How to Resolve Error "CS0243: The Conditional attribute is not valid on 'method' because it is an override method" in C#

The Compiler Error CS0243 is an attribute usage error. The message reads: "The Conditional attribute is not valid on 'MethodName' because it is an override method."

The [Conditional] attribute allows you to tell the compiler: "Only compile calls to this method if a specific symbol (like DEBUG) is defined." However, this attribute affects the call site (where the method is used), not the method implementation itself.

Because Overriding works via runtime polymorphism (the Virtual Method Table), allowing a child class to conditionally "disappear" while the parent class remains creates a broken contract. Therefore, C# enforces that the conditional behavior must be defined on the Base method, not the Override.

This guide explains how to correctly implement conditional logic within overridden methods.

Understanding the Restriction

When you mark a method with [Conditional("DEBUG")], the method itself is compiled into the DLL, but the compiler removes calls to that method if "DEBUG" is not defined.

If you were allowed to put this on an override method but not the base method:

1. Code calling BaseClass.Method() expects it to run.
2. At runtime, the object might actually be ChildClass.
3. If ChildClass.Method() was "conditional'd out," the runtime flow breaks.

To maintain consistency, the "Conditional" status belongs to the Base declaration. If the base is conditional, the override is implicitly conditional. If the base is not, the override cannot be.

Scenario: Misplacing the Attribute

This error happens when you want a specific overridden method to only execute logic during debugging, so you attempt to tag the override directly.

Example of error:

using System.Diagnostics;

public class Logger
{
    public virtual void Log(string message)
    {
        // Base implementation
    }
}

public class FileLogger : Logger
{
    // ⛔️ Error CS0243: The Conditional attribute is not valid on 'FileLogger.Log' 
    // because it is an override method.
    [Conditional("DEBUG")] 
    public override void Log(string message)
    {
        System.Console.WriteLine($"File Log: {message}");
    }
}

Solution 1: Use Preprocessor Directives (#if)

If you want the contents of the overridden method to run only when a symbol is defined, use standard preprocessor directives. This keeps the method signature valid (satisfying the override) but removes the code body when compiling for Release.

Solution:

public class FileLogger : Logger
{
    // ✅ Correct: The method always exists, but the logic is conditional.
    public override void Log(string message)
    {
#if DEBUG
        System.Console.WriteLine($"File Log: {message}");
#endif
    }
}

note

This approach means the method call overhead still exists in Release mode (the method is called, does nothing, and returns).

Solution 2: Call a Private Conditional Method

A cleaner, more "C#-style" approach is to create a separate private method that handles the logic. You can apply the [Conditional] attribute to this private method because it is not an override.

Solution: this keeps your code clean and avoids ugly #if blocks inside the logic.

using System.Diagnostics;

public class FileLogger : Logger
{
    // ✅ Correct: The override simply calls a helper.
    public override void Log(string message)
    {
        LogInternal(message);
    }

    // ✅ Correct: Private methods can be Conditional.
    // If 'DEBUG' is missing, the compiler removes the call inside 'Log' above.
    [Conditional("DEBUG")]
    private void LogInternal(string message)
    {
        System.Console.WriteLine($"File Log: {message}");
    }
}

tip

Why this works: The compiler sees the call to LogInternal inside Log. If DEBUG is not defined, it simply deletes that line of code. The Log method remains empty but valid.

Conclusion

CS0243 ensures that inheritance contracts remain consistent.

1. Rule: You cannot put [Conditional] on an override method.
2. If you own the Base: You can put [Conditional] on the virtual base method (this applies to all overrides automatically).
3. If you don't own the Base: Use a private helper method marked with [Conditional], and call it from your override.