Using LINQ to convert List to List

Answer 1

  var iweil = sil.Select(item=> new InvoiceWithEntryInfo {
                 IdIWEI = item.ID,
                 AmountIWEI = item.Amount,
                 DateIWEI = item.Date}).ToList();

Answer 2

Just use Select:

if(sil != null)
{
   var iweil = sil.Select(item=>new InvoiceWithEntryInfo()
            {
                IdIWEI = item.ID,
                AmountIWEI = item.Amount,
                DateIWEI = item.Date
            }).ToList();
}

Answer 3

You need a function to convert a T instance to a U instance:

ResultType ConvertMethod(StartType input)

and you need to write this. Then

outputList = inputList.Select(ConvertMethod).ToList();

will apply it to the whole input collection. The conversion function can be a lambda written inline but doesn't need to be (if the function has the right signature, like ConvertMethod then the compiler will convert it correctly to pass to Select).

Answer 4

Your regular C# code and LINQ are not equivalent. In the regular C# you instantiate a new instance of the other class and initialize the properties, whereas you try to cast (well convert) from one to the other; however, since they are not in the same class hierarchy you can't cast, and as you haven't defined a conversion operator, you can't convert (using cast syntax) either.

You either have to define a conversion operator

public static explicit operator InvoiceWithEntryInfo(ServiceInfo item){
     return new InvoiceWithEntryInfo {
             IdIWEI = item.ID,
             AmountIWEI = item.Amount,
             DateIWEI = item.Date};
 }

or a creation method using regular method signature. I'd suggest the latter since the former pretend to be something it's not. It's not a cast and I'd personally like to be able to see that the code creates a new instance based on some input.

Answer 5

var iweilCopy = sil.Select(item => new InvoiceWithEntryInfo()
{
  IdWEI = item.Id,
  NameWEI = item.Name,
  ....
}).ToList();