---
source: https://iinventedlight.com/library/notes/csharp-interfaces.md
title: "C# interfaces: contracts for many types"
license: CC-BY-4.0
license_url: https://creativecommons.org/licenses/by/4.0/
source_license: "CC BY 4.0 (dotnet/docs); prose restates cited source lines"
attribution: "Summary by iinventedlight (AI-assisted). Sources: Microsoft Learn (.NET docs), Interfaces - define behavior for multiple types - C#, CC BY 4.0 (dotnet/docs); Microsoft Learn (.NET docs), interface keyword - C# reference, CC BY 4.0 (dotnet/docs)."
author: "iinventedlight (synthesis of cited sources); text drafted with AI assistance"
date: 2026-11-04
status: published
checked_on: 2026-11-04
as_of: 2026-11-04
confidence: medium
untested: "No code was run for this note; only the fetched documentation pages were read."
category: coding-references
topic_area: "code-ai-eng: C# and .NET"
tags: csharp, dotnet, interfaces
sources:
  - id: S1
    title: "Interfaces - define behavior for multiple types - C#"
    publisher: Microsoft Learn (.NET docs)
    url: https://learn.microsoft.com/en-us/dotnet/csharp/fundamentals/types/interfaces
    accessed: 2026-11-04
    license: "CC BY 4.0 (dotnet/docs)"
  - id: S2
    title: "interface keyword - C# reference"
    publisher: Microsoft Learn (.NET docs)
    url: https://learn.microsoft.com/en-us/dotnet/csharp/language-reference/keywords/interface
    accessed: 2026-11-04
    license: "CC BY 4.0 (dotnet/docs)"
---
# C# interfaces: contracts for many types

## 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]

```csharp
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).