How to Resolve Error "CS0537: The class System.Object cannot have a base class or implement an interface" in C#

The Compiler Error CS0537 is a foundational type system error. The message reads: "The class System.Object cannot have a base class or implement an interface".

In the .NET type system, System.Object is the Root of the entire class hierarchy. Every other class implicitly inherits from it. Because it is the absolute beginning of the chain:

1. It cannot have a Base Class (there is nothing "above" it).
2. It cannot implement an Interface (because interfaces themselves are types that exist within the hierarchy derived from Object).

This error only occurs in specialized scenarios where you are building a custom Core Library (typically with the /nostdlib compiler option) and defining your own version of mscorlib or System.Runtime.

Understanding the Root of the Hierarchy

In a standard C# application, you never see this error because you use the built-in System.Object provided by Microsoft. However, if you are writing a custom runtime, operating system kernel, or embedded framework in C#, you must define System.Object yourself.

The runtime expects System.Object to be a pure, standalone class definition. Attempting to make it inherit logic from elsewhere creates a logical paradox (Circular Dependency) in the type loader.

Scenario: Incorrect Definition of System.Object

This error happens when you try to apply standard OOP practices (like inheriting from a helper class or implementing a common interface) to the Object class itself.

Example of error:

// You are building a custom core library (NoStdLib)

namespace System
{
    public interface IKernelObject { }

    // ⛔️ Error CS0537: The class System.Object cannot have a base class 
    // or implement an interface.
    public class Object : IKernelObject 
    {
    }
}

Solution: Define Object as Standalone

The definition of System.Object must be absolutely minimal in terms of inheritance syntax. It cannot have a colon : after the class name.

Solution: remove any base classes or interfaces.

namespace System
{
    // ✅ Correct: No inheritance. This is the root.
    public class Object
    {
        // Typically contains core logic like:
        public virtual string ToString() => GetType().ToString();
        public virtual int GetHashCode() => /* ... */;
        public virtual bool Equals(Object obj) => /* ... */;
        // ...
    }
}

Conclusion

CS0537 is a rule for the architects of the .NET runtime environment.

1. Context: This error only appears if you are defining System types manually (using <NoStdLib>).
2. The Rule: System.Object must stand alone.
3. The Fix: Remove : BaseClass or : IInterface from the class declaration.