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How to Resolve Error "CS0689: Cannot derive from 'identifier' because it is a type parameter" in C#

The Compiler Error CS0689 is an inheritance restriction error related to Generics. The message reads: "Cannot derive from 'T' because it is a type parameter".

In C#, you cannot inherit from a Generic Type Parameter (T). When you define a class like class MyClass<T> : T, you are asking the compiler to create a class that inherits from whatever T turns out to be at runtime. Since T could be a sealed class (like string), a struct (value type), an interface, or an array, the compiler cannot guarantee that the inheritance is legal or structurally possible. Therefore, C# forbids deriving directly from a generic placeholder.

This guide explains why this restriction exists and how to work around it using composition or interfaces.

Understanding the Inheritance Constraint​

Inheritance determines the fundamental layout of an object in memory. A class must know exactly what its parent is at compile-time to determine its memory size, method table (vtable), and field offsets.

If you write:

class Wrapper<T> : T { }
  • If T is int, you are trying to inherit from a struct (Forbidden).
  • If T is string, you are trying to inherit from a sealed class (Forbidden).
  • If T is List<int>, it might be valid.

Because the compiler cannot enforce validity for all possible T, it blocks the syntax entirely.

Scenario: Attempting Dynamic Inheritance​

This error often occurs when developers want to create a "Decorator" or "Extension" class that adds functionality to any class passed in as a generic argument.

Example of error:

// ⛔️ Error CS0689: Cannot derive from 'T' because it is a type parameter
public class LoggingWrapper<T> : T
{
    public void Log() 
    {
        Console.WriteLine("Logging...");
    }
}

The intent is usually: "I want LoggingWrapper to have all the methods of T, plus a Log method."

Solution 1: Use Interfaces (Constraint-Based)​

You cannot inherit from T, but T can be an Interface, and your class can implement that same interface. This allows your generic class to "look like" the underlying type.

Solution:

public interface IService
{
    void DoWork();
}

// ✅ Correct: We inherit from the interface IService, not T itself.
// We verify T implements IService using the 'where' clause.
public class ServiceWrapper<T> : IService where T : IService, new()
{
    private T _inner;

    public ServiceWrapper()
    {
        _inner = new T();
    }

    public void DoWork()
    {
        Console.WriteLine("Before Work");
        _inner.DoWork(); // Delegate to the inner object
        Console.WriteLine("After Work");
    }
}

Solution 2: Use Composition (Wrapping)​

If you don't have a shared interface, or if you want to wrap classes that you don't control, use Composition. Instead of being a T (Inheritance), you hold a T (Composition).

This doesn't give you automatic access to T's members (you can't call wrapper.MethodOnT() directly), but it allows you to manage instances of T.

Solution:

public class SmartContainer<T>
{
    // ✅ Correct: Composition. We contain T.
    public T Value { get; set; }

    public SmartContainer(T value)
    {
        Value = value;
    }

    public void CustomFeature()
    {
        Console.WriteLine($"Managing: {Value}");
    }
}
tip

Dynamic Proxy: If you truly need a class that dynamically inherits from another class at runtime (e.g., for mocking libraries like Moq or Castle DynamicProxy), you must use Reflection.Emit or intermediate libraries to generate IL code on the fly. You cannot do it with standard C# syntax.

Conclusion​

CS0689 is a fundamental limitation of static compilation.

  1. Inheritance requires certainty: The compiler needs to know the exact base class layout.
  2. Use Interfaces: If you want to extend behavior, define an interface and implement it.
  3. Use Composition: If you want to wrap data, hold T as a field or property.

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