Variance
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Description
Covariance enable implicit reference conversion for array types, delegate types, and generic type arguments.
Covariance preserves assignment compatibility and contravariance reverses it.
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Since .NET Framework 4, C# supports covariance and contravariance in generic interfaces and delegates and allows for implicit conversion of generic type parameters. |
Assignment compatibility
Allow an object of a more derived type (child class, ex: string) to be assigned to an object of a less derived type (parent class, ex: object).
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string s = "test"; object o = s; |
Covariance
Allow a generic object of child class type (ex: string) to be assigned to a generic object of parent class type (ex: object).
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It works only with covariant interface |
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var strings = new List<string>(); List<object> objects = strings; // Cannot convert type List<string> to List<object> because there is no covariance with List<T> IList<object> objects = strings; // Cannot implicitly convert type List<string> to IList<object>. An explicit conversion exists IEnumerable<object> objects = strings; // There is covariance with IEnumerable<T> var objects = (IList<object>)strings; // There is covariance with IList<T> |
