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How to Resolve Error "CS1025: Single-line comment or end-of-line expected" in C#

The Compiler Error CS1025 is a syntax error related to Preprocessor Directives. The message reads: "Single-line comment or end-of-line expected".

In C#, lines starting with a hash symbol (#) are processed by the preprocessor before the actual compilation. Unlike standard C# code, which ends with semicolons, preprocessor directives end at the new line. This error occurs when the compiler finds text, code, or invalid characters on the same line after a directive that doesn't accept arguments (like #else or #endif), or when using an invalid comment style.

This guide explains the strict formatting rules for preprocessor directives.

Understanding Preprocessor Line Rules​

Directives like #region accept text arguments (the region name). However, directives like #endif, #else, and #endregion do not accept arguments.

The only thing allowed after a no-argument directive is a Single-Line Comment (//).

  • Valid: #endif // Debug mode
  • Invalid: #endif Debug mode (The compiler thinks "Debug mode" is code, which isn't allowed here).

Scenario 1: Text After #endif or #else​

This is the most common cause. Developers often label their closing directives to indicate what block is ending, similar to how one might comment a closing brace } // End if. However, if you forget the //, the compiler rejects the text.

Example of error​

#if DEBUG
    public void Test() { }
// ⛔️ Error CS1025: Single-line comment or end-of-line expected.
// The compiler does not know what 'End of Debug' means here.
#endif End of Debug

Solution: Use a Comment​

Add double slashes // before the label.

#if DEBUG
    public void Test() { }
// ✅ Correct: The text is now properly commented out.
#endif // End of Debug

Scenario 2: Using Multi-line Comments​

While C# code supports block comments (/* ... */), preprocessor directives strictly require single-line comments (//) if you want to add text at the end of the line. Using block comment syntax on a directive line is invalid in many contexts or strictly interpreted as "unexpected text" by the preprocessor parser.

Example of error​

// ⛔️ Error CS1025: Preprocessor directives generally do not support /* */ syntax
// on the same line in this manner.
#region Settings /* Configuration Area */

Solution: Use Single-line Comments​

Switch to //.

// ✅ Correct
#region Settings // Configuration Area

Scenario 3: Code on the Same Line​

Unlike standard statements, you cannot put code on the same line as a directive. A directive consumes the entire line.

Example of error​

Trying to condense code to one line.

// ⛔️ Error CS1025: The code 'Console.WriteLine...' is treated as garbage text 
// belonging to the #if directive.
#if DEBUG Console.WriteLine("Debug Mode");
#endif

Solution: Use New Lines​

Directives must be on their own lines.

// ✅ Correct
#if DEBUG
    Console.WriteLine("Debug Mode");
#endif
note

Semicolons: Even adding a semicolon immediately after a directive (e.g., #define MY_FLAG;) will trigger this error or CS1024, because the semicolon is treated as invalid text, not a terminator.

Conclusion​

CS1025 is the preprocessor telling you: "I stopped reading the command, but there is still stuff on this line."

  1. Check #endif / #else: These take NO arguments. If you have text after them, ensure it starts with //.
  2. Check Comments: Always use // for comments on preprocessor lines, never /* */.
  3. Check Formatting: Never put C# code (like methods or variables) on the same line as a # directive.

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