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Short answer

An interface is a contract: a set of methods, properties, events and indexers that any implementing class, record or struct must provide. C# has no multiple inheritance of classes, but one type can implement many interfaces. [S1][S2]

interface IShape { double Area(); }
record Square(double Side) : IShape { public double Area() => Side * Side; }

Modern features

• Interfaces can include default implementations for members. [S2]
• They can declare static abstract or static virtual members that implementers must supply. [S2]

Claims

• id: C1 claim: An interface is a group of members a class or struct must implement. source: S1 evidence: "a group of related methods, properties, events, and indexers that a class or struct must"
• id: C2 claim: A single type can implement multiple interface contracts. source: S1 evidence: "Interfaces let a single type implement multiple contracts"
• id: C3 claim: An interface can define a default implementation for a member. source: S2 evidence: "An interface can define a default implementation for a member."
• id: C4 claim: Interfaces can define static abstract or static virtual members. source: S2 evidence: "An interface can define static abstract or static virtual members"

Sources

• [S1] Interfaces - define behavior for multiple types - C#. Microsoft Learn (.NET docs). https://learn.microsoft.com/en-us/dotnet/csharp/fundamentals/types/interfaces . License: CC BY 4.0 (dotnet/docs). Accessed 2026-11-04.
• [S2] interface keyword - C# reference. Microsoft Learn (.NET docs). https://learn.microsoft.com/en-us/dotnet/csharp/language-reference/keywords/interface . License: CC BY 4.0 (dotnet/docs). Accessed 2026-11-04.

License: this note is CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). Summary by iinventedlight (AI-assisted).

@misc{iil_note_t0p6jxbz,
  author = {{iinventedlight}},
  title = {{C\# interfaces: contracts for many types}},
  howpublished = {iinventedlight Library},
  year = {2026},
  date = {2026-11-04},
  url = {https://iinventedlight.com/library/notes/csharp-interfaces/},
  note = {Permanent ID iil:note:t0p6jxbz. License: CC-BY-4.0.},
}

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