dotnet/maui · error · BuildException

XC0125

XC0125

Error message

"A resource with the key "{0}" is already present in the ResourceDictionary.

What it means

Raised by the XAML compiler (Build.Tasks) when two resources inside the same ResourceDictionary declare the same x:Key. The build task keeps a name set via GetResourceNamesInUse(parent) and rejects any key that already appears in it, so the XAML cannot compile until the collision is removed.

Source

Thrown at src/Controls/src/Build.Tasks/SetPropertiesVisitor.cs:1936

			return CanAddToResourceDictionary(parent, propertyType, elementNode, lineInfo, context);
		}

		static bool CanAddToResourceDictionary(VariableDefinition parent, TypeReference collectionType, ElementNode node, IXmlLineInfo lineInfo, ILContext context)
		{
			if (parent is null)
				return false;

			if (collectionType.FullName != "Microsoft.Maui.Controls.ResourceDictionary"
				&& !collectionType.InheritsFromOrImplements(context.Cache, context.Module.ImportReference(context.Cache, ("Microsoft.Maui.Controls", "Microsoft.Maui.Controls", "ResourceDictionary"))))
				return false;

			if (node.Properties.TryGetValue(XmlName.xKey, out INode value))
			{
				var valueNode = value as ValueNode ?? throw new BuildException(XKeyNotLiteral, lineInfo, null);
				var key = valueNode.Value as string;
				var names = context.Cache.GetResourceNamesInUse(parent);
				if (names.Contains(key))
					throw new BuildException(ResourceDictDuplicateKey, lineInfo, null, key);
				return true;
			}

			//is there a RD.Add() overrides that accepts this ?
			var nodeTypeRef = context.Variables[node].VariableType;
			var module = context.Body.Method.Module;
			if (module.ImportMethodReference(context.Cache,
											 module.GetTypeDefinition(context.Cache, ("Microsoft.Maui.Controls", "Microsoft.Maui.Controls", "ResourceDictionary")),
											 methodName: "Add",
											 parameterTypes: [nodeTypeRef]) != null)
				return true;
			throw new BuildException(ResourceDictMissingKey, lineInfo, null);
		}

		static IEnumerable<Instruction> Add(VariableDefinition parent, XmlName propertyName, INode node, IXmlLineInfo iXmlLineInfo, ILContext context)
		{
			var module = context.Body.Method.Module;
			var elementNode = node as ElementNode;

View on GitHub (pinned to f377ff1c5e)

Solutions

1. Search the whole XAML file (and any MergedDictionaries) for the duplicate x:Key and rename one of the two occurrences to a unique value.
2. If the duplicate comes from a merged dictionary, override it intentionally with MergedWith/Source semantics or remove it from the source dictionary instead of redeclaring.
3. Use a unique prefix per feature area (e.g. 'LoginPrimaryButton') to avoid accidental collisions as dictionaries grow.

Example fix

<!-- before -->
<ResourceDictionary>
    <x:String x:Key="Greeting">Hello</x:String>
    <x:String x:Key="Greeting">Hi</x:String>
</ResourceDictionary>
<!-- after -->
<ResourceDictionary>
    <x:String x:Key="Greeting">Hello</x:String>
    <x:String x:Key="GreetingAlt">Hi</x:String>
</ResourceDictionary>

// Before shipping XAML, assert every x:Key in a ResourceDictionary is unique.
// Pseudo: gather all x:Key literal strings per RD scope and assert no duplicates.
static void AssertUniqueKeys(IEnumerable<string> keys)
{
    var dup = keys.GroupBy(k => k).Where(g => g.Count() > 1).Select(g => g.Key);
    if (dup.Any()) throw new InvalidOperationException("Duplicate keys: " + string.Join(", ", dup));
}

When it happens

Trigger: The SetPropertiesVisitor sees a node whose parent collection is (or inherits from) Microsoft.Maui.Controls.ResourceDictionary, the node carries an x:Key whose string value is already present in context.Cache.GetResourceNamesInUse(parent), and the duplicate key throw fires.

Common situations: Copying a Style/Color/OnPlatform block and forgetting to rename its x:Key; merging MergedDictionaries that both define 'DefaultStyle'; refactoring resources between App.xaml and page-level dictionaries where the same key leaks in.

Related errors

• XC0126
• XC0128
• XC0127
• XC0066
• XC0000

AI-assisted analysis of dotnet/maui@f377ff1c5e (2026-08-13). Data as JSON: /api/errors/df6faa13f515b9dd. Report an issue: GitHub.