Why are interfaces not able to be marked as sealed?

The purpose of sealing a class, or a virtual method of a class, is to lower your costs . Designing for inheritance is expensive and if you do not do it correctly, it is dangerous . There are security, correctness and robustness consequences to improperly designing for inheritance, so if you do not intend to design for inheritance, it is wise to seal your class and thereby avoid the costs associated with designing for inheritance.

It would just be confusing. Using the standard syntax, it would imply that you cannot implement the interface. Also, interfaces don't contain any functionality or fields, so there is no practical use in sealing it.

An interface is more or less a contract.

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