How can I add a type parameter to my DAL's implementation of ExecuteScalar?

Question

So as everyone knows, .NET's SqlClient class offers as a way of getting data from the database the ExecuteScalar method, which returns an object . While it's not a big deal to just cast appropriately, I wanted to write a strongly typed version that returns a properly typed object. So, I wrote the following C#:

public T ExecuteScalar<T>(IDbCommand cmd) where T : struct
{
    cmd.Connection = this.conn;
    object o = cmd.ExecuteScalar();
    return (T)o;
}

This works very well for booleans and DateTime s. However, for integers, it throws an InvalidCastException . So, I did what anyone armed with a copy of Reflector would do and dove into the Field<T>(this DataRow, string columnName) extension method. I ripped out the internal class that it uses to convert values to the requested type and tested. However, for value types, the code is just return (T)value; - which is of course not helpful.

So, my question: anyone have any thoughts about how I can get my method to properly return values for all value types and strings? Even just string, boolean, DateTime and int would be fine.

Thanks in advance!

Answer 1

Your query is returning a boxed instance of a different numeric type (probably decimal ).

You cannot unbox it and convert it to int in one operation .

Instead, you can call Convert.ChangeType :

return o is t ? (T)o : (T)Convert.ChangeType(o, typeof(T));

Answer 2

Looking at your code the cast should be OK, if the value returned is in fact an integer.

Perhaps your database returns NULL ? In that case, o will be DBNull.Value , which you will have to take into account before casting. An easy way would be to use int? , or you could decide to throw an exception in that case.

If null is not the issue, then try posting the concrete type of o after the ExceuteScalar call.

Answer 3

If the object returned was not an integer (if it is a float, double, byte, etc) it was boxed to an object and can only be cast back to its original type. check the type of o .