Why I should go for Interfaces in C# when I can implement the methods directly

Interfaces are contracts that implementers must follow. Abstract classes allow contracts plus shared implementations - something that Interfaces cannot have. Classes can implement and inherit multiple interfaces. Classes can only extend a single abstract class. Why Interface You don't have default or shared code implementation You want to share data contracts (web services, SOA) You have different implementations for each interface implementer (IDbCommand has SqlCommand and OracleCommand which implement the interface in specific ways) You want to support multiple inheritance.  Allow for multiple methods with the same name between interfaces, because hey, you have no conflicting implementation, just a signature. Why Abstract You have default or shared code implementation You want to minimize code duplication You want to easily support versioning