Preprocessor errors and warnings

2025-10-09

The compiler generates the following errors for incorrect use of preprocessor directives:

CS1024: Preprocessor directive expected
CS1025: Single-line comment or end-of-line expected
CS1027: #endif directive expected
CS1028: Unexpected preprocessor directive
CS1029: #error: 'text'
CS1030: #warning: 'text'
CS1032: Cannot define/undefine preprocessor symbols after first token in file
CS1038: #endregion directive expected
CS1040: Preprocessor directives must appear as the first non-white-space character on a line
CS1517: Invalid preprocessor expression
CS1560: Invalid filename specified for preprocessor directive. Filename is too long or not a valid filename
CS1576: The line number specified for #line directive is missing or invalid
CS1578: Filename, single-line comment or end-of-line expected
CS1633: Unrecognized #pragma directive
CS1634: Expected disable or restore
CS1635: Cannot restore warning 'warning code' because it was disabled globally
CS1691: 'number' is not a valid warning number
CS1692: Invalid number
CS1694: Invalid filename specified for preprocessor directive. Filename is too long or not a valid filename
CS1695: Invalid #pragma checksum syntax; should be #pragma checksum "filename" "{XXXXXXXX-XXXX-XXXX-XXXX-XXXXXXXXXXXX}" "XXXX..."
CS1696: Single-line comment or end-of-line expected
CS1709: Filename specified for preprocessor directive is empty
CS7009: Cannot use #r after first token in file
CS7010: Quoted file name expected
CS7011: #r is only allowed in scripts
CS8097: #load is only allowed in scripts
CS8098: Cannot use #load after first token in file
CS8938: The #line directive value is missing or out of range
CS8939: The #line directive end position must be greater than or equal to the start position
CS8996: Raw string literals are not allowed in preprocessor directives
CS9028: The #line span directive requires space before the first parenthesis, before the character offset, and before the file name
CS9297: #: directives cannot be after first token in file
CS9298: #: directives can be only used in file-based programs (-features:FileBasedProgram)
CS9299: #: directives cannot be after #if directive
CS9314: #! directives can be only used in scripts or file-based programs

Invalid preprocessor directive syntax

CS1024: Preprocessor directive expected
CS1025: Single-line comment or end-of-line expected
CS1027: #endif directive expected
CS1028: Unexpected preprocessor directive
CS1038: #endregion directive expected
CS1040: Preprocessor directives must appear as the first non-white-space character on a line
CS1517: Invalid preprocessor expression
CS1633: Unrecognized #pragma directive
CS1696: Single-line comment or end-of-line expected
CS8996: Raw string literals are not allowed in preprocessor directives

These errors indicate that you used invalid syntax for preprocessor directives. Common causes include:

Using an unrecognized directive after # (CS1024, CS1633).
Including multiline comments on directive lines (CS1025, CS1696).
Using directives in unexpected locations (CS1028).
Missing required matching directives (CS1027, CS1038).
Not defining or undefining symbols as the first token in the file (CS1032)
Not placing the directive as the first token on a line (CS1040).
Using invalid expressions in conditional compilation (CS1517).
Using raw string literals in preprocessor directives (CS8996).

CS1024 example - Preprocessor directive expected:

#import System   // CS1024 - "import" is not a valid directive

CS1025 example - Single-line comment or end-of-line expected:

#if true /* hello  
*/   // CS1025 - multiline comment not allowed
#endif

CS1027 example - #endif directive expected:

#if true   // CS1027 - missing #endif
class Test { }

CS1028 example - Unexpected preprocessor directive:

#endif   // CS1028 - no matching #if

CS1032 example - #define and #undef preprocessor directives must appear before other tokens:

/* Comment */ 
#define X   // CS1032 - directive not first token in file

CS1038 example - #endregion directive expected:

#region testing
class Test { }
// CS1038 - missing #endregion

CS1040 example - Preprocessor directives must appear as the first non-white-space character:

/* Comment */ #define X   // CS1040 - directive not first on line

CS1517 example - Invalid preprocessor expression:

#if 1           // CS1517 - numeric literals not allowed
#endif
#if ~symbol     // CS1517 - bitwise operators not allowed  
#endif

CS1633 example - Unrecognized #pragma directive:

#pragma unknown  // CS1633 - "unknown" is not a valid pragma

CS8996 example - Raw string literals are not allowed in preprocessor directives:

// CS8996.cs
#pragma checksum """raw_string""" "{406EA660-64CF-4C82-B6F0-42D48172A799}" "hash"  // CS8996
class Test { }

To fix this error, use a regular string literal:

#pragma checksum "filename.cs" "{406EA660-64CF-4C82-B6F0-42D48172A799}" "hash"  // OK
class Test { }

To fix these errors, ensure your preprocessor directives follow the correct syntax rules described in the preprocessor directives documentation.

#error and #warning directive errors

CS1029: #error: 'text'
CS1030: #warning: 'text'
CS1634: Expected disable or restore
CS1635: Cannot restore warning 'warning code' because it was disabled globally
CS1691: 'number' is not a valid warning number
CS1692: Invalid number

These errors occur when the compiler processes #error, #warning, and #pragma warning directives. These directives allow you to generate custom error and warning messages during compilation and control warning behavior.

CS1029 displays the text of an error defined with the #error directive:

// CS1029.cs
class Sample
{
   static void Main()
   {
      #error Let's give an error here   // CS1029
   }
}

Compilation produces the following output:

example.cs(9,8): error CS1029: #error: 'Let's give an error here   // CS1029  '

CS1030 displays the text of a warning defined with the #warning directive:

// CS1030.cs
class Sample
{
   static void Main()
   {
      #warning Let's give a warning here
   }
}

Compilation produces the following output:

example.cs(6,16): warning CS1030: #warning: 'Let's give a warning here'

CS1634 occurs when a #pragma warning clause is badly formed, such as when disable or restore is omitted:

// CS1634.cs
// compile with: /W:1

#pragma warning   // CS1634
// Try this instead:
// #pragma warning disable 0219

class MyClass
{
  public static void Main()
  {
  }
}

CS1635 occurs when you use the /nowarn command line option or project setting to disable a warning globally, but use #pragma warning restore to attempt to restore that warning:

// CS1635.cs
// compile with: /w:1 /nowarn:162

enum MyEnum {one=1,two=2,three=3};

class MyClass
{
    public static void Main()
    {
#pragma warning disable 162

    if (MyEnum.three == MyEnum.two)
        System.Console.WriteLine("Duplicate");

#pragma warning restore 162  // CS1635
    }
}

CS1691 occurs when a number passed to the #pragma warning directive is not a valid warning number:

// CS1691.cs
public class C
{
    int i = 1;
    public static void Main()
    {
        C myC = new C();
#pragma warning disable 151  // CS1691
// Try the following line instead:
// #pragma warning disable 1645
        myC.i++;
#pragma warning restore 151  // CS1691
// Try the following line instead:
// #pragma warning restore 1645
    }
}

CS1692 occurs when a number in preprocessor directives like #pragma and #line is invalid because it's too large, in the wrong format, or contains illegal characters:

// CS1692.cs

#pragma warning disable a  // CS1692
// Try this instead:
// #pragma warning disable 1691

class A
{
    static void Main()
    {
    }
}

These directives are useful for conditional compilation scenarios where you want to alert developers about specific conditions in the code, or control which warnings are displayed during compilation. For more information about using these directives, see the preprocessor directives documentation and #pragma warning.

`#line` and file directive errors

CS1560: Invalid filename specified for preprocessor directive. Filename is too long or not a valid filename
CS1576: The line number specified for #line directive is missing or invalid
CS1578: Filename, single-line comment or end-of-line expected
CS1694: Invalid filename specified for preprocessor directive. Filename is too long or not a valid filename
CS1695: Invalid #pragma checksum syntax; should be #pragma checksum "filename" "{XXXXXXXX-XXXX-XXXX-XXXX-XXXXXXXXXXXX}" "XXXX..."
CS1709: Filename specified for preprocessor directive is empty
CS8938: The #line directive value is missing or out of range
CS8939: The #line directive end position must be greater than or equal to the start position
CS9028: The #line span directive requires space before the first parenthesis, before the character offset, and before the file name

These errors indicate incorrect usage of the #line directive or file-related preprocessor directives. Common causes include:

Invalid or missing filenames (CS1560, CS1694, CS1709).
Incorrect line number format or values (CS1576, CS8938, CS8939).
Missing proper syntax for filenames and comments (CS1578).
Malformed #pragma checksum syntax (CS1695).
Improper spacing in #line span directives (CS9028).

CS1560/CS1694 example - Invalid filename specified:

#line 100 "MyFile1234567890MyFile1234567890MyFile1234567890MyFile1234567890MyFile1234567890MyFile1234567890MyFile1234567890MyFile1234567890MyFile1234567890MyFile1234567890MyFile1234567890MyFile1234567890MyFile1234567890MyFile1234567890MyFile1234567890MyFile1234567890.txt"   // CS1560/CS1694 - filename too long

CS1576 example - Line number missing or invalid:

#line "abc.sc"         // CS1576 - missing line number
#line abc "file.cs"    // CS1576 - invalid line number format

CS1578 example - Filename, comment or end-of-line expected:

#line 101 abc.cs   // CS1578 - filename not quoted

CS1695 example - Invalid #pragma checksum syntax:

#pragma checksum "12345"  // CS1695 - missing GUID and checksum

CS1709 example - Empty filename:

#pragma checksum "" "{406EA660-64CF-4C82-B6F0-42D48172A799}" ""  // CS1709 - empty filename

CS8938 example - #line directive value missing or out of range:

#line   // CS8938 - missing value
#line 0  // CS8938 - out of range (must be 1-16,707,566)

CS8939 example - #line end position error:

#line (1, 10) - (1, 5) "file.cs"  // CS8939 - end column < start column

CS9028 example - #line span directive spacing:

#line(1, 1) - (1, 10)"file.cs"  // CS9028 - missing spaces

To fix these errors, ensure your #line directives and file-related preprocessor directives follow the correct syntax as described in the preprocessor directives documentation.

Incorrect use of file-based apps directives

CS9297: #: directives cannot be after first token in file
CS9298: #: directives can be only used in file-based programs (-features:FileBasedProgram)
CS9299: #: directives cannot be after #if directive
CS9314: #! directives can be only used in scripts or file-based programs

These errors indicate that you used the #: directives for a file-based app incorrectly. You can learn more about the syntax for these directives in the article on preprocessor directives in the section on file-based apps. Or, you can explore file based apps by following the tutorial on file-based apps.

File contains script.cs directives

The following errors indicate script.cs syntax in a compiled C# file:

CS7009: Cannot use #r after first token in file
CS7010: Quoted file name expected
CS7011: #r is only allowed in scripts
CS8097: #load is only allowed in scripts

These directives aren't supported in compiled C#. You must remove them, or use script.cs.

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