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How to Resolve Error "CS0582: The Conditional attribute is not valid on interface members" in C#

The Compiler Error CS0582 is a syntax restriction regarding the [Conditional] attribute. The message reads: "The Conditional attribute is not valid on interface members".

In C#, an Interface represents a strict contract. It defines exactly what members a class must implement to satisfy that contract. The [Conditional] attribute works by instructing the compiler to completely remove calls to a method if a specific symbol (like DEBUG) is not defined.

If an interface member were conditional, the "shape" of the interface would change depending on build configuration. A class compiled in DEBUG mode would see an interface with the method, while a class compiled in RELEASE mode would see an interface without it. This would break binary compatibility and the stability of the contract. Therefore, C# forbids applying this attribute directly to interface definitions.

Understanding the Restriction​

The [Conditional] attribute allows the compiler to strip out call sites: Logger.Log("Info"); -> becomes (nothing) if the symbol isn't defined.

However, interfaces are used for polymorphism (e.g., ILogger log = new FileLogger()). If you invoke log.Log(), the compiler issues a virtual call (callvirt). The compiler cannot skip generating this call based on an attribute on the implementation, and it cannot conditionally remove the slot from the interface definition itself without breaking the rules of inheritance and type loading.

Scenario: The Invalid Interface​

This error typically occurs when developers want to create a logging or diagnostic interface that automatically does nothing in Release mode.

Example of error:

using System.Diagnostics;

public interface IDiagnostic
{
    // ⛔️ Error CS0582: The Conditional attribute is not valid on interface members.
    // You cannot make the contract itself conditional.
    [Conditional("DEBUG")]
    void CheckSystem();
}

public class SystemCheck : IDiagnostic
{
    public void CheckSystem()
    {
        System.Console.WriteLine("Checking...");
    }
}

Solution 1: Use Preprocessor Directives in Implementation​

To fix the error, you must remove the attribute from the interface. To achieve the behavior of running code only in Debug mode, you wrap the implementation logic inside the class with #if directives.

Note that the method call will still happen at runtime (it is not stripped), but the method will be empty and return immediately, resulting in minimal overhead.

Solution:

using System.Diagnostics;

public interface IDiagnostic
{
    // ✅ Correct: The interface defines the method permanently.
    void CheckSystem();
}

public class SystemCheck : IDiagnostic
{
    public void CheckSystem()
    {
        // ✅ Correct: The LOGIC is conditional, not the definition.
#if DEBUG
        System.Console.WriteLine("Checking...");
#endif
    }
}
warning

Why not put [Conditional] on the class method? If you implement the interface and put [Conditional] on the class method, the code will compile. However, calls made via the interface (IDiagnostic d = ...; d.CheckSystem()) will NOT be stripped out. Calls made via the concrete class (SystemCheck s = ...; s.CheckSystem()) WILL be stripped. This inconsistency creates bugs. The #if approach above is safer for interfaces.

Solution 2: Use Conditional Extension Methods​

If your goal is to truly strip the method calls from the compiled binary (for performance reasons), you can use a clever workaround involving Extension Methods.

  1. Keep the Interface clean (or empty).
  2. Create a static Extension Method on that interface.
  3. Apply [Conditional] to the Extension Method.

Since extension methods are resolved at compile-time (static binding), the compiler can successfully remove calls to them.

Solution:

using System.Diagnostics;

// 1. The Interface (Can be empty or contain other real methods)
public interface IDiagnostic
{
}

// 2. The Extension Method
public static class DiagnosticExtensions
{
    // ✅ Correct: Extension methods are static, so they support [Conditional]
    [Conditional("DEBUG")]
    public static void CheckSystem(this IDiagnostic diag)
    {
        System.Console.WriteLine("Checking system via extension...");
    }
}

public class Program
{
    static void Main()
    {
        IDiagnostic diag = new SystemCheck();

        // 3. Usage
        // If DEBUG is defined: Calls the extension method.
        // If DEBUG is NOT defined: This line is erased by the compiler.
        diag.CheckSystem(); 
    }
}

public class SystemCheck : IDiagnostic { }

Conclusion​

CS0582 protects the stability of your code contracts.

  1. Remove the Attribute: Delete [Conditional(...)] from the interface definition immediately.
  2. Choose a Strategy:
    • Logic Only: Use #if DEBUG inside the implementing class methods. The call happens, but does nothing.
    • Call Stripping: Use a Conditional Extension Method on the interface. The call is removed from the binary.

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