check if appropriate ctor exists
 if (ctor == null)
     throw something

 T t = (T)Activator.CreateInstance(typeof(T), data);
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1 Comment

Right. I was trying to figure out if a built in method exists, instead of invoking a LINQ query.
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When you look at the Activator.CreateInstance(Type, Object[]) method, for the Object[] paramter :

An array of arguments that match in number, order, and type the parameters of the constructor to invoke. If args is an empty array or null, the constructor that takes no parameters (the default constructor) is invoked.

So maybe in your case, your data object is not typed for each value (if getting from a DB or a file). You need to find a way to "type" your objects in your data array.

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7 Comments

@JNF are you sure about that? In particular, mismatches are (I say with confidence from my micro-ORM work) common between single / double / decimal, and short / int / long. I strongly suggest looking at what the Array.ConvertAll(data, x => x.GetType()) says before boldly saying "No. Types are OK.". If the types were OK, Activator would have already found the constructor.
@MarcGravell - sure. I checked my data. Didn't yet run into the problem you're describing, so it's not the issue here. Will be, perhaps another time. :)
@JNF in that case: is the constructor non-public? Put simply, it is either a data mismatch, or a non-public constructor.
@MarcGravell, to make it clearer - sometimes I would explicitly use the function for a type with no constructors. Say int.
@JNF then... why do you have code that is looking for a constructor? Also: dapper's Query<int>(sql, args) would handle that fine
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Just use Type.GetTypeArray(object[]).

https://msdn.microsoft.com/en-us/library/system.type.gettypearray(v=vs.110).aspx

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