dotnet/wpf · error · ArgumentNullException

ArgumentNullException: fileName

Error message

ArgumentNullException: fileName

What it means

The InternalsVisibleTo target string is present but malformed: constructing new AssemblyName(assemblyName) throws ArgumentException, and LoadInternalsVisibleToHelper wraps it in XamlSchemaException with SR.BadInternalsVisibleTo2, naming both the bad string and the owning assembly.

Solutions

1. Correct the InternalsVisibleTo string to a valid assembly name (e.g. "MyApp.Tests"), matching the friend assembly's simple name.
2. Check build/MSBuild properties feeding the attribute for empty values.
3. Remove or update stale entries after assembly renames.
4. Validate with AssemblyName.TryParse-equivalent logic before shipping the attribute.

Example fix

// before
[assembly: InternalsVisibleTo("")]
// after
[assembly: InternalsVisibleTo("MyApp.Tests")]

foreach (var a in asm.GetCustomAttributes<InternalsVisibleToAttribute>())
{
    try { new AssemblyName(a.AssemblyName); } catch (ArgumentException) { /* fix attribute */ }
}

try { new XamlSchemaContext(); } catch (XamlSchemaException ex) when (ex.InnerException is ArgumentException) { /* correct the malformed InternalsVisibleTo string */ }

When it happens

Trigger: new AssemblyName(assemblyName) raising ArgumentException because the InternalsVisibleTo value is not a parseable assembly name (e.g. empty string, illegal characters, missing name), during LoadInternalsVisibleTo.

Common situations: Typos in the friend assembly name; attributes generated from build variables left empty ('' rather than null); renamed assemblies leaving stale InternalsVisibleTo entries; strong-name strings with corrupt public-key blobs.

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

• Animation_ChildMustBeKeyFrame
• Animation_ChildMustBeKeyFrame
• Animation_NoTextChildren
• Animation_NoTextChildren
• ArgumentException: path

AI-assisted analysis of dotnet/wpf@81131a70a4 (2026-09-14). Data as JSON: /api/errors/968562cba2d5619e. Report an issue: GitHub.

Appendix: source

Thrown at src/Microsoft.DotNet.Wpf/src/PresentationBuildTasks/MS/Internal/Tasks/TaskFileService.cs:243

            else
            {
                lastChangeDT = File.GetLastWriteTime(srcFile);
            }

            return lastChangeDT;
        }


        //
        // Checks to see if file exists
        //
        public bool Exists(string fileName)
        {
            bool fileExists;

            if (fileName == null)
            {
                throw new ArgumentNullException(nameof(fileName));
            }

            if (HostFileManager != null)
            {
                fileExists = HostFileManager.Exists(fileName, IsRealBuild);
            }
            else
            {
                fileExists = File.Exists(fileName);
            }

            return fileExists;
        }

        //
        // Deletes the specified file
        //
        public void Delete(string fileName)

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