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Use an abstract class when related types need shared instance state, constructors, or protected behavior. Use an interface when you need to describe a capability or contract that unrelated types can implement, or when a type needs to satisfy multiple contracts. Modern C# interfaces can also provide default implementations, but they still cannot store instance fields or declare constructors.
What is the difference between an abstract class and an interface in C#?
An abstract class is a base class that cannot be instantiated directly. It can combine shared implementation with abstract members that derived classes must implement. An interface defines a contract: classes, records, and structs can implement it, whether or not they belong to the same class family.
The key design difference is that an abstract class can own instance state and constructor logic, while an interface describes behavior without instance storage. A class can derive from only one direct base class, but it can implement multiple interfaces. Microsoft’s guidance is to use abstract classes for related types that share state, constructors, or non-public members, and interfaces for capabilities across otherwise unrelated types or for multiple contracts (Microsoft Learn: inheritance; .NET API compatibility).
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1Repair Windows errors before they cause bigger problems2Fix the driver behind crashes, sound loss and screen glitches3Clear out junk files and repair common Windows errors| Decision point | Abstract class | Interface |
|---|---|---|
| Primary role | Shared base for related types; can leave some behavior for derived types to implement. | Contract or capability that different types can implement. |
| Instance fields and state | Can declare fields and provide shared instance state. | Cannot declare instance fields or store instance state. |
| Constructors | Can define constructors used by derived classes. | Cannot define constructors. |
| Inheritance | A class has one direct base class. | A class or struct can implement multiple interfaces; an interface can inherit multiple interfaces. |
| Implementation | Can contain concrete, virtual, and abstract members. | Can declare required members and, in modern C#, default implementations and certain static abstract or virtual members. |
What an abstract class gives you
Use an abstract class when a group of types is meaningfully related and should share the same base implementation or rules. It can centralize common data, initialize that data in a constructor, and expose protected or other non-public helpers to derived classes. It can also declare abstract members to require each concrete derived class to supply type-specific behavior.
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An abstract class does not have to declare any abstract members. It may simply prevent direct construction while serving as a reusable base. A non-abstract derived class must implement inherited abstract members before it can be instantiated.
What an interface gives you
An interface is useful when the important relationship is “can do this,” rather than “is a kind of this base class.” For example, an ICompressible capability could apply to an archive device, a network payload, or an in-memory object even if those types have different base classes. Interfaces also let a type satisfy multiple independent contracts.
An interface property declaration does not create an automatically backed property or hidden instance field. If an implementation needs stored state, that state belongs in the implementing type or an appropriate base class. A type may implement an interface member explicitly, in which case the member is accessed through the interface rather than directly through the class instance.
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Yes. Starting with C# 8.0, an interface can provide a default implementation for a member. This means an interface is not limited to signatures, although it still cannot hold instance fields or declare constructors. A default interface member is accessed through an interface instance; it is not simply exposed as an ordinary member on the implementing class.
Default implementations can help evolve an existing interface by adding a member with behavior that existing implementers can inherit or override. They are not universally frictionless: the runtime must support default interface members, some languages do not support them, and member dispatch can be ambiguous in some cases. Microsoft notes that support requires .NET Core 3.0 or later, and that ref struct implementers must explicitly implement every instance interface member rather than rely on defaults (Microsoft Learn: safely updating interfaces with default interface methods; .NET API compatibility). Check the language version, target runtime, and downstream language consumers before using this feature in a public API.
How to choose between an abstract class and an interface
- Choose an abstract class when related types need common instance data, constructor-enforced initialization, shared implementation, or protected/internal helpers.
- Choose an interface when you are describing a capability that can cross unrelated type hierarchies, or when a class needs to implement several independent contracts.
- Use a default interface implementation selectively when shared behavior helps evolve a contract, after verifying runtime and language compatibility for every intended consumer.
- Combine them when both roles are useful: derive from one abstract class for shared state or base behavior, and implement one or more interfaces for additional capabilities.
Example: shared base state plus an independent capability
public abstract class StorageDevice
{
protected StorageDevice(string name) => Name = name;
public string Name { get; }
public abstract void Save(ReadOnlySpan<byte> data);
}
public interface ICompressible
{
byte[] Compress(ReadOnlySpan<byte> data);
}
public sealed class ArchiveDevice : StorageDevice, ICompressible
{
public ArchiveDevice() : base("archive") { }
public override void Save(ReadOnlySpan<byte> data) { /* device-specific save */ }
public byte[] Compress(ReadOnlySpan<byte> data) => data.ToArray();
}StorageDevice supplies a constructor-backed name and requires concrete devices to implement Save. ICompressible marks an additional capability that could belong to types outside the storage-device family. The example illustrates the design distinction; it is not a benchmark or a claim about performance.
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Further reading
- Microsoft Learn:
interfacekeyword in the C# reference - Microsoft Learn: object-oriented inheritance in C#
- Microsoft Learn: interfaces define behavior for multiple types
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