Question

Ever since getting Resharper I’ve been a fan of using var inside methods wherever possible. So later if you pass in a different type the change cascades down through your code without having to change every int declaration to a double.
Which brings me to my question. I’m retrieving a list of DataListItems from a DataList

var daysWorked = dlResourcesAllocated.Items;

Then stepping through the List it returns

foreach (var d in daysWorked)
{
     var c = d.FindControl("ucSomethingSomething"); //Doesn't compile d has no methods
}

And my d is suddenly cast to an Object that has no methods

foreach (DataListItem d in daysWorked)
{
     var c = d.FindControl("ucSomethingSomething");
}

However works just fine.

I’m just curious how come “var” can’t figure out it’s a collection of DataListItem. Intellisense seems to know it.
I’m sure there’s a perfectly simple one liner explanation for this. The skeet dude is probably gonna show up and point out I shoulda paid better attention when reading C# in depth…

Answer 1

This is because DataListItemCollection only implements IEnumerable, not IEnumerable<DataListItem>, as it predates generics.

Since it doesn’t implement IEnumerable<T>, the runtime uses Object in your foreach loop unless you explicitly provide the type.

By writing this:

foreach (var d in daysWorked)

The compiler sees that daysWorked implements IEnumerable, and rewrites this as:

foreach (object d in daysWorked)

When using foreach with IEnumerable (non-generic), you are allowed to provide the type explicitly:

foreach (DataListItem d in daysWorked)

Doing this effectively casts the results from the IEnumerable for you, providing the proper type in your loop.