Question

I have the following code:

  var commitmentItems = new List<CommitmentItem<ITransaction>>();
  commitmentItems.Add(new CapitalCallCommitmentItem());

And I get the following error:

Argument '1': cannot convert from 'Models.CapitalCallCommitmentItem' to
'Models.CommitmentItem<Models.ITransaction>'

However, CapitalCallCommitmentItem inherits from CommitmentItem<CapitalCall>, and CapitalCall implements ITransaction. So why the error?

Here is a better example:

CapitalCall implements ITransaction

            var test = new List<ITransaction>();
            test.Add(new CapitalCall());
            var test2 = new List<List<ITransaction>>();
            test.Add(new List<CapitalCall>()); // error.

Answer 1

Understanding why this doesn’t work can be kind of tricky, so here’s an analogous example, replacing the classes in your code with some well-known classes from the framework to act as placeholders and (hopefully) illustrate the potential pitfalls of such desired functionality:

// Note: replacing CommitmentItem<T> in your example with ICollection<T>
// and ITransaction with object.
var list = new List<ICollection<object>>();

// If the behavior you wanted were possible, then this should be possible, since:
// 1. List<string> implements ICollection<string>; and
// 2. string inherits from object.
list.Add(new List<string>());

// Now, since list is typed as List<ICollection<object>>, our innerList variable
// should be accessible as an ICollection<object>.
ICollection<object> innerList = list[0];

// But innerList is REALLY a List<string>, so although this SHOULD be
// possible based on innerList's supposed type (ICollection<object>),
// it is NOT legal due to innerList's actual type (List<string>).
// This would constitute undefined behavior.
innerList.Add(new object());