In C#, covariance and contravariance enable implicit reference conversion for array types, delegate types, and generic type arguments. Covariance preserves assignment compatibility and contravariance reverses it.
Covariance and contravariance support for method groups allows for matching method signatures with delegate types. This enables you to assign to delegates not only methods that have matching signatures, but also methods that return more derived types (covariance) or that accept parameters that have less derived types (contravariance) than that specified by the delegate type. For more information, see Variance in Delegates (C#) and Using Variance in Delegates (C#).
In .NET Framework 4 and later versions, C# supports covariance and contravariance in generic interfaces and delegates and allows for implicit conversion of generic type parameters. For more information, see Variance in Generic Interfaces (C#) and Variance in Delegates (C#).
A generic interface or delegate is called variant if its generic parameters are declared covariant or contravariant. C# enables you to create your own variant interfaces and delegates. For more information, see Creating Variant Generic Interfaces (C#) and Variance in Delegates (C#).
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This feature was added in C# 2.0. When you instantiate a delegate, you can assign it a method that has a more derived return type than that specified in the delegate (covariance). You can also assign a method that has parameter types less derived than those in the delegate (contravariance).
This is a new feature in C# 4.0. Now, when creating a generic interface, you can specify whether there is an implicit conversion between interface instances that have different type arguments. For example, you can use an interface instance that has methods with more derived return types than originally specified (covariance) or that has methods with less derived parameter types (contravariance). The same rules are applied to generic delegates.
If you extend a variant interface, the extending interface is invariant by default. You must explicitly specify whether the type parameters are covariant or contravariant by using the out or in keyword. Here is a quick example from MSDN:
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In all our donors, renal vascular imaging was performed as part of a thorough standard medical screening by the nephrologist. Hereafter, each live donor is assessed by a dedicated transplant surgeon involved in live donor nephrectomy. The transplant surgeon determines which kidney will be retrieved based on the principle that the donor should be left with the best kidney and the side of the donor nephrectomy should be the least complicated one to avoid additional risk to the donor. Then, every donor is discussed prior to donation in a multidisciplinary team discussion involving (transplant) surgeons, nephrologists, anesthetists, donor coordinators, nurse practitioners and a urologist or psychologist if necessary. To discover any discrepancies in vascular anatomy between imaging and surgery, radiological and surgical reports were compared regarding vascular anatomy. The policy of our center in case of AM is that there is no contraindication for living donation, regarding the choice for left versus right kidney; the kidney with least arteries is chosen unless other factors determine that the other kidney should stay with the donor. In case of AM all transplant surgeons always aim for reconstructing the renal arteries in an end-to-side or side-to-side fashion to enable one arterial anastomosis in the recipient in order to keep the second WIT short. Only if the distance between the arteries is too large to enable safe reconstruction, separate arterial anastomoses are created in the recipient. Small (
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