How to Resolve Error "CS0555: User-defined operator cannot convert a type to itself" in C#

The Compiler Error CS0555 is a logic redundancy error regarding Operator Overloading. The message reads: "User-defined operator cannot convert an object of the enclosing type to itself".

In C#, assigning a variable to another variable of the exact same type is called an Identity Conversion. This is handled natively by the language (copying the value for structs, or copying the reference for classes). You are forbidden from redefining this behavior using implicit or explicit operators because doing so would create ambiguity between standard assignment and your custom logic, potentially leading to infinite recursion or unreachable code.

This guide explains why this restriction exists and how to implement data copying or cloning correctly.

Understanding Identity Conversion

When you write:

int a = 5;
int b = a; // Copy the value

Or:

MyClass x = new MyClass();
MyClass y = x; // Copy the reference

The C# compiler handles this automatically. It does not look for an operator method to perform this task. If you try to define public static implicit operator MyClass(MyClass x), you are trying to intercept an operation that the compiler treats as intrinsic.

Scenario: Defining a Self-Conversion

This error usually occurs when a developer tries to implement "Deep Copying" or "Cloning" logic using casting syntax, thinking that (MyClass)myObj should return a fresh copy.

Example of error: attempting to convert Money to Money.

public class Money
{
    public decimal Amount { get; set; }

    // ⛔️ Error CS0555: User-defined operator cannot convert an object 
    // of the enclosing type to itself.
    // You cannot tell the compiler how to convert 'Money' into 'Money'.
    public static implicit operator Money(Money m)
    {
        return new Money { Amount = m.Amount };
    }
}

Solution 1: Remove the Operator (Standard Assignment)

If your goal was simply to assign one variable to another, you do not need to write any code.

Solution: delete the operator method.

public class Money
{
    public decimal Amount { get; set; }
}

public class Program
{
    static void Main()
    {
        Money wallet1 = new Money { Amount = 100 };
        
        // ✅ Correct: Standard assignment works automatically.
        // Note: For classes, this copies the reference, not the data.
        Money wallet2 = wallet1; 
    }
}

Solution 2: Use a Clone Method (Data Copying)

If your intention was indeed to create a new object with the same data (Cloning) rather than just copying the reference, you cannot use operator overloading. You must use a standard method, conventionally named Clone, Copy, or Duplicate.

Solution: implement a copy method or the ICloneable interface.

public class Money
{
    public decimal Amount { get; set; }

    // ✅ Correct: Use a method for Deep Copy logic
    public Money Clone()
    {
        return new Money { Amount = this.Amount };
    }
    
    // Alternative: Copy Constructor
    public Money(Money other)
    {
        this.Amount = other.Amount;
    }
    
    // Default constructor needed if you add others
    public Money() { }
}

public class Program
{
    static void Main()
    {
        Money m1 = new Money { Amount = 100 };

        // Usage via Method
        Money m2 = m1.Clone();

        // Usage via Constructor
        Money m3 = new Money(m1);
    }
}

note

Why is the operator forbidden? If operator Money(Money m) were allowed, the statement Money m2 = m1; would be ambiguous. Should the compiler just copy the memory pointer (standard behavior), or call your potentially expensive function? C# avoids this ambiguity by enforcing standard behavior.

Conclusion

CS0555 prevents you from redefining the fundamental assignment logic of the language.

1. Check the Operator: Does the return type match the parameter type? (e.g., operator Type(Type x)).
2. Delete it: If you just want assignment, the language does it for you.
3. Rename it: If you want cloning/copying logic, rename the operator to a method like Clone().