How to Resolve Error "CS0553: User-defined conversions to or from a base type are not allowed" in C#

The Compiler Error CS0553 is a restriction on Operator Overloading. The message reads: "User-defined conversions to or from a base type are not allowed".

In C#, inheritance automatically provides conversions between a derived class and its base class.

• Upcasting (Derived $\rightarrow$ Base): Implicit and always safe.
• Downcasting (Base $\rightarrow$ Derived): Explicit and checked at runtime.

Because these behaviors are built into the language itself, you are forbidden from overriding them with custom implicit or explicit operators. Doing so would create ambiguity: when you cast (Base)child, the compiler wouldn't know if it should perform a standard reference cast (identity preservation) or run your custom method (which might create a new object).

This guide explains the built-in casting rules and how to implement custom conversion logic properly.

Understanding Built-in Inheritance Conversions

If Dog inherits from Animal, C# automatically allows the following:

Dog myDog = new Dog();

// Implicit Upcast (Automatic)
Animal myAnimal = myDog; 

// Explicit Downcast (Requires cast syntax)
Dog backToDog = (Dog)myAnimal;

These operations do not change the object itself; they only change the type of the reference pointing to the object. CS0553 prevents you from interfering with this fundamental mechanism.

Scenario 1: Redefining Implicit Upcasting

You might try to define an implicit operator to convert a child class to its parent, thinking it's required.

Example of error:

public class BaseClass { }

public class DerivedClass : BaseClass
{
    // ⛔️ Error CS0553: User-defined conversions to or from a base type are not allowed.
    // C# already does this automatically. You cannot redefine it.
    public static implicit operator BaseClass(DerivedClass d)
    {
        return d;
    }
}

Scenario 2: Redefining Explicit Downcasting

Similarly, you might try to define how a Base class converts down to a Derived class.

Example of error:

public class BaseClass { }

public class DerivedClass : BaseClass
{
    // ⛔️ Error CS0553: Conversion from base to derived is already reserved 
    // by the language for Type Casting/Checking.
    public static explicit operator DerivedClass(BaseClass b)
    {
        return new DerivedClass(); // Custom logic forbidden here
    }
}

Solution 1: Remove the Operator (Use Standard Casting)

If your intention was simply to allow assignment between the types, you don't need to write any code. C# handles it.

Solution: delete the operator method entirely.

public class BaseClass { }
public class DerivedClass : BaseClass { }

public class Program
{
    static void Main()
    {
        DerivedClass d = new DerivedClass();

        // ✅ Correct: Works natively
        BaseClass b = d; 
        
        // ✅ Correct: Works natively (throws exception if b is not actually a DerivedClass)
        DerivedClass d2 = (DerivedClass)b;
    }
}

Solution 2: Use Factory Methods or Constructors

If your intention was to create a new object or perform complex transformation logic (e.g., creating a new DerivedClass instance based on data in a BaseClass instance), you cannot use casting syntax.

Instead, use a Constructor or a Factory Method. This makes it clear that a new object is being created, rather than just casting a reference.

Solution:

public class BaseClass 
{ 
    public int Id { get; set; }
}

public class DerivedClass : BaseClass
{
    public string ExtraData { get; set; }

    // Option A: Constructor
    public DerivedClass(BaseClass parent)
    {
        this.Id = parent.Id;
        this.ExtraData = "Default";
    }

    // Option B: Static Factory Method
    public static DerivedClass FromBase(BaseClass parent)
    {
        return new DerivedClass(parent);
    }
}

public class Program
{
    static void Main()
    {
        BaseClass b = new BaseClass { Id = 10 };

        // ✅ Correct: Clear indication of creating a new object from a base object.
        DerivedClass d = DerivedClass.FromBase(b);
    }
}

note

The as Operator: Remember that C# also provides the as operator (var d = b as DerivedClass;). This is another built-in mechanism for base-to-derived conversion that would conflict with custom operators.

Conclusion

CS0553 preserves the integrity of C# Inheritance.

1. Check Inheritance: Does the source or destination of your operator inherit from the other?
2. If Yes: Delete the operator.
3. For Transformation: If you need to convert data (not just references), use a Constructor or a method like ToDerived().