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How to Resolve Warning "CS0618: 'member' is obsolete: 'text'" in C#

The Compiler Warning CS0618 is a specific deprecation warning. The message reads: "'Member' is obsolete: 'Specific Message'".

This warning is very similar to CS0612, but with one crucial difference: the author of the deprecated code has provided a custom message explaining why the code is obsolete or what you should use instead. This makes CS0618 one of the most helpful warnings in the C# compiler, as the solution is often embedded directly in the error message itself.

This guide explains how to read these deprecation messages and how to handle them properly.

Understanding the Custom Message​

When a developer marks code as obsolete, they can use the [Obsolete] attribute in two ways:

  1. [Obsolete] -> Triggers CS0612 (Generic warning).
  2. [Obsolete("Use method B instead")] -> Triggers CS0618 (Warning with instructions).

The text string passed into the attribute is what appears in your Error List. It serves as a direct communication channel from the library author to you.

Scenario: Using a Method Marked with a Message​

Suppose you are using a cryptography library or a helper class, and the author has decided that ComputeHash is unsafe and should be replaced.

Definition​

public class CryptoHelper
{
    // The author provides specific instructions in the string
    [Obsolete("ComputeHash is insecure. Please use ComputeSHA256 instead.")]
    public void ComputeHash(string data)
    {
        Console.WriteLine("Computing legacy hash...");
    }

    public void ComputeSHA256(string data)
    {
        Console.WriteLine("Computing secure hash...");
    }
}

Usage with Warning​

public class Program
{
    static void Main()
    {
        var crypto = new CryptoHelper();

        // ⛔️ Warning CS0618: 'CryptoHelper.ComputeHash(string)' is obsolete: 
        // 'ComputeHash is insecure. Please use ComputeSHA256 instead.'
        crypto.ComputeHash("secret"); 
    }
}

The primary way to fix CS0618 is to read the string provided in the warning ("...Please use ComputeSHA256 instead") and update your code accordingly.

public class Program
{
    static void Main()
    {
        var crypto = new CryptoHelper();

        // ✅ Correct: Using the modern replacement suggested by the warning text.
        crypto.ComputeSHA256("secret"); 
    }
}
tip

Always check the IntelliSense tooltip when typing a method name. If it is crossed out (MethodName), hovering over it will display the CS0618 message before you even compile.

Solution 2: Suppress the Warning​

If you are maintaining a legacy application and cannot switch to the new method yet (perhaps due to backward compatibility requirements), you can suppress the warning locally.

Using Pragma​

Wrap the specific line of code to silence the warning just for that instance.

public class Program
{
    static void Main()
    {
        var crypto = new CryptoHelper();

        // ✅ Correct: Warning suppressed. 
        // We acknowledge the risk but need the old behavior for now.
        #pragma warning disable CS0618
        crypto.ComputeHash("secret");
        #pragma warning restore CS0618
    }
}
warning

The "True" Flag: If the author marked the attribute as [Obsolete("Message", true)], this becomes Error CS0619. In that case, the compiler treats it as a hard error, and #pragma suppression will not work. You strictly must update the code.

Conclusion​

CS0618 is a friendly warning that includes the solution instructions.

  1. Read the Message: The text after "is obsolete:" tells you exactly what to do.
  2. Refactor: Switch to the recommended replacement method or property.
  3. Suppress: Use #pragma warning disable CS0618 only if you are forced to use legacy code.

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