. net--covariant and inverse variation

1, Description: Covariance and contravariance can only appear in the interface and the delegation.

To modify by means of an out, inversion by in.

Valid for reference type, invalid for value type

2, need to solve the problem:

3. Solution:

ienumerable<parent> _parents = new list<child> ();

View the definition of IEnumerable discovery

An out parameter was added before the generic parameter.

4. Supplement:

Out covariant: The modified parameter can only appear where the return value is.

In contravariance: The modified parameter can only appear in the position of the parameter.

5, the case of the inverse change:

6. Write yourself an example of contravariance and covariance:

. net--covariant and inverse variation