How to Resolve Error "CS0460: Constraints for override and explicit interface implementation methods are inherited from the base method, so they cannot be specified directly, except for either a 'class', or a 'struct' constraint." in C#

The Compiler Error CS0460 is a syntax error regarding Generics and Inheritance. The message reads: "Constraints for override and explicit interface implementation methods are inherited from the base method, so they cannot be specified directly, except for either a 'class', or a 'struct' constraint."

In C#, when a generic method is declared in a base class or interface with constraints (e.g., where T : new()), those constraints become part of the method's contract. Any derived class overriding that method, or implementing that interface, automatically inherits those constraints. You are not allowed to restate them. This prevents mismatched constraints between the base definition and the implementation.

This guide explains how to properly override or implement generic methods without triggering this redundancy error.

Understanding Constraint Inheritance

When a method is polymorphic (virtual or interface-based), the "Contract" is defined at the source.

• Source: virtual void Method<T>() where T : IComparable
• Implied Contract: Any version of this method must enforce IComparable.

Because this is guaranteed by the compiler, C# forbids you from typing where T : IComparable again on the overriding method. Repeating it is considered a syntax error because it implies you might be able to change it (which you can't).

Scenario 1: Overriding Virtual Methods

This is the most common occurrence. You copy-paste the signature from the base class to the child class to override it, including the where clause.

Example of error

public class BaseRepository
{
    // The constraint is defined here
    public virtual void Save<T>(T item) where T : new()
    {
    }
}

public class SqlRepository : BaseRepository
{
    // ⛔️ Error CS0460: Constraints are inherited from the base method, 
    // so they cannot be specified directly.
    public override void Save<T>(T item) where T : new()
    {
    }
}

Solution: Remove the Constraint

Simply delete the where clause from the overriding method. The compiler knows it exists because of the override keyword linking back to the base.

public class SqlRepository : BaseRepository
{
    // ✅ Correct: No 'where' clause. 
    // The compiler implicitly enforces 'where T : new()' from BaseRepository.
    public override void Save<T>(T item)
    {
        // We can still use 'new T()' here because the constraint is inherited!
        T temp = new T(); 
    }
}

Scenario 2: Implementing Interface Methods

The same rule applies when implementing interfaces. The interface defines the constraints; the class implementation must not repeat them.

Example of error

using System;

public interface IProcessor
{
    // Interface defines the constraint
    void Process<T>(T input) where T : IDisposable;
}

public class DataProcessor : IProcessor
{
    // ⛔️ Error CS0460: Cannot specify constraints on implementation.
    public void Process<T>(T input) where T : IDisposable
    {
        input.Dispose();
    }
}

Solution: Remove the Constraint

Keep the method signature identical in terms of name and parameters, but omit the constraints.

using System;

public class DataProcessor : IProcessor
{
    // ✅ Correct: Constraints are implied by the interface.
    public void Process<T>(T input)
    {
        // We can still call .Dispose() because the compiler knows T is IDisposable
        input.Dispose();
    }
}

note

Explicit Interface Implementation: If you implement the interface explicitly (e.g., void IProcessor.Process<T>(T input)), the same rule applies. Do not add the where clause.

Conclusion

CS0460 is a "Don't Repeat Yourself" rule enforced by the compiler to ensure consistency in inheritance.

1. Check the Source: Look at the Base Class or Interface.
2. Clean the Override: Remove the where T : ... clause from your override method or your interface implementation.
3. Trust the Inheritance: Remember that inside your method, you can still use the features guaranteed by the constraints (like calling new() or interface methods), even though you didn't type the constraint in that specific file.