dotnet/wpf · error · ArgumentException

' ' cannot contain the path delimiter: ' '.

Error message

'{0}' cannot contain the path delimiter: '{1}'.

What it means

CheckStringForEmbeddedPathSeparator rejects strings containing the compound-file path separator (backslash). Compound-file element names are single path segments; embedded separators would imply a path, which is not allowed within one name.

Solutions

1. Pass only the final path segment, not a full path, as the name
2. Split the path into segments yourself and create intermediate storages for each level
3. Catch ArgumentException and detect the embedded delimiter to give a clearer message

Example fix

// before
CreateStream("folder\\file.bin");
// after
string name = Path.GetFileName("folder\\file.bin");
CreateStream(name); // create storages for intermediate segments separately

if (name.Contains('\\')) throw new ArgumentException("Name must be a single path segment, not a path");

static bool IsSingleSegment(string s) => !string.IsNullOrEmpty(s) && !s.Contains('\\');

try { CreateStream(name); } catch (ArgumentException ex) when (ex.Message.Contains("delimiter")) { /* split path and create storages */ }

When it happens

Trigger: Calling ConvertStringArrayPathToBackSlashPath or related compound-file APIs with a name string containing '\'.

Common situations: Passing a full path (e.g. "folder\file") where only a single segment name is expected.

Understand the failure class

Background: "Invalid ... format", "must be in format X", "does not look like a ..." — invalid argument format errors across CLI tools and libraries — this error's family across 17 libraries.

Related errors

• ' ' cannot start with the reserved character range…
• ' ' ID is not a valid XSD ID.
• ' ' is not a valid value for ' '.
• Can only countersign parts with Digital Signature…
• Cannot countersign an unsigned package.

AI-assisted analysis of dotnet/wpf@81131a70a4 (2026-09-14). Data as JSON: /api/errors/e678e6ec91bb7525. Report an issue: GitHub.

Appendix: source

Thrown at src/Microsoft.DotNet.Wpf/src/WindowsBase/MS/Internal/IO/Packaging/CompoundFile/ContainerUtilities.cs:453

            CheckStringAgainstNullAndEmpty(nameString, "nameString");

            return (nameString[0] >= '\x0001'
                    &&
                    nameString[0] <= '\x001F');
        }

        /// <summary>
        /// Checks for embedded delimiter in the string (as well as Null and Empty)
        /// </summary>
        /// <param name="testString">string to test</param>
        /// <param name="testStringIdentifier">message for exception</param>
        internal static void CheckStringForEmbeddedPathSeparator(string testString,
            string testStringIdentifier)
        {
            CheckStringAgainstNullAndEmpty(testString, testStringIdentifier);

            if (testString.Contains(PathSeparator))
                throw new ArgumentException(
                    SR.Format(SR.NameCanNotHaveDelimiter,
                        testStringIdentifier,
                        PathSeparator), nameof(testString));
        }

#endif
    }
}

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