PrismLibrary/Prism · error · ArgumentException

Resources.RegionNotFound (formatted with regionName)

Error message

Resources.RegionNotFound (formatted with regionName)

What it means

RegionManager.AddToRegion(string regionName, string targetName) throws ArgumentException (parameter regionName) when no region with that name is registered — Regions.ContainsRegionWithName(regionName) is false. Resources.RegionNotFound is formatted with the requested name.

Solutions

  1. Verify the region name matches exactly the RegionName registered in XAML/code (case-sensitive).
  2. Ensure the view containing the region has been created/loaded before calling AddToRegion, or use RegisterViewWithRegion instead.
  3. Check that a region adapter exists for the hosting control so the region is actually created.

Example fix

// before
regionManager.AddToRegion("MainreGion", "MyView"); // typo/case mismatch
// after
if (regionManager.Regions.ContainsRegionWithName("MainRegion"))
    regionManager.AddToRegion("MainRegion", "MyView");
Defensive patterns

Strategy: validation

Validate before calling

if (!regionManager.Regions.ContainsRegionWithName(regionName))
    throw new InvalidOperationException($"Region '{regionName}' is not registered");
regionManager.AddToRegion(regionName, targetName);

Type guard

bool RegionExists(IRegionManager rm, string name) => rm.Regions.ContainsRegionWithName(name);

Try / catch

try { regionManager.AddToRegion(regionName, targetName); }
catch (ArgumentException ex) { Console.WriteLine($"Region '{regionName}' not found: {ex.Message}"); }

Prevention

When it happens

Trigger: Calling regionManager.AddToRegion("SomeRegion", "ViewName") when no region named "SomeRegion" has been registered or the XAML region name differs in case/spelling.

Common situations: Typo or case mismatch between the RegionName in XAML and the string used in code; region not yet created because the containing view hasn't loaded; Prism.MongoDb... no — region adapters missing for the target control so no region was instantiated.

Understand the failure class

Background: 'Could not be found', 'does not exist', 'not found in database': the resource-not-found family when an ID, slug, key, or URI lookup comes back empty — this error's family across 20 libraries.

Related errors

AI-assisted analysis of PrismLibrary/Prism@358118cd64 (2026-09-15). Data as JSON: /api/errors/93df5540df5af05a. Report an issue: GitHub.

Appendix: source

Thrown at src/Maui/Prism.Maui/Navigation/Regions/RegionManager.cs:37

        Regions = new RegionCollection(this);
    }

    /// <summary>
    /// Gets a collection of <see cref="IRegion"/> that identify each region by name. You can use this collection to add or remove regions to the current region manager.
    /// </summary>
    /// <value>A <see cref="IRegionCollection"/> with all the registered regions.</value>
    public IRegionCollection Regions { get; }

    /// <summary>
    /// Add a view to the Views collection of a Region. Note that the region must already exist in this <see cref="IRegionManager"/>.
    /// </summary>
    /// <param name="regionName">The name of the region to add a view to</param>
    /// <param name="targetName">The view to add to the views collection</param>
    /// <returns>The RegionManager, to easily add several views. </returns>
    public IRegionManager AddToRegion(string regionName, string targetName)
    {
        if (!Regions.ContainsRegionWithName(regionName))
            throw new ArgumentException(string.Format(Thread.CurrentThread.CurrentCulture, Resources.RegionNotFound, regionName), nameof(regionName));

        var region = Regions[regionName];
        return region.Add(targetName);
    }

    /// <summary>
    ///     Add a view to the Views collection of a Region. Note that the region must already exist in this <see cref="IRegionManager"/>.
    /// </summary>
    /// <param name="regionName">The name of the region to add a view to</param>
    /// <param name="view">The view to add to the views collection</param>
    /// <returns>The RegionManager, to easily add several views. </returns>
    public IRegionManager AddToRegion(string regionName, object view)
    {
        if (!Regions.ContainsRegionWithName(regionName))
            throw new ArgumentException(string.Format(Thread.CurrentThread.CurrentCulture, Resources.RegionNotFound, regionName), nameof(regionName));

        return Regions[regionName].Add(view);
    }

View on GitHub (pinned to 358118cd64)