dotnet/wpf · error · ArgumentException

SR.JumpItemsRejectedEventArgs_CountMismatch

Error message

SR.JumpItemsRejectedEventArgs_CountMismatch

What it means

The JumpItemsRejectedEventArgs constructor requires the rejectedItems and reasons collections to be null together or non-null with equal counts, since each rejected jump item needs exactly one rejection reason. Any mismatch throws ArgumentException with JumpItemsRejectedEventArgs_CountMismatch.

Solutions

1. Ensure reasons.Count equals rejectedItems.Count, one reason per rejected item, before constructing the args.
2. Pass null for both parameters together if there are no rejections.
3. Build the lists in the same loop so index counts stay synchronized.

Example fix

// before
var args = new JumpItemsRejectedEventArgs(rejected, reasons.Take(1).ToList()); // count mismatch
// after
if (rejected.Count == reasons.Count)
    var args = new JumpItemsRejectedEventArgs(rejected, reasons);

bool IsValidPair(System.Collections.Generic.IList<JumpItem> items, System.Collections.Generic.IList<JumpItemRejectionReason> reasons) =>
    (items == null && reasons == null) || (items != null && reasons != null && items.Count == reasons.Count);

try { var args = new JumpItemsRejectedEventArgs(rejected, reasons); }
catch (ArgumentException ex) when (ex.Message.Contains("CountMismatch")) { /* resync lists */ }

When it happens

Trigger: `new JumpItemsRejectedEventArgs(items, reasons)` where one is null and the other is not, or where items.Count != reasons.Count (e.g. one reason omitted, or an empty reasons list with non-empty items).

Common situations: Custom JumpList.RejectItems handling code that aggregates reasons but drops entries for items filtered out earlier; hand-rolling event args when forwarding rejection data.

Understand the failure class

Background: "Must be a positive integer", "Invalid value", "Unsupported": the invalid-argument-value error family, when a library rejects the value you pass — this error's family across 35 libraries.

Related errors

• SR.JumpList_CantApplyUntilEndInit
• SR.JumpList_CantCallUnbalancedEndInit
• SR.JumpList_CantNestBeginInitCalls
• " }} " element found. Expected fixed page element ( }} ).
• ' ' can only host a ' ' or a ' '. ' ' is an invalid…

AI-assisted analysis of dotnet/wpf@81131a70a4 (2026-09-14). Data as JSON: /api/errors/02c3162246d734ab. Report an issue: GitHub.

Appendix: source

Thrown at src/Microsoft.DotNet.Wpf/src/PresentationFramework/System/Windows/Shell/JumpList.cs:62

    }

    /// <summary>
    /// EventArgs for JumpList.JumpItemsRejected event.
    /// </summary>
    public sealed class JumpItemsRejectedEventArgs : EventArgs
    {
        public JumpItemsRejectedEventArgs()
            : this(null, null)
        { }

        public JumpItemsRejectedEventArgs(IList<JumpItem> rejectedItems, IList<JumpItemRejectionReason> reasons)
        {
            // If one of the collections is null then the other has to be, too.
            if ((rejectedItems == null && reasons != null)
                || (reasons == null && rejectedItems != null)
                || (rejectedItems != null && reasons != null && rejectedItems.Count != reasons.Count))
            {
                throw new ArgumentException(SR.JumpItemsRejectedEventArgs_CountMismatch);
            }

            // We don't want the contents of the list getting modified in the event handler,
            // so use a read-only copy
            if (rejectedItems != null)
            {
                RejectedItems = new List<JumpItem>(rejectedItems).AsReadOnly();
                RejectionReasons = new List<JumpItemRejectionReason>(reasons).AsReadOnly();
            }
            else
            {
                RejectedItems = new List<JumpItem>().AsReadOnly();
                RejectionReasons = new List<JumpItemRejectionReason>().AsReadOnly();
            }
        }

        public IList<JumpItem> RejectedItems { get; private set; }
        public IList<JumpItemRejectionReason> RejectionReasons { get; private set; }

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