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Convert a List from One Type to Another

You have a List of one object but you really want to have a List of some other object. If it was a single object, you could cast from one to the other with a single line of code:

Dim pCust As PremiumCustomer = New PremiumCustomer
Dim cust As Customer 
cust = pCust

But, because you have a collection of objects, you might assume that you have to write a loop and transfer your objects from one List to the other individually. The good news is that you can do that in one line of code using two LINQ functions: Cast and ToList. Here's the one line solution that creates a list of Customers from a list of a derived class, PremiumCustomers:

Dim pCusts As List(Of CustomerPremium) pCusts = GetPremiumCustomers()
Dim custs As List(Of Customer)
custs = pCusts.Cast(Of Customer)().ToList()

Unlike some "clever" LINQ solutions this one is, I think, actually readable. Under the hood, of course, that loop is probably still being created by LINQ and the compiler…but you don't have to write it. Besides, with this single line of code, the compiler might even be able to invoke some clever optimization to speed up the process.

I didn't come up with this myself: If you're interested there are six more useful LINQ tips where this one came from, all thanks to Igor Ostrovsky.

Posted by Peter Vogel on 06/01/2016 at 11:51 AM


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