PrismLibrary/Prism · error · InvalidOperationException

Resources.RegionBehaviorRegionCannotBeSetAfterAttach

Error message

Resources.RegionBehaviorRegionCannotBeSetAfterAttach

What it means

RegionBehavior.Region throws InvalidOperationException if you set the Region property after Attach() was called. A behavior is designed to be attached to exactly one region; re-pointing it afterward would corrupt region state.

Solutions

Create a new behavior instance per region instead of reusing one
Only set Region before calling Attach()
If using RegionBehaviorCollection.Add, let it set Region and call Attach for you

Example fix

// before
sharedBehavior.Region = newRegion; // already attached
// after
var behavior = new MyBehavior { Region = newRegion };
behavior.Attach();

Defensive patterns

Strategy: validation

Validate before calling

if (behavior.IsAttached) throw new InvalidOperationException("Behavior already attached; create a new instance.");

Type guard

bool Reusable(IRegionBehavior b) => !b.IsAttached;

Try / catch

try { behavior.Region = r; } catch (InvalidOperationException) { behavior = NewBehavior(r); }

Prevention

Never share behavior instances between regions
Set Region only before Attach
Let RegionBehaviorCollection.Add handle wiring

When it happens

Trigger: Assigning behavior.Region = someRegion after behavior.Attach() has already run — commonly when a behavior instance is shared/reused across two regions.

Common situations: Reusing a singleton IRegionBehavior for multiple regions; re-registering behaviors in a custom region adapter; region re-creation reusing old behavior instances.

Understand the failure class

Background: "Invalid state transition" errors: "status must be X, actually Y", "already rejected/charging/uninstalled", "cannot ... while running" — what they mean when a library rejects your call — this error's family across 31 libraries.

Related errors

AI-assisted analysis of PrismLibrary/Prism@358118cd64 (2026-09-15). Data as JSON: /api/errors/8805bc077b3e7e68. Report an issue: GitHub.

Appendix: source

Thrown at src/Prism.Core/Navigation/Regions/RegionBehavior.cs:26

    /// </summary>
    public abstract class RegionBehavior : IRegionBehavior
    {
        private IRegion region;

        /// <summary>
        /// Behavior's attached region.
        /// </summary>
        public IRegion Region
        {
            get
            {
                return region;
            }
            set
            {
                if (this.IsAttached)
                {
                    throw new InvalidOperationException(Resources.RegionBehaviorRegionCannotBeSetAfterAttach);
                }

                this.region = value;
            }
        }

        /// <summary>
        /// Returns <see langword="true"/> if the behavior is attached to a region, <see langword="false"/> otherwise.
        /// </summary>
        public bool IsAttached { get; private set; }

        /// <summary>
        /// Attaches the behavior to the region.
        /// </summary>
        public void Attach()
        {
            if (this.region == null)
            {

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