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How to Resolve Error "CS0057: Inconsistent accessibility: parameter type 'type' is less accessible than operator 'operator'" in C#

The Compiler Error CS0057 is an Encapsulation error related to Operator Overloading. The message reads: "Inconsistent accessibility: parameter type 'MyType' is less accessible than operator 'MyOperator'".

In C#, all overloaded operators (such as +, -, ==, or conversion operators) must be declared as public static. Because the operator itself is strictly public, the arguments (parameters) you pass to it must also be public. You cannot define a public operation that requires a hidden (internal or private) object as an input, because the outside world would be unable to provide that input.

This guide explains the visibility rules for operators and how to resolve this conflict.

Understanding Operator Visibility Rules​

When you overload an operator, the syntax is strict: public static ReturnType operator +(TypeA a, TypeB b)

You cannot make an operator private or internal. Since the operator is forced to be public:

  1. The Return Type must be public (see CS0056).
  2. The Parameter Types (TypeA, TypeB) must be public.

If one of the operands is internal (visible only inside your project), an external caller would see the + operator but would be unable to use it because they cannot access the required internal type.

Scenario 1: Public Operator with Internal Parameter​

This commonly happens when you define a helper class (defaulting to internal) and try to use it in an arithmetic or conversion operation inside a public class.

Example of error:

// No modifier = 'internal' (visible only inside this project)
class CurrencyRate
{
    public decimal Rate;
}

public class Money
{
    public decimal Amount;

    // ⛔️ Error CS0057: Inconsistent accessibility
    // The operator is PUBLIC (mandatory).
    // But the second parameter 'CurrencyRate' is INTERNAL.
    // External code cannot pass a 'CurrencyRate' object to this operator.
    public static Money operator *(Money m, CurrencyRate rate)
    {
        return new Money { Amount = m.Amount * rate.Rate };
    }
}
note

This error also applies to Implicit and Explicit conversion operators. For example, public static implicit operator Money(CurrencyRate r) would trigger the same error because the input r is internal.

Solution 1: Make the Parameter Type Public​

If the operator represents a valid interaction that the outside world should perform, the type of the parameter must be visible to them.

Solution: add the public keyword to the internal class.

// ✅ Correct: Explicitly mark the class as public
public class CurrencyRate
{
    public decimal Rate;
}

public class Money
{
    public decimal Amount;

    // Now valid because both the operator and the parameter 'CurrencyRate' are public.
    public static Money operator *(Money m, CurrencyRate rate)
    {
        return new Money { Amount = m.Amount * rate.Rate };
    }
}

Solution 2: Replace the Operator with a Method​

If the parameter type (CurrencyRate) is supposed to be internal (a hidden implementation detail), then you should not be using a public operator. Since you cannot make an operator internal, you must replace it with a standard Method.

Methods allow you to control visibility levels (e.g., internal, private).

Solution: remove the operator and create an internal method instead.

internal class CurrencyRate // Remains internal
{
    public decimal Rate;
}

public class Money
{
    public decimal Amount;

    // ✅ Correct: We use a named method instead of an operator.
    // We can mark this method 'internal' to match the parameter's visibility.
    internal Money MultiplyByRate(CurrencyRate rate)
    {
        return new Money { Amount = this.Amount * rate.Rate };
    }
}

Conclusion​

CS0057 enforces consistency in your public API.

  1. Remember: Operators are always public.
  2. Check the Parameters: Look at the types inside the parentheses operator op(TypeA, TypeB).
  3. Align Them:
    • If the operation is for public use, make the parameter class public.
    • If the operation involves internal data, replace the operator with an internal method.

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