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 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.