C# generics problem

Question

hi, I have a method (see below)

    public T ExecuteScalar<T>(string sSql, params SqlParameter[] parameters)
    {
        if (!string.IsNullOrEmpty(sSql))
        {
            DataAccess dal = new DataAccess(ConnectionString);
            DebugOutput_SQL(sSql, parameters);
            object value = null;
            value = dal.ExecuteScalar(sSql, parameters);

            if (value != null && value != DBNull.Value)
                return (T)value;
        }
        return default(T);
    }

and I call this

 int32 WorkFlowID = DataProvider.ExecuteScalar<Int32>(sSql);

then it give me a error "Don't unbox to value" in line "return (T)value", any suggestion for this.

Answer 1

"Don't unbox to value" sounds like an odd error message. Is that exactly what it said?

My guess is that ExecuteScalar was returning a different type (long, double etc) rather than int. Try logging value.GetType() before you convert to T.

Just as a side-note, this:

object value = null;
value = dal.ExecuteScalar(sSql, parameters);

would be simpler (IMO) as:

object value = dal.ExecuteScalar(sSql, parameters);

There's no need to put it in two statements here.

EDIT: Just to clarify, the reason it's failing is that when you unbox a value, you have to unbox to the actual type, not just one which has a conversion available. For example, if you want to get an int out eventually, you could do:

int x = (int) (decimal) value;

which would unbox to decimal and then convert the decimal to an int. In this case though, I'd just specify the right type to start with if possible :)

Answer 2

Maybe the value object that you receive is a Nullable in which case you could do

return ((Nullable<T>)value).Value;

Answer 3

You could try:

public T ExecuteScalar<T>(string sSql, params SqlParameter[] parameters)
{
    if (!string.IsNullOrEmpty(sSql))
    {
        DataAccess dal = new DataAccess(ConnectionString);
        DebugOutput_SQL(sSql, parameters);
        object value = null;
        value = dal.ExecuteScalar(sSql, parameters);

        if (value != null && value != DBNull.Value)
            return (T) Convert.ChangeType(value, typeof(T));
    }
    return default(T);
}