Extension Method ConvertAll

Question

What is the proper use of ConverAll ? Will it convert one type to another type?

like

List<int> intList = new List<int>();
intList.Add(10);
intList.Add(20);
intList.Add(30);
intList.Add(33);

var query= intList.ConvertAll(x=>(double)x);

for this i can use cast or OfType<>.

Answer 1

ConvertAll will just call your delegate/anonymous method for each element of the list. What this does is entirely up to you.

In the example code you posted, it will attempt to cast each element to a double and return that, which means you'll get a List<Double> in return.

You should not use OfType<T>, since this will filter the elements based on the type, and will only return a different type than the original if it is type compatible due to inheritance or interface implementation.

In other words, .OfType<Double> will return no elements, since none of the ints are also doubles.

Answer 2

ConvertAll is a projection operator and maps most closely to LINQ's Select. LINQ's Cast is a specific projection operator and represents doing what you did [via projection] - or it would (as pointed out in Luke's answer [and comment], which I +1'd) if you weren't converting to a value type.

In general, LINQ has a more complete and well-thought-through set of operators, which makes older stuff like ConvertAll look a bit silly at times [like this]. (or @stoopid :D).

Answer 3

to my knowledge, OfType<T> will only return the elements in the collection that are of the specified type T. ConvertAll allows you to convert the elements to another type.

Answer 4

ConvertAll isn't an extension method, it's a real method on List<T> itself.

It returns a new list containing the converted elements. So in your example, the query variable isn't actually a query, it's a List<double>.

Cast and OfType are extension methods that operate on IEnumerable and return an IEnumerable<T>. However they're not suitable for your stated purpose: Cast can convert reference types but cannot convert value types, only unbox them. OfType doesn't perform any conversion, it just returns any elements that are already of the specified type.