dotnet/wpf · error · ArgumentException

Expected a VersionPair object.

Error message

Expected a VersionPair object.

What it means

VersionPair.CompareTo(object) throws ArgumentException when the passed object is not null/other VersionPair and its runtime type differs from VersionPair's. IComparable semantics require comparing only compatible types, so a foreign type is rejected rather than mis-compared.

Solutions

1. Only pass VersionPair instances to CompareTo; convert System.Version to VersionPair first
2. Use generic collections (Sort<VersionPair>) to get compile-time type safety
3. Check obj is VersionPair before calling CompareTo

Example fix

// before
object v = Version.Parse("1.2");
versionPair.CompareTo(v); // throws
// after
if (v is VersionPair vp)
    versionPair.CompareTo(vp);
else
    throw new ArgumentException("Expected a VersionPair");

if (obj is not VersionPair) throw new ArgumentException("Expected a VersionPair object.", nameof(obj));

static bool IsVersionPair(object obj) => obj is VersionPair;

try { result = versionPair.CompareTo(obj); }
catch (ArgumentException ex) when (ex.Message.Contains("VersionPair")) { result = -1; /* define ordering for foreign types */ }

When it happens

Trigger: Passing a non-VersionPair object (e.g. System.Version, string, int) to CompareTo or to sorted collections/calls that invoke it, such as List<VersionPair>.BinarySearch with a boxed non-VersionPair value.

Common situations: Sorting mixed-type collections with ArrayList or non-generic Sort; calling CompareTo(obj) with a System.Version assuming interchangeability.

Understand the failure class

Background: Type mismatch errors: IllegalArgumentException, TypeError and type guards across 150 open-source libraries — this error's family across 150 libraries.

Related errors

• Cannot compare different types.
• SR.CanNotCompareDiffTypes
• ' ' is not a Visual or Visual3D.
• 0x80070057
• Animation_AnimationTimelineTypeMismatch

AI-assisted analysis of dotnet/wpf@81131a70a4 (2026-09-14). Data as JSON: /api/errors/2459bcf7a2f01a32. Report an issue: GitHub.

Appendix: source

Thrown at src/Microsoft.DotNet.Wpf/src/WindowsBase/System/IO/Packaging/CompoundFile/VersionPair.cs:286

#if !PBTCOMPILER
        /// <summary>
        /// Compare this instance to the object
        /// </summary>
        /// <param name="obj">Object to compare</param>
        /// <returns>Less than 0 - This instance is less than obj
        /// Zero - This instance is equal to obj
        /// Greater than 0 - This instance is greater than obj</returns>
        public int CompareTo(Object obj)
        {
            if (obj == null)
            {
                return 1;
            }

            if (obj.GetType() != GetType())
            {
                throw new ArgumentException(SR.ExpectedVersionPairObject);
            }

            VersionPair v = (VersionPair) obj;

            if (this.Equals(obj))   // equal
            {
                return 0;
            }
            // less than
            else if (this < v)
            {
                return -1;
            }

            // greater than
            return 1;
        }
#endif

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