What causes "{osVer} is not a valid OS!" in dotnet/wpf?

Only pass values from the OperatingSystemVersion enum — never cast arbitrary ints to it.

dotnet/wpf · warning · ArgumentException

is not a valid OS!

Error message

{osVer} is not a valid OS!

What it means

OSVersionHelper.IsOsVersionOrGreater maps an OperatingSystemVersion enum value to the corresponding verify-version API. If the enum value falls outside every known case (an undefined or out-of-range cast value), it throws ArgumentException("{osVer} is not a valid OS!", nameof(osVer)). This guards internal callers against invalid enum values rather than silently returning false.

Solutions

Only pass values from the OperatingSystemVersion enum — never cast arbitrary ints to it.
Call OSVersionHelper.GetOsVersion() and compare the returned enum instead of hand-rolling version checks.
Update WPF references so the enum and helper come from the same build.
If extending the enum, add the matching case (and its IsOs*OrGreater property) to the switch.

Example fix

// before
var ver = (OperatingSystemVersion)99;
bool ok = OSVersionHelper.IsOsVersionOrGreater(ver); // throws ArgumentException
// after
bool ok = OSVersionHelper.IsOsVersionOrGreater(OperatingSystemVersion.Windows10RS5);

Defensive patterns

Strategy: type-guard

Validate before calling

static bool IsDefinedOsVersion(OperatingSystemVersion v) => Enum.IsDefined(typeof(OperatingSystemVersion), v);

Type guard

static bool TryGetOsCheck(OperatingSystemVersion v, out Func<bool> check)
{ check = null; if (!Enum.IsDefined(typeof(OperatingSystemVersion), v)) return false; check = () => OSVersionHelper.IsOsVersionOrGreater(v); return true; }

Try / catch

try { ok = OSVersionHelper.IsOsVersionOrGreater(ver); }
catch (ArgumentException) { ok = false; /* unknown version value */ }

Prevention

Never cast raw ints to OperatingSystemVersion
Use Enum.IsDefined before passing enum values across boundaries
Keep WPF reference assemblies and runtime assemblies in sync

When it happens

Trigger: Passing an OperatingSystemVersion value not present in the switch (e.g. a value cast from an int that is not a defined enum member, or an enum member added by a newer WPF but handled by an older helper).

Common situations: Reflection/dynamic code casting raw ints to OperatingSystemVersion; code compiled against a different WPF build than the one loaded (enum member drift); copy-pasted switch logic in forks missing a newly added case.

Understand the failure class

Background: Invalid enum value errors: "Unknown type", "Invalid scope", "must be one of" — when a string is not on the library's allowed list — this error's family across 23 libraries.

Related errors

AI-assisted analysis of dotnet/wpf@81131a70a4 (2026-09-14). Data as JSON: /api/errors/09a644d37b49121b. Report an issue: GitHub.

Appendix: source

Thrown at src/Microsoft.DotNet.Wpf/src/Shared/System/Windows/Interop/OSVersionHelper.cs:248

                case OperatingSystemVersion.Windows8:
                    return IsOsWindows8OrGreater;
                case OperatingSystemVersion.Windows7SP1:
                    return IsOsWindows7SP1OrGreater;
                case OperatingSystemVersion.Windows7:
                    return IsOsWindows7OrGreater;
                case OperatingSystemVersion.WindowsVistaSP2:
                    return IsOsWindowsVistaSP2OrGreater;
                case OperatingSystemVersion.WindowsVistaSP1:
                    return IsOsWindowsVistaSP1OrGreater;
                case OperatingSystemVersion.WindowsVista:
                    return IsOsWindowsVistaOrGreater;
                case OperatingSystemVersion.WindowsXPSP3:
                    return IsOsWindowsXPSP3OrGreater;
                case OperatingSystemVersion.WindowsXPSP2:
                    return IsOsWindowsXPSP2OrGreater;
            }

            throw new ArgumentException($"{osVer} is not a valid OS!", nameof(osVer));
        }

        internal static OperatingSystemVersion GetOsVersion()
        {
            if (IsOsWindows10RS5OrGreater)
            {
                return OperatingSystemVersion.Windows10RS5;
            }
            else if (IsOsWindows10RS4OrGreater)
            {
                return OperatingSystemVersion.Windows10RS4;
            }
            else if (IsOsWindows10RS3OrGreater)
            {
                return OperatingSystemVersion.Windows10RS3;
            }
            else if (IsOsWindows10RS2OrGreater)
            {

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